BUKU I BAHAN AJAR - Perpustakaan Digital Polbandigilib.polban.ac.id/files/disk1/83/jbptppolban-gdl-radisuradi...CARA PENGGUNAAN ... data sheets allow for quick reference in case of

BUKU I BAHAN AJAR

TEKNIK PERAWATAN PESAWAT UDARA

Penyusuan Bahan Ajar Dalam Kurikulum Berbasis

Kompetensi (Kurikulum 2011) ini dibiayai dari DIPA

Politeknik Negeri Bandung

Departemen Pendidikan Nasional

Tahun Anggaran 2012

Disusun Oleh : Radi Suradi, Dipl. Ing, M.Eng.

NIP : 19640503 1992011 001

PROGRAM STUDI TEKNIK AERONAUTIKA JURUSAN TEKNIK MESIN

POLITEKNIK NEGERI BANDUNG 2012

BA 12 KBAE 3042 17

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HALAMAN PENGESAHAN

1. Identitas Bahan Ajar

a. Judul Bahan Ajar : TEKNIK PERAWATAN PESAWAT UDARA b. Mata Kuliah/Semester : Teknik Perawatan Pesawat Udara / V c. SKS (T-P)/Jam (T-P) : 2/6 d. Jurusan : Teknik Mesin e. Program Studi : Aeronautika e. Nomor Kode Mata Kuliah : KBAE 3042

2. Penulis

a. Nama : Radi Suradi, Dipl. Ing, M.Eng. b. NIP : 19640503 1992011001 c. Pangkat / Golongan : III/c d. Jabatan Fungsional : Lektor e. Program Studi : Aeronautika f. Jurusan : Teknik Mesin

Mengetahui,

Ketua KBK

Nur Rachmat, Dipl., Ing,, M.Sc.

NIP. 1960119 199102 1001

Bandung, 08 Desember 2012

Penulis,

Radi Suradi, Dipl. Ing, M.Eng.

NIP.19640503 1992011001

Menyetujui,

Ketua Jurusan / Program Studi

Ir. Ali Mahmudi, M.Eng. NIP. 19580606 199102 1001

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KATA PENGANTAR

Puji syukur penulis panjatkan kepada Alloh SWT. atas selesainya penulisan buku ajar “TEKNIK PERAWATAN PESAWAT UDARA” dalam rangka untuk meningkatkan kinerja proses belajar mengajar. Buku ajar ini dikhususkan untuk digunakan pada Program Studi Aeronautika, Jurusan Teknik Mesin Polban dan dengan buku ini dimaksudkan untuk dapat dipergunakan sebagai acuan bagi Mahasiswa dalam mempelajari sistem instrumentasi dan teknik digital yang digunakan pada pesawat terbang.

Isi materi buku ini ditujukan untuk mahasiswa diploma III dan dengan waktu pelaksanaan perkuliahan selama 80 jam. Sehubungan dengan teknologi perawatan pesawat terbang telah berkembang sedemikian pesat, maka isi buku ini dititik beratkan pada aspek pengetahuan secara umum dan pada aspek perawatan secara praktis.

Apabila ada saran, kritik, diskusi, koreksi ataupun masukan dari pembaca terkait dengan penyempurnaan lebih lanjut, maka penulis akan sangat berterima kasih dan terbuka untuk dihubungi secara langsung ataupun melalui e-mail : [email protected]

Semoga dengan bertambahnya ilmu dari buku ajar ini Alloh SWT. selalu memberkati perjalanan hidup mahasiswa Aeronautika khususnya dan seluruh keluarga besar Program Studi Aeronautika pada umumnya

Bandung, 08 Nopember 2012

Penyusun

Radi Suradi, Dipl. Ing, M.Eng.

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DAFTAR ISI

HALAMAN PENGESAHAN .......................................................................................... ii

DAFTAR ISI .................................................................................................................... iv

DESKRIPSI MATA KULIAH ......................................................................................... 1

CARA PENGGUNAAN ................................................................................................... 3

BAB I ................................................................................................................................ 4

SAFETY PRECAUTIONS AIRCRAFT AND WORKSHOP ......................................... 4

1.1 General, Material Safety Data Sheets ................................................................ 4

1.2 Container Labelling, Fire Regulations, First Aid ............................................... 9

1.3 Electricity, High-Pressure Gases, Hazards ....................................................... 12

1.4 Aviation Oils & Fuels, Chemical & Physiological Hazards ............................ 19

BAB II ............................................................................................................................. 22

WORKSHOP PRACTICES ........................................................................................... 22

2.1 Care and Control of Tools ................................................................................ 22

2.2 Use of Workshop Materials ............................................................................. 23

2.3 Tool Calibration ............................................................................................... 24

2.4 Standards of Workmanship .............................................................................. 27

2.5 Standards of Dimension, Allowances, and Tolerances .................................... 34

BAB III ........................................................................................................................... 38

ELECTRICAL WIRING INTERCONNECTION SYSTEMS (EWIS) ......................... 38

3.1 Continuity and Insulation Testing .................................................................... 38

3.2 Use of Crimping Tools ..................................................................................... 46

3.3 Testing of Crimped Joints ................................................................................ 59

3.4 Connector Pin Removal and Insertion ............................................................. 61

3.5 Coaxial Cables, Wiring Protection Technics ................................................... 74

BAB IV ........................................................................................................................... 95

PIPES AND HOSES ....................................................................................................... 95

4.1 Bending and Flaring Aircraft Pipes .................................................................. 96

4.2 Inspection and Testing of Pipes and Hoses .................................................... 104

4.3 Installation and Clamping of Pipes ................................................................ 107

BAB V .......................................................................................................................... 111

CONTROL CABLES ................................................................................................... 111

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5.1 Swaging of End Fitting .................................................................................. 111

5.2 Handling, Inspection and Testing of Ctrl Cable&Associated Hardware ....... 115

5.3 Bowden and Teleflex Cable Systems ............................................................. 124

BAB VI ......................................................................................................................... 129

WELDING, BRAZING, SOLDERING, AND BONDING ......................................... 129

6.1 Welding .......................................................................................................... 129

6.2 Soldering & Brazing ..................................................................................... 138

6.3 Bonding .......................................................................................................... 145

BAB VII ........................................................................................................................ 150

DISASSEMBLY, INSPECTION, REPAIR AND ASSEMBLY TECHNIQUES ....... 150

7.1 Types of Defect .............................................................................................. 150

7.2 Visual Inspections .......................................................................................... 154

7.3 Corrosion Removal Assessment and Reprotection ........................................ 161

7.4 General Repair Method .................................................................................. 173

7.5 NDT Technics ................................................................................................ 185

7.6 Disassembly and Re-assembly Technics ....................................................... 188

BAB VIII ...................................................................................................................... 192

AIRCRAFT HANDLING AND STORAGE ............................................................... 192

8.1 Aircraft Towing and Taxiing ......................................................................... 192

8.2 Aircraft Jacking, Chocking, Hoisting and Securing ....................................... 201

8.3 Aircraft Storage .............................................................................................. 218

REFERENSI ................................................................................................................. 222

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DESKRIPSI MATA KULIAH

Identitas Mata Kuliah Judul Mata Kuliah : Teknik Perawatan Pesawat Terbang

Nomor Kode / SKS : KBAE3042/ 2

Semester / Tingkat : V / III

Prasyarat : Dasar Teknik Listrik dan Elektronika

Sistem Pesawat Terbang

Jumlah Jam/Minggu : 6 jam/minggu

Ringkasan Topik / Silabus Teknik Perawatan Pesawat Terbang mempelajari dasar-dasar keteknikan tentang bagaimana merawat terbang secara teoritis yang diantaranya mendiskripsikan tentang safety precautions aircraft & workshop, workshop practices, electrical wiring interconnection systems, pipes & hoses, control cables, welding, brazing, soldering, & bonding, dan disassembly, inspection, repair, & assembly techniques, serta maintenance procedures. Mata kuliah Teknik Perawatan Pesawat Terbang dilaksanakan dalam 1 semester yaitu pada semester III. Materi pada Mata kuliah Teknik Perawatan Pesawat Terbang ini merupakan mata kuliah penunjang dari mata kuliah Sistem Pesawat Terbang sehingga dalam pembahasannya terkait dengan materi-materi lain yang tergabung didalam mata kuliah sistem pesawat terbang.

Kompetensi Yang Ditunjang Teknik Perawatan Pesawat Udara dalam kaitannya untuk mendukung pengetahuan dasar tentang perawatan pesawat udara serta sebagai pembekalan pengetahuan dalam memperoleh sertifikasi A1/A4 tentang kerangka dan mesin pesawat terbang. Hal lain yang diharapkan dapat diperoleh pada matakuliah ini adalah agar mahasiswa mampu mendapatkan gambaran secara menyeluruh tentang bagaimana merawat pesawat udara untuk mendukung ilmu dibidang Sistem Pesawat Udara yang kelak akan berguna bila mahasiswa lulus dan bekerja sebagai teknisi pesawat udara.

Tujuan Pembelajaran Umum Memahami fungsi, aplikasi, tata-letak komponen, dengan benar dan sehingga diakhir pembelajaran mahasiswa akan memahami gambaran terperinci terhadap konsep dan aplikasinya dalam merawat pesawat udara.

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Tujuan Pembelajaran Khusus Mahasiswa mampu: memilih hardware & tools dengan benar, memahami dokumen perawatan yang terkait (maintenance manual, task card, dsb), menerapkan teknik perawatan pesawat udara dan melaksanakan perawatan dengan mengindahkan prosedur keselamatam kerja yang telah dibakukan.

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CARA PENGGUNAAN

Pedoman Mahasiswa Pada setiap pertemuan proses belajar mengajar untuk mata kuliah Teknik Perawatan Pesawat Udara setiap mahasiswa wajib membawa buku ajar terkait dan perlengkapan belajar baku lainnya, mengerjakan tugas yang diberikan pengajar secara mandiri.

Pedoman Pengajar Pada setiap pertemuan proses belajar mengajar untuk mata kuliah Teknik Perawatan Pesawat Udara, pengajar wajib membawa buku ajar terkait juga harus menjelaskan secara aktif tentang pemahaman konsep serta detailnya yang terkait dengan yang diutarakan maupun tertulis serta pada slide power point. Pengajar wajib memberikan tugas mandiri kepada mahasiswa dan memeriksanya, hal tersebut dimaksudkan untuk mempermudah mahasiswa dalam memahami, menyerap dan mengerti materi yang diberikan.

Penggunaan Ilustrasi dalam Bahan Ajar Competency Base Training, Microsoft Power Point, Microsoft Excel & Microsoft Words

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BAB I SAFETY PRECAUTIONS AIRCRAFT AND WORKSHOP

Aircraft, by their very nature and design, make for a dangerous working

environment. The danger is further increased by the wide variety of machines, tools and

materials required to support and maintain aircraft.

Personal safety starts with being appropriately dressed for the work being

undertaken, combined with the correct use of eye and ear protection whenever

necessary

1.1 General, Material Safety Data Sheets

Technicians should only operate equipment with which they are familiar and

which they can operate safely. Hand tools should be kept in good working order.

Good 'housekeeping' in workshops, hangars, and on flight line ramps is essential

to safe and efficient maintenance. Pedestrian and fire lanes should be clearly marked

and NEVER obstructed. They should always be used to keep non-technical personnel

clear from the work area.

Any spillage of oils, greases and fuels should be immediately covered with

absorbent material and cleaned up, to prevent fire or injury. Spillage should be

prevented, from running into floor drains.

It is very important, that all personnel know the location of the fixed points

where fire fighting equipment and First Aid treatment are available. They must also be

aware of the types of emergency that can occur in the workplace (whether in the

workshop, hangar or on the ramp), and of the procedures to be followed in any

emergency.

While the goal of an aviation technician is to maintain aircraft in such a manner

as to assure safe flight, you must also be concerned with creating a safe environment

while an aircraft is on the ground. For example, the fuel tanks of transport aircraft

contain large amounts of highly flammable fuel and, therefore, can pose a considerable

risk of fire. In addition, rotating propellers and operating turbojet engines present a

serious risk of injury or death to ground personnel. Therefore, you must make every

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effort to prevent injury to personnel and damage to aircraft while maintenance and

servicing are being performed.

Accidents in the workplace are one of the leading causes of death and. One

reason for this is that after working at a job for a period of time, many people become

complacent and do not give workplace safety the attention it requires. Aircraft operation

areas contain many dangers to personnel, but a sound safety program and an aware

workforce can reduce these dangers dramatically. Make workplace safety one of your

primary job duties

COSHH regulations require an employer to have copies of relevant Material

Safety Data Sheets that are readily available to all shop personnel at all times. These

data sheets allow for quick reference in case of a chemical spill or injury. In the case of

a chemical injury, a copy of the pertinent data sheet(s) should be sent along to the

emergency room to ensure immediate medical attention.

A Material Safety Data Sheet consists of nine basic sections:

1) Product identification including trade name, and the address and

emergency phone number of the manufacturer/supplier.

2) Principal ingredients including percentages of mixture by weight.

3) Physical data describing the substances appearance, odour, and specific

technical information such as boiling point, vapour pressure, solubility,

etc.

4) Fire and explosion hazard potential.

5) Reactivity data including stability and incompatibility with other

substances.

6) First aid and health hazard data.

7) Ventilation and personal protection gloves, goggles, respirator. etc.

8) Storage and handling precautions.

9) Spill, leak, and disposal procedures.

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Figure 1-1 Material Safety Data Sheets (MSDS) provide information on hazardous materials that are present in the workplace, Furthermore, all employers must maintain current copies of the Material Safety Data Sheets for reference at any time.

Safety Around Machine Tools

Many kinds of high-speed cutting tools are commonly found in aviation

maintenance shops and can be dangerous if misused. However. these tools pose little

threat when used for their intended purpose and reasonable safety precautions are

observed. For example, do not use any machine tools with which you are not familiar,

or any tool whose safety features you are unfamiliar with. The guards and safety covers

found on many tools have been put there to protect the operator. Some of these guards

may appear to interfere with the operation of the equipment. However, they must never

be removed or disabled. The slight inconvenience they cause is more than compensated

for by the added safety they provide.

Dull cutting tools present a greater threat of injury than sharp tools since a dull

or improperlv sharpened tool requires excessive forces to do its job. As a result, the

work can be grabbed or thrown out of the machine. Therefore, always make sure the

cutting tool is sharp and serviceable before you use it.

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Figure 1-2 Be sure the eye-protection shields are in place when using a bench grinder.

When using a drill press be sure that the material being worked is securely

clamped to the drill press table before you begin drilling a hole. If this is not done, it is

possible for the drill or the cutter to grab the metal and spin it around, effectively slicing

anything in its way. Furthermore, never leave a chuck key in a drill motor or a drill

press. If the switch is accidentally tumed on. the key will be thrown out with

considerable force.

The use of eye protection cannot be overstressed. Chips coming off

metalworking tools can easily penetrate deeply into your eves. In addition, if someone

working near you is using compressed air, a blast of air can easily pick up dirt or dust

and spray into your face. To prevent eye injuries, always wear eye protection when

using power tools. or when you must enter areas where they are being used.

In addition to eye protection. you should always wear the appropriate clothing

when in the shop. For example. you should never wear ties or other clothing that could

get caught in a spinning tool. Furthermore, if you wear your hair long, tie it back to

keep it out of the way.

When adjusting or changing the blade or hit on a power tool, disconnect the tool

from its power source. When maintenance is performed on a power tool that cannot be

disconnected from its power source, the electrical junction box for that tool should he

tumed off and locked out, to prevent someone from accidentally tuming the power back

on.

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Figure 1-3 Face shields or goggles should be used when drilling, grinding, or sawing.

Power tools are one of the greatest timesavers found in a maintenance shop. but

you must not allow their convenience to cause you to misuse the tool. In other words,

never be in a hurry around a power tool and never use a tool for a purpose for which it

is not intended. Most important of all, think before using any tool.

Welding

Welded repairs are common in aircraft maintenance and shops should provide a

means of safely accomplishing the task. Welding should be performed only in areas that

are designated for the purpose. If a part needs to be welded, remove it and take it to the

welding area. Welding areas should be equipped with proper tables. ventilation, tool

storage. and fire extinguishing equipment. If welding is to be accomplished in a hangar,

no other aircraft should be within 35 feet of the hanger, and the area should be roped off

and clearly marked.

Fire Safety

Aviation maintenance shops harbor ail of the requirements for fires, so fire

prevention is a vital concem. All combustible materials should be stored in proper

containers in areas where spontaneous combustion cannot occur. Since dope and paint

solvents are so volatile. They should he stored in a cool, Ventilated area outside of the

shop.

Spilled gasoline, sanding dust, and dried paint overspray should never be swept

with a dry broom, since static electricity can cause a spark and ignite them. Always

flush these combustible products with water before sweeping them.

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Always be aware of the possibility of fire and pro-vide for exits when putting

aircraft in the hangar. Be sure that fire extinguishers are properly serviced, clearly

marked and never obstructed. The key to fire safety is a knowledge of what causes fire,

how to prevent them, and how to put them out

1.2 Container Labelling, Fire Regulations, First Aid

1.2.1 Container Labelling

Chemical hazard labels vary in size, style, and the amount of information they

convey. However, all hazardous materials utilize the same colour coding and hazard

indexing information. A typical hazard label consists of four colour-coded diamonds

arranged into one large diamond. The colours used in the table are red, blue, yellow, and

white. Within three of the coloured areas a number from zero to four appears. The label

area coloured red indicates a materials flammability hazard. A zero indicates materials

that are normally stable and that do not burn unless heated. A rating of four, however,

applies to highly combustible gases and volatile liquids with flash points below 73°F

and boiling points below 100°F. The blue area of the label rates a substances health

hazard from no significant risk (0), to life threatening or permanently damaging with

single or repeated exposures (4). The yellow area of the label rates a substances

reactivity. A zero rating applies to materials which are normally stable, even under fire

conditions, and which do not react with water. On the other hand, materials rated at four

are readily capable of detonation or explosive decomposition at normal temperatures

and pressures.

The white area of the label indicates a personal protection index. Unlike the

other three ratings given, the personal protection index incorporates an alphabetical

rating system using the letters 'A' through 'K'. Each letter indicates different

combinations of protective equipment to be worn when working with hazardous

materials. For example, the letter A indicates the minimum required equipment,

including safety glasses or goggles. A "K" rating requires the use of a full body suit,

boots and head mask with independent air supply. You should always use the

recommended safety equipment when handling hazardous materials.

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Figure 1-4 The material described by this sample hazard label presents a serious flammability hazard (red), a moderate hazard to health (blue), and a minimum stability hazard (yellow). When handling this material, the use of goggles, gloves and respirator IG) is required for personal protection.

1.2.2 Fire Regulations

Whenever possible portable fire-fighting equipment should be grouped to form a

fire point. The fire point should be clearly indicated so that it can be readily

identified. In premises that are uniform in layout extinguishers should, whenever

possible, be located at the same point on each floor.

If for any reason extinguishers are placed in positions hidden from direct view

their position should be indicated by suitable signs, as described in BS 5499 :

Parts 1 and 3.

Figure 1-5 Fire signs

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• No person should have to travel more than 30 metres from the site of a fire to

reach an extinguisher.

1.2.3 Type and Number of Portable Fire-fighting Equipment

The basic provision of extinguishers within a building should be one

extinguisher, of at least 13A rating, for every 200m2 of floor area, or part

thereof, with at least two per floor. However, for those buildings where an upper

floor area is less than 100m2, one single 13A rated extinguisher may be

acceptable on each floor.

Should the premises contain risks from flammable liquids, then B rated

extinguishers should be provided. Under normal circumstances a 34B rated

extinguisher would equate to a 13A in this respect.

1.2.4 Location of Portable Fire-fighting Equipment

Extinguishers should be sited in conspicuous positions where they are visible to

anyone using an escape route. They should be mounted on brackets fixed to the

wall or some other convenient structural feature.

Ideally, larger extinguishers should be mounted so that they can be conveniently

demounted for use without undue effort or risk of injury. In effect, this will

mean mounting the extinguisher with the handle no more than 1 m from the

floor but, circumstances may dictate variations from this recommendation.

Smaller and more easily handled extinguishers may be mounted with the handle

about 1.5m from the floor level.

Suitably constructed floor cradles may be acceptable in certain locations.

Where there are special risks, extinguishers should be grouped conveniently in

positions where any user will not be placed in danger whilst attempting to use

them.

Other suitable positions include near exits to rooms or store's, corridors or

lobbies, stairways and landings. Extinguishers should not be sited behind doors

or inaccessible positions such as deep recesses or in cupboards.

It is also necessary to consider their exposure to extremes of heat or cold as well

as the risk of accidental damage during the normal day-to-day use of the

premises. The operation of an extinguisher will be affected by temperature and

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this could even cause accidental discharge. In premises where theft and

vandalism are a problem, extinguishers may be located in secure areas, but

always under the supervision of trained staff.

1.2.5 First Aid

It has been previously discussed that, when working indoors, whether it is in an

office, a workshop or a hangar, there will be fixed points where fire-fighting equipment

is available. Similarly, there will be First Aid points where emergency kits, eye washing

equipment and call bells are installed and there will be trained First Aid personnel to

assist in the treatment of injuries. It is the responsibility of every person at work to

know:

The location of the First Aid Points

The methods of calling for help

The locations of alarm bells, and the siting of appropriate telephones which may

be used to summon help in an emergency

The identity of the trained First Aid personnel in their vicinity

In the event of an injury (however slight), it is important that the injured person,

or the attending First Aider, should complete an entry in the Accident Book, which is

usually kept near the First Aid Point.

1.3 Electricity, High-Pressure Gases, Hazards

1.3.1 Electricity

Every aircraft maintenance shop uses electrical power for day to day activities.

While electricity performs many useful functions, you must remember that it can injure

or kill if mishandled. Therefore, it is the responsibility of everyone that uses electrical

power to be aware of the safety procedures regarding it.

The human body conducts electricity. Furthermore, electrical current passing

through the body disrupts the nervous system and causes burns at the entry and exit

points. Common 220/240- volt, AC house current is particularly dangerous because it

affects nerves in such a way that a person holding a current-carrying wire is unable to

release it. Since water conducts electricity, you must avoid handling electrical

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equipment while standing on a wet surface or wearing wet shoes. The water provides a

path to ground and heightens the possibility of electric shock.

To understand how common hand tools can create an electrical hazard. consider

a typical electric drill that has an AC motor inside a metal housing. One wire is

connected to the power terminal of the motor, and the other terminal connects to ground

through a white wire. there are only two wires in the cord and the power lead becomes

shorted to the housing, the return current flows to ground through the operators body.

However, if the drill motor is wired with a three-conductor cord, return current flows

through the third (green) wire to ground.

To minimize the risk of shock, make sure that ail electrical equipment is

connected with threewire extension cords of adequate capacity. Furthermore, do not use

cords that are frayed, or that have any of the wires exposed, and be sure to replace any

plugs that are cracked.

The human body conducts electricity. Furthermore, electrical current, passing

through the body, disrupts the nervous system and causes burns at the entry and exit

points. The current, used in domestic 220-240 volt, 50Hz ac electricity, is particularly

dangerous because it affects nerves in such a way that a person, holding a current-

carrying conductor, is unable to release it. Table 6-1 shows some typical harmful values

and effects of both ac and dc electricity supplies.

Table 1 Harmful Values of Electricity

Since water also conducts electricity, great care must be taken to avoid handling

electrical equipment of all kinds when standing on a wet surface or when wearing wet

shoes. The water provides a path to earth and heightens the possibility of electric shock.

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To ensure that equipment is safe, the minimum requirement is through the use of three-

core cable (which includes an earth lead) and, possibly, a safety cut-out device.

In conjunction, more often than not, with ignorance or carelessness, electrical

hazards generally arise due to one or more of the following factors:

Inadequate or non-existent earthing

Worn or damaged wiring, insulation, plugs, sockets and other installations

Bad wiring systems and the misuse of good systems

Incorrect use of fuses

Inadequate inspection and maintenance of power tools and equipment

All electrical equipment must be regularly checked and tested for correct

operation and electrical safety. To show that this has been done, a dated label should be

attached, showing when the equipment was last tested and when the next inspection is

due.

Any new item of equipment must have a test label attached. The presence of a

test label does not, however, absolve the user from checking the equipment for any

external signs of damage, such as a frayed power cord (or missing safety devices)

before use.

In the event of a person witnessing another person receiving an electric shock,

the basic actions, to be followed by the witness, are:

Shout for help and ensure there is no danger of also becoming a victim

Switch off the electrical current or remove the victim from the supply by means

of insulated material

If the victim has ceased breathing, initiate resuscitation

Call for professional medical help

If the victim is suffering from burns, exclude air from wounds

Treat for shock by keeping the victim warm

The approved methods of artificial resuscitation must, by law, be displayed on

wall charts in workplaces.

1.3.2 High-Pressure Gases

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Compressed gases are frequently used in the maintenance and servicing of

aircraft. The use of compressed gases requires a special set of safety measures. The

following rules apply for the use of compressed gases:

Cylinders of compressed gas must be handled in the same way as any high-

energy (and therefore potentially explosive) sources

Eye protection must always be worn when handling compressed gases

Never use a cylinder that cannot be positively identified

When storing or moving a cylinder, have the cap securely in place to protect the

valve stem

When large cylinders are moved, ensure that they are securely attached to the

correct trolley or vehicle

Never direct high-pressure gases at a person

Do not use compressed gas or compressed air to blow away dust and dirt, as the

resulting flying particles are dangerous

Release compressed gas slowly. The rapid release of a compressed gas will

cause an unsecured gas hose to whip about and even build up a static charge,

which could ignite a combustible gas

Keep gas cylinders clean. Oil or grease on an oxygen cylinder can cause

spontaneous combustion and explosions

1.3.2.1 Safety Around Compressed Gases

Compressed gases are found in all aircraft maintenance shops. For example,

compressed air powers pneumatic drill motors, rivet guns. paint spray guns, and

cleaning guns. In addition, compressed nitrogen is used to inflate tires and shock struts

while compressed acetylene is used in welding.

Most shop compressed air is held in storage tanks and routed throughout the

shop in high pressure lines. This high pressure presents a serious threat of injury. For

example, if a concentrated stream of compressed air is blown across a cut in the skin, it

is possible for the air to enter the bloodstream and cause severe injury or death. For this

reason, air dusting guns are usually equipped with a restrictor that reduces the pressure

at their discharge to 30 PSI or less.

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Be very careful when using compressed air not to blow dirt or chips into the face

of anyone standing nearby. To prevent eye injury. you should wear eye protection when

using pneumatic tools. To prevent injury from a ruptured hose, always keep air hoses

and fittings in good condition.

Far too many accidents occur when inflating or deflating tires. Therefore, wheel

assemblies being worked on should be placed in a safety cage to minimize injury if the

wheel or tire fails during inflation. Always use calibrated tyre gauges, and make certain

to use a regulator that is in good working condition.

High-pressure compressed gases are especially dangerous if they are

mishandled. Oxygen and nitrogen are often found in aviation maintenance shops stored

in steel cylinders under a pressure of around 1,800 PSI. These cylinders have brass

valves screwed into them. If a cylinder should be knocked over and the valve broken

off, the escaping high-pressure gas would propel the tank like a rocket. Because this

would create a substantial hazard, you should make sure that all gas cylinders are

properly supported. A common method of securing high pressure cylinders in storage is

by chaining them to a building. Furthermore, a cap should be securely installed on any

tank that is not connected into a system. This protects the valve from damage.

It is extremely important that oxygen cylinders be treated with special care. In

addition to having all of the dangers inherent with other high-pressure gases, oxygen

always possesses the risk of explosion and combustion. For example. you must never

allow oxygen to come in contact with petroleum products such as oil or grease, since

oxygen causes these materials to ignite spontaneously and burn. Furthermore, never use

an oily rag, or tools that are oily or greasy, to install a fitting or a regulator on an oxygen

cylinder.

To minimize the risk of fire. use only an approved MIL Specification thread

lubricant when assembling oxygen system components. When checking oxygen systems

for leaks, use only an approved leak check solution that contains no oil.

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Figure 1- 6 Be sure the protective cap is screwed on a cylinder containing high-pressure gas to prevent damage to the valve when the cylinder is not connected into a system

1.3.2.2 Gas Bottle Identification

High-pressure gas cylinders contain various types of gas, the most common used

on commercial aircraft being nitrogen and oxygen. To ensure correct identification of

these containers, they are colour coded and the name of the gas is stencilled on the side.

In the UK, gas containers use BS 381 C as the standard to determine the correct

colour and shade for each gas type. Nitrogen bottles are painted grey on the body with a

black neck, whilst oxygen bottles are black with a white neck. Be aware that bottles of

US manufacture use an alternative system, the main difference being oxygen bottles are

painted green all over.

1.3.2.3 High Pressure Gas Replenishing

When replenishing aircraft services such as tyres and hydraulic accumulators

with highpressure gas, care must be taken to ensure that only the required pressure

enters the container. When full, a gas storage bottle can hold as much as 200 bar (3000

PSI) whilst an aircraft tyre pressure may only require 7 bar (100 PSI).

To safely control the gas, two pressure regulating valves are used, one at the

storage bottle end and one at the delivery end of the system. If one valve fails the other

will prevent the receiving vessel from taking the full bottle pressure with the

consequence of an explosion.

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For added safety the gas bottle valve opening key should be left in the valve

whilst decanting operations are completed. If problerns occur then the high-pressure

bottle can be quickly isolated before the situation becomes dangerous.

The transfer of high-pressure gases from a large storage bottle to the aircraft

component is often called decanting and must be done at a very slow rate. If the gas is

decanted rapidly the temperature of the receiving component will increase in

accordance with the gas laws.

Again using the same gas laws the temperature of the gas in the container will

drop to ambient, and the pressure in that vessel will reduce. The component pressure

will now be incorrect and require the decanting process to be repeated.

In workshops, compressed air is, sornetimes, produced by a compressor (which

is housed in a remote building), and fed, via galleries, to work stations. Care rnust be

taken to ensure that no damage occurs to the piping whilst in use.

If a concentrated stream of compressed air is blown across a cut in a person's

skin, then air can enter the blood stream and cause injury or death. For this reason, air-

dusting guns are restricted to about 2000 kPa (30 PSI).

Aircraft tyres can require very high pressures and must be inflated inside a

strong cage. This cage would protect the personnel working on the wheels in the event

of a tyre or wheel bursting.

1.3.2.4 Oxygen Systems

Modern aircraft fly at altitudes where life support systems are needed. Even

though most of these aircraft are pressurised, emergency oxygen must be carried in the

event that the pressurisation system fails. Smaller aircraft can carry oxygen in cylinders

whilst the larger, civil aircraft have individual oxygen generator units.

These units are stowed in the overhead cargo bins, above the passenger seats,

and are known as the passenger service units or PSUs. A PSU produces oxygen, by

means of a chemical reaction, and is activated when its mask (which drops from the

overhead bin in an emergency) is pulled by a passenger.

Note: When PSUs reach their life expiry and have to be returned to their

manufacturer, it is vital that all precautions are followed to make the units 'safe' for

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transit. PSUs get very hot when working and have caused the destruction, due to fire, of

an aircraft, which was carrying these units as cargo.

The main oxygen systems are serviced from trolleys or vehicles that carry a

number of highpressure bottles of oxygen, which can be at 140 bar (2,000 PSI) or more.

Some trolleys may also have a bottle of nitrogen, to allow the replenishment of

hydraulic accumulators and landing gears. The two types of bottles must be separated,

in order to prevent the accidental mixing of the gases.

It is extremely important that oxygen cylinders be treated with special care,

because, in addition to having all the dangers inherent with all other high-pressure

gases, oxygen always possesses the risk of combustion and explosion.

As previously stated, oxygen must never be allowed to come into contact with

petroleum products such as oil and grease, since oxygen causes these materials to ignite

spontaneously and to burn. Furthermore, an oil-soaked rag, or tools that are oily or

greasy (or badly oil-stained overalls), must never be used when installing an adapter or

a regulator on an oxygen cylinder.

Due to the risk of fire and explosion, replenishing trolleys must never be parked

close to hydraulic oil replenishing rigs, or in any area where petroleum products are

likely to come into contact with the oxygen servicing equipment. Similarly only

specially approved thread lubricants can be used when assembling oxygen components.

1.4 Aviation Oils & Fuels, Chemical & Physiological Hazards

1.4.1 Aviation Oils & Fuels

Aviation oils, generally, are a fairly low-risk material when compared with the

more volatile, higher distillates of petroleum such as the aviation fuels - petrol

(gasoline) and paraffin (kerosene). Most lubricating oils are flammable, if enough heat

is generated but, when the materials are kept away from excessive heat sources, they are

(comparatively) quite begin.

Synthetic lubricating oils, methanol and some hydraulic oils may be harmful or

even toxic if their vapours are inhaled. Also, if they come into contact with the skin or

eyes, they can cause injury or blindness. Particular note should be taken of any wamings

of dangers to health that may be contained in the relevant maintenance manuals and the

recommended procedures for the handling of these liquids should always be observed.

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Oils and fuels also have an adverse effect on paintwork, adhesives and sealants

and, thus, may inhibit corrosion-prevention schemes. Care should, therefore, be taken

not to spill any of these liquids but, if a spillage should occur, it must be cleaned up

immediately.

Note: Sweeping up gasoline spillage with a dry broom can cause a build up of

static electricity, with the accompanying risk of explosion.

With gasoline and kerosene there is a much greater chance of fire, so more

thorough precautions are required. These start with the basic rules, such as not wearing

footwear with nails or studs (to prevent sparks), not carrying matches or cigarette

lighters and ensuring that ALL replenishing equipment is fully serviceable.

Note: All references to refuelling, normally, also include the action of de-

fuelling and both are considered under the common term of fuelling.

During any fuelling operation, in a workshop, a hangar or on the flight line, the

relevant fire extinguishers must be in place

1.4.2 Chemical & Physiological Hazards

Many chemical compounds, both liquid and solid, are used in aircraft

maintenance and these may need specific precautions. Any precautions can be found in

the relevant maintenance manuals and in the Control of Substances Hazardous to Health

(COSHH) leaflets applicable to those materials.

The range of adhesives used for repair and sealing during the maintenance of

aircraft is vast. A large number of these produce vapours which, generally, can be

dangerous in any enclosed space, both from the results of inhalation of narcotic fumes

and from the fire risk associated with those which give off volatile, flammable, vapours.

Surface finishes present another area where the various types of material used

(etchants, celluloses, acrylics, enamels, polyurethanes etc.), dictate specific precautions.

The solvents used, before the actual painting and afterwards, need safety precautions

with regards to ventilation, reaction with other materials and, most importantly, their

possible corrosive, toxic, irritant and addictive effects on personnel.

Some materials have a mildly radioactive property, although they emit little

ionising radiation in normal circumstances. These materials are sometimes referred to as

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'heavy metals' and can be found in balance-weights as well as in smoke detectors,

luminescent 'EXIT' signs and instruments.

This radiation differs from that used for non-destructive testing (NOT)

procedures, where high levels of radiation are employed, by specially trained personnel,

and which, therefore, require many safety precautions to avoid personal injury. The

safety precautions for NOT procedures will be found within the manuals applicable to

their employment.

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BAB II

WORKSHOP PRACTICES

Despite the enormous advances in the mechanisation and computerisation of the

engineering industry in general, there remains the requirement for a high degree of hand

skills on the part of technicians who are engaged in the day-to-day maintenance of

aircraft and their associated components.

While the majority of aerospace components are manufactured under stringent

standards, in factory (and laboratory) conditions, it is necessary to remove many items

of equipment for cleaning, inspection, overhaul and, if needed, repair before they are,

subsequently, re-installed in their appointed locations.

These actions may entail the use of many specialist tools and materials, which

are used while following written procedures, while it is quite possible that some,

comparatively simple, repairs may call upon such basic hand skills as the cutting, filing,

drilling, riveting and painting of metals or other materials.

No matter whether there are specialist or basic skills required, all will demand a

certain quality of the work practices (and of the work-force) involved.

2.1 Care and Control of Tools

Engineers are responsible for the maintenance of their personal tools, whilst

other personnel, in designated Tool Stores, must care for all the different, specialist

tools for which they have the responsibility. It is also the responsibility of engineers to

ensure that any tools, or other items of equipment they use, are not left in an aircraft or

associated components.

The care required for different tools can vary. Ordinary hand tools may merely

require racking or locating within sturdy tool boxes, with careful, daily, maintenance

restricted to little more than a visual check.

Precision instruments however, require great care both in storage and in use.

They may need to be kept in special, soft-lined, boxes within other storage facilities.

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Prior to use they should have a 'zero' check or calibration. Some tools require that they

have a light coating of machine oil, to prevent the onset of corrosion.

Each tool (whether it be a hammer or a micrometer), will require some special

care, to ensure its optimum performance for, without this care, even the most expensive

tools very quickly become second rate and useless.

2.2 Use of Workshop Materials

Many of the wide variety of materials, used in workshops, require some form of

control in their handling, This control can involve:

Safety: relating to such topics as the toxicity, corrosiveness or other health risks

associated with the use of certain materials

Management: referring to the storage, use and correct handling of all materials

whether they are solid, liquid, or, in some instances, gaseous

Economy: involving such matters as to the using of the correct dosage or

proportions when mixing compounds, using only as much material as required

for a specific task and to the keeping in stock of only sufficient materials and

thus avoiding 'lifed' items reaching their expiry dates before being used.

Abrasive papers, solder and brazing materials, wire wool, tyre powder, oil spill

powder and so on, all require control of issue and use, though they may not, normally,

require stringent safety precautions.

A huge range of liquids can be used in the workshop situation, some of which

are harmless and some of which are extremely toxic. It is vital that the work-force make

themselves aware of the risks involved when dealing with ANY materials, and

especially when working within enclosed areas.

Some materials are flammable and must, therefore, be stored outdoors. These

include oils, greases, some adhesives, sealing and glazing compounds in addition to

many paints, enamels and epoxy surface finishes, which are stored in metal cabinets

and, usually, located (in the Northern hemisphere) on the North side of a workshop or

hangar. This ensures that the cabinet remains in the shade of the building and does not

get exposed to the sun's hot rays during the day. It is also important that only the

minimum amount of these materials is taken indoors for the work which is being done.

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When handling materials that give off fumes, it may be necessary to have the

area well ventilated and/or have the operator wearing a mask or some form of remote

breathing apparatus. The finished work may also give off fumes for some time

afterwards, so care must be taken to keep it ventilated if necessary.

Obviously all liquids must only be used for the purpose for which they are

designed and never mixed together, unless the two materials are designed to be mixed,

such as with two part epoxy adhesives and sealants. Many liquids used in workshops

and in the hangar have (as mentioned earlier) a fixed 'life'. This date is printed on the

container and must be checked before use, because many materials are unsafe if used

beyond their expiry date.

The disposal of liquids is a critical operation, and must only be carried out in

accordance with company (and, often, national or international) regulations. Liquids

must never be disposed of by pouring them into spare or unidentified containers and

they must not be allowed to enter the 'domestic' drains systems. The working with, and

the use of, high pressure gas containers and oxygen systems, was adequately discussed

in the Safety Precautions topic.

2.3 Tool Calibration

Requirements within the relevant airworthiness codes, applicable to the United

Kingdom Civil Aviation Industry, such as the British Civil Aviation Requirements

(BCARs), Joint Aviation Requirements (JARs), and Air Operators Certificates,

prescribe that where necessary, tools, equipment and, in particular test equipment are all

calibrated to acceptable standards.

Figure 2-1 All measuring equipment must be regularly calibrated

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This topic provides an overall picture of the types of requirements and tests

required in establishing and maintaining an effective calibration system. It takes into

account factors such as the degree of accuracy required, frequency of use and the

reliability of the equipment.

The key factor is the need to establish confidence in the accuracy of the

equipment when it is required for use. The required calibration frequency for any

particular piece of test equipment is that which will ensure it is in compliance with the

standards applicable to its intended use. In all cases, standards used are attributed upon

the need for ultimate traceability to one of the following:

The standard specified by the equipment manufacturer/design organisation

The appropriate National/International Standards.

The appropriate standards are used to achieve consistency between

measurements made in different locations, possibly using alternate measuring

techniques. The calibration of test equipment is best achieved by the operation of a

methodical system of control.

This system should be traceable by an unbroken chain of comparisons, through

measurement standards of successively better accuracy up to the appropriate standard.

Where recommendations for calibration standards are not published, or where they are

not specified, calibration should be carried out, in the UK, in accordance with British

Standard EN 30012-1: Quality Assurance Requirements for Measuring Equipment.

As an alternative to operating an internal Measurement and Calibration System,

an Approved Organisation or an Approved/Licensed Engineer may enter into a sub-

contracting arrangement to use an Appliance Calibration Service. This arrangement

does not absolve the contractors of the service from maintaining standards as if they

were carrying out the work themselves.

In all instances, it is the responsibility of the user to be satisfied that the

unbroken traceability chain is in place. External organisations, which supply an external

Calibration Service, should be those holding accreditation of the National Accreditation

of Measurement and Sampling, (NAMAS).

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2.3.1 Procedures

Requirements within the relevant airworthiness codes applicable to the UK

aviation industry such as Part 145, or JAR OPS prescribe that where necessary, tools,

equipment, and in particular test equipment are calibrated to standards acceptable to the

authority. The key factor, however is the need to establish confidence in the accuracy of

the equipment when required for use. A clear system of labelling calibrated appliances

is necessary setting out when the next inspection, service or calibration is due and

indicating the serviceability, particularly where it may not be obvious.

Some tools require calibration on a regular basis, and some every time before

use. As an example micrometers would require the calibration of the zero settings using

a test piece where necessary. Tools such as torque wrenches require calibration before

use on every occasion using a torque test gauge. Equipment such as tyre pressure

gauges require calibration by the manufacturer on a predetermined basis.

A record would be kept for all tools that require calibration, detailing when last

done, when next due, and the requirements of the calibration. A sticker would be

attached to the tool detailing the due date of the next calibration. This should always be

checked before use, and no tool should be used if it out of calibration.

Calibration records or certificates should as a minimum, contain the following

information:

Identification of equipment

Standard used

Results obtained

Uncertainty of measurement

Assigned calibration interval

Limits of permissible error

The authority under which the release document was issued

Any limitation in the use of the equipment

Date on which each calibration was conducted

Any appliance whose serviceability is in doubt, should be removed from service,

and labelled accordingly. Such equipment shall not be retumed to service until the

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reasons for the unserviceability have been eliminated and its continued calibration is re-

validated.

2.3.2 Calibration Standard

In all cases, standards used are predicated upon the need for ultimate traceability

to one of the following:

The standard specified by the appliance manufacturer/design organisation

The appropriate national/intemational standard

National Accreditation of Measurement and Sampling. (NAMAS)

United Kingdom Accreditation

2.4 Standards of Workmanship

Standards are so much a part of our daily routine that we use them without even

being aware of doing so, and without giving thought to how they are created or the

benefits they provide. Intemational Standards are basic technology and economic

building blocks similar to DNA because they affect everything we do. It is estimated

that more than 500,000 Standards exist in the world today to support the global

marketplace.

Standards have existed for thousands of years. For example, the first long

distance roads in Europe were built by Imperial Rome for the benefit of their legions.

The ruts created by the Roman chariots were then used by all other wagons and later

became a gauge for laying the first railway lines.

A Standard is an agreed way of doing something. It can be recorded and

published formally, or may simply be a company's informal unwritten procedure.

Formal Standards, such as British, European or intemational Standards vary

according to what they provide. For example, they may specify requirements for the

features or characteristics of a product, such as the components for solar heating

equipment, or recoml\lend the best way of doing something, such as the service

supplied by fumiture removal companies, or for a system such as a company's system

for managing information security.

Standards provide benefits to business and to individuals, by defining accurate

measurements and lowering production costs; improving product performance, quality,

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uniformity, interoperability and functionality; and providing a method to improve

health, safety, the environment, communications, competition, intemational trade, and

improving the quality of life

Standards may be one of the following types:

Private Standards are used only by the organization that developed them

Open source Standards are made available for anyone to use and. An example

is computer software that is freely available on the Intemet, and which provides

the original source code so advanced users can modify it.

Formal Standards are produced through a Standards organisation for national,

European or international use, for example:

National Standards, e.g. British Standards (suffix BS) are produced by a

country's National Standards Body (NSB). In the UK, British Standards are

developed together with the UK government, businesses and society. Some are

enforced by regulation, but most Standards are voluntary.

Publicly Available Standards (PAS) can be, for example, a Standard produced

by a trade association that may be used by any organization, Under the PAS

scheme, BSI helps organizations to draft best-practice methodologies for

products or processes.

European Standards (suffix EN) are produced by CEN, the European

Committee for Standardization, whose members are the national Standards

bodies of the European Union countries.

International Standards (suffix ISO) are produced by ISO, the International

Organization for Standardization, whose members are the national Standards

bodies of countries all over the world.

International Electro-technical Standards (suffix lEe) are produced by the

Electrotechnical Commission. Formal Standards are created following

discussions with a variety of interested organizations and groups.

2.4.1 National Standards

A country's National Standards Body (NSB) is usually its biggest producer of

formal Standards. The NSB brings together representatives from relevant sections of

business, government and society in technical committees that develop the Standards.

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BSI British Standards is the national Standards body in the UK. It produces British

Standards and ensures the representation of UK interests in European and international

forums. Other NSBs include AFNOR in France, DIN in Germany and ANSI in the US.

2.4.2 European Standards

CEN (European Committee for Standardization) members are the national

Standards bodies of countries in the European Union, including BSI. CEN promotes

voluntary technical harmonization to reduce trade barriers in Europe and worldwide. All

European Standards are adopted by the EU countries. In the UK, they are adopted as

British Standards (BS EN). An example of this is the toys Standard BS EN 71, which

relates to the EU Directive for the trade of new toys in Europe.

2.4.3 International Standards

ISO (International Organization for Standardization) is the world's largest

developer of Standards. Its membership comprises the National Standards Bodies of

countries around the world. BSI is a leading member of ISO and represents the UK's

interest in the development of international standards. BSI also decides which

international Standards to adopt as British Standards (BS ISO).

IEC is the International Electro-technical Commission. IEC oversees the

development of electrical and electronic Standards for participating countries. Most

Standardization work takes place through groups of experts (known as technical

committees). At national, European or international level, committees are made up of

representatives from businesses, trade associations, government, academia, consumer

and other groups.

BSI does not write the Standards but ensures that all organizations represented

are involved in discussions until a consensus is reached and that all stages of

development of a Standard are followed.

Reliable and well-defined procedures are essential to ensure agreement is

reached on Standards, such as fairness, openness, transparency, and methods to ensure

consideration of the views for all interested parties.

Before final publication, formal Standards are made publicly available for

review. Standards are vital tools of industry and trade because they promote

understanding between buyers and sellers. For example, buyers may not be able to tell

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whether a product is suitable to meet their needs. Information on whether a product,

service or system meets a particular Standard can tell a buyer about the product's safety

and suitability.

In the UK we have the Kite mark scheme, which is a mark of quality and

indicates that a product meets the requirements of a particular Standard. Most Standards

are voluntary and not legal documents although they may be called up in legal cases.

They are often linked to UK legislation or EC directives.

Companies that rely on conformity assessment results (for example, to know that

a product they trade meets the requirements of a supplier) need to know and understand

which types of conformity assessment activities were included in the process.

Conformity assessment activities typically include:

inspection

testing

laboratory accreditation

certification programs and their accreditation

management system assessment/registration and accreditation

recognition of the competence of accreditation programs

In the UK, conformity assessment is regulated by UKAS (United Kingdom

Accreditation Service).

2.4.4 International Organization for Standardization

ISO (International Organization for Standardization) is the world's largest

developer of standards. Although ISO's principal activity is the development of

technical standards, ISO standards also have important economic and social

repercussions. ISO standards make a positive difference, not just to engineers and

manufacturers for whom they solve basic problems in production and distribution, but

to society as a whole.

The International Standards which ISO develops are very useful. They are useful

to industrial and business organizations of all types, to governments and other

regulatory bodies, to trade officials, to conformity assessment professionals, to suppliers

and customers of products and services in both public and private sectors, and,

ultimately, to people in general in their roles as consumers and end users.

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ISO standards contribute to making the development, manufacturing and supply

of products and services more efficient, safer and cleaner. They make trade between

countries easier and fairer. They provide governments with a technical base for health,

safety and environmental legislation. They aid in transferring technology to developing

countries. ISO standards also serve to safeguard consumers, and users in general, of

products and services - as well as to make their lives simpler.

When things go well - for example, when systems, machinery and devices work

well and safely - then it is because they conform to standards. And the organization

responsible for many thousands of the standards which benefit society worldwide is

ISO.

ISO is a network of the national standards institutes of 156 countries, on the

basis of one member per country, with a Central Secretariat in Geneva, Switzerland,

that coordinates the system.

ISO is a non-govemmental organization: its members are not, as is the case in

the United Nations system, delegations of national govemments. Nevertheless, ISO

occupies a special position between the public and private sectors. This is because, on

the one hand, many of its member institutes are part of the govemmental structure of

their countries, or are mandated by their govemment. On the other hand, other members

have their roots uniquely in the private sector, having been set up by national

partnerships of industry associations.

Therefore, ISO is able to act as a bridging organization in which a consensus can

be reached on solutions that meet both the requirements of business and the broader

needs of society, . such as the needs of stakeholder groups like consumers and users.

Because "International Organization for Standardization" would have different

abbreviations in different languages ("IOS" in English, "OIN" in French for

Organisation intemationale de normalisation), it was decided at the outset to use a word

derived from the Greek isos, meaning "equal". Therefore, whatever the country,

whatever the language, the short form of the organization's name is always ISO.

International standardization began in the electro-technical field: the

International Electrotechnical Commission (IEC) was established in 1906. Pioneering

work in other fields was carried out by the International Federation of the National

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Standardizing Associations (ISA), which was set up in 1926. The emphasis within ISA

was laid heavily on mechanical engineering. ISA's activities came to an end in 1942.

In 1946, delegates from 25 countries met in London and decided to create a new

international organization, of which the object would be "to facilitate the international

coordination and unification of industrial standards". The new organization, ISO,

officially began operations on 23 February 1947.

When the large majority of products or services in a particular business or

industry sector conform to International Standards, a state of industry-wide

standardization can be said to exist. This is achieved through consensus agreements

between national delegations representing all the economic stakeholders concerned -

suppliers, users, government regulators and other interest groups, such as consumers.

They agree on specifications and criteria to be applied consistently in the classification

of materials, in the manufacture and supply of products, in testing and analysis, in

terminology and in the provision of services. In this way, International Standards

provide a reference framework, or a common technological language, between suppliers

and their customers - which facilitates trade and the transfer of technology.

For businesses, the widespread adoption of International Standards means that

suppliers can base the development of their products and services on

specifications that have wide acceptance in their sectors. This, in turn, means

that businesses using International Standards are increasingly free to compete on

many more markets around the world.

For customers, the worldwide compatibility of technology which is achieved

when products and services are based on International Standards brings them an

increasingly wide choice of offers, and they also benefit from the effects of

competition among suppliers,

For governments, International Standards provide the technological and

scientific bases underpinning health, safety and environmental legislation,

For trade officials negotiating the emergence of regional and global markets,

International Standards create "a level playing field" for all competitors on those

markets. The existence of divergent national or regional standards can create

technical barriers to trade, even when there is political agreement to do away

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with restrictive import quotas and the like. International Standards are the

technical means by which political trade agreements can be put into practice.

For developing countries, International Standards that represent an international

consensus on the state of the art constitute an important source of technological

knowhow. By defining the characteristics that products and services will be

expected to meet on export markets, International Standards give developing

countries a basis for making the right decisions when investing their scarce

resources and thus avoid squandering them.

For consumers, conformity of products and services to International Standards

provides assurance about their quality, safety and reliability.

For everyone, International Standards can contribute to the quality of life in

general by ensuring that the transport, machinery and tools we use are safe.

For the planet we inhabit, International Standards on air, water and soil quality,

and on emissions of gases and radiation, can contribute to efforts to preserve the

environment.

Every participating ISO member institute (full members) has the right to take

part in the development of any standard which it judges to be important to its country's

econorny. No matter what the size or strength of that economy, each participating

member in ISO has one vote. ISO's activities are thus carried out in a democratic

framework where each country is on an equal footing to influence the direction of ISO's

work at the strategic level, as well as the technical content of its individual standards,

Voluntary

ISO standards are voluntary. As a non-governmental organization, ISO has no

legal authority to enforce their implementation. A certain percentage of ISO standards -

mainly those concerned with health, safety or the environment - has been adopted in

some countries as part of their regulatory framework, or is referred to in legislation for

which it serves as the technical basis.

Such adoptions are sovereign decisions by the regulatory authorities or

governments of the countries concerned; ISO itself does not regulate or legislate.

However, although ISO standards are voluntary, they may become a market

requirement, as has happened in the case of ISO 9000 quality management systems, or

of dimensions of freight containers and bank cards.

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2.5 Standards of Dimension, Allowances, and Tolerances

2.5.1 Associated Terms (British Standard)

Feature: Physically identifiable portion of a part, such as a hole, slot, pin, or

chamfer.

Nominal Size: An approximate dimension that is used for the purpose of general

identification.

Basic Size: The theoretical size from which limits of size are derived by

applying allowances and tolerances.

Actual size: The measured size of a finished part.

Tolerance: The total amount by which a given dimension is allowed to vary.

Limit dimensions: The maximum and minimum dimensions of a machined part.

Bilateral Tolerance: Deviation (plus or minus) from the basic size.

Unilateral Tolerance: Deviation in one direction only from the basic size.

Clearance: The space between mating parts

Interference: Negative clearance.

Maximum Material Condition: The condition where a feature of a certain size

contains the maximum amount of material within the stated limits of that size.

Allowance: The minimum clearance space (or maximum interference) intended

between the maximum material condition of mating parts.

allowance = smallest hole - largest shaft.

Fit: The looseness or tightness that can result from the application of a specific

combination of allowance and tolerance in the design of mating part features.

Clearance Fit: Fit that allows for sliding or rotation between mating parts.

Interference Fit: A force or shrink fit which results in surface contact and

surface forces due to the overlap of physical material for the entire range of

tolerances between mating parts.

Transition Fit: A fit between mating parts which may be a clearance or

interference fit used for accurate location of parts.

Geometric Tolerancing: Tolerances that involve shape features of the part.

Datum: A theoretically exact axis, point, line, or plane.

2.5.2 Tolerances

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Quality and accuracy are major considerations in making machine parts or

structures . Interchangeable parts require a high degree of accuracy to fit together.

Dimensions of parts given on blueprints and manufactured to those dimensions should

be exactly alike and fit properly. Unfortunately, it is impossible to make things to an

exact or dimension. Most dimensions have a varying degree of accuracy and a means of

specifying acceptable limitations in dimensional variance that an object will tolerate and

still function.

Tolerance - Tolerance is the total amount a specific dimension may vary stated

as a minimum and maximum limitation. To understand tolerances, you should

understand some of the definitions associated with the determination of a tolerance.

These definitions may be generally categorized as relating to size, allowance, or fit. .

Size: The size of an object or its mate is known as nominal, basic, or design size.

Allowance: The maximum and minimum allowable dimensions are known as

limit, allowance, unilateral, and bilateral tolerances.

Fit: Fit, clearance, interference, or transition fit refer to how the object fits an

assembly.

Size -To specify the size of an object, you dimension it with a nominal size,

basic size, or design size.

Nominal size: Nominal size generally identifies the overall size of an object.

Basic Size: The basic size is the decimal equivalent of a nominal or numerically

stated size. It is the dimension from which you derive the limits of size by the

application of allowances and tolerances.

Design size: The size from which you derive the limits of size by the use of

tolerances.

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Figure 2-2 Normal, Basic and Design size

Allowance - Limits, allowance, unilateral tolerance, and bilateral tolerance

refer to size allowable variations.

Limits: The maximum and minimum sizes indicated by a toleranced dimension.

For example, the limits for a hole are 1.500 and 1 .501 inches and for a shaft 1

.498 and 1.497 inches.

Allowance: The intentional difference between the maximum material limits of

mating parts. This is a minimum clearance (positive allowance) or maximum

interference (negative allowance) between mating parts.

Unilateral tolerance: Unilateral tolerances indicate variation from the design

size in one direction.

Bilateral tolerance: Bilateral tolerances indicate variation from the design size

in both directions. The actual size of the object may be larger or smaller than the

stated size limitation if there can be equal variation in both directions. The plus

and minus limitations combine to form a single value.

Figure 2-3 Limit Allowance, Unilateral Tolerance, Bilateral Tolerance

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POLBAN

BAB III ELECTRICAL WIRING INTERCONNECTION SYSTEMS

(EWIS)

This module discusses the basic and more sophisticated instruments for carrying

out these tests. The methods of testing remain the same whichever instrument is used.

There is a great variety of crimping tools at present on the market. The following

pages are extracts from the BAe 146 Wiring Manual. References are made to

appendices not contained herein but procedures and processes are described for a

variety of hand and hydraulic crimping tools.

3.1 Continuity and Insulation Testing

3.1.1 Continuity Testing

3.1.1.1 Using a Voltmeter (continuous-circuits)

If a voltmeter is connected across the lamp, as shown in Figure 3-1, the

voltmeter will read zero. Since no current can flow in the circuit because of the open

resistor, there is no voltage drop across the lamp. This illustrates a troubleshooting rule

that should be remembered: -

When a voltmeter is connected across a good (not defective) component in

an open circuit, the voltmeter will read zero.

Figure 3-1 Voltmeter across Lamp in an Open Circuit

3.1.1.2 Using a Voltmeter (open-circuits)

In this case the voltmeter is connected across the open resistor, as shown in

Figure 3-2. The voltmeter has closed the circuit by shunting (paralleling) the burned-out

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resistor, allowing current to flow. Current will flow from the negative terminal of the

battery, through the switch, through the voltmeter and the lamp, back to the positive

terminal of the battery.

Figure 3-2 Voltmeter across Resistor in an Open Circuit

3.1.1.3 Using an Ohmmeter (open-circuit fault)

This type of open circuit malfunction can also be traced by using an ohmmeter.

When an ohmmeter is used, the circuit component to be tested must be isolated and the

power source removed from the circuit. In this case, as shown in Figure 3-3, these

requirements can be met by opening the circuit switch.

Figure 3-3 Using an Ohmmeter to check a Circuit Component

The ohmmeter is zeroed and placed across (in parallel with) the lamp. In this

circuit, some value of resistance is read. This illustrates another important

troubleshooting point: -

When an ohmmeter is properly connected across a circuit component and a

resistance reading is obtained, the component has continuity and is not open.

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When the ohmmeter is connected across the open resistor, as shown in Figure 3-4, it

indicates infinite resistance, or a discontinuity. Thus, the open circuit has been located

with both a voltmeter and an ohmmeter.

Figure 3-4 Using an Ohmmeter to Locate an Open in a Circuit Component

An open in a series circuit will cause the current flow to stop. A short circuit, or

'short', will cause the opposite effect. A short across a series circuit produces a greater

than normal current flow.

3.1.1.4 A Shorted Resistor

Figure 3-5 is a circuit designed to light a lamp. A resistor is connected in the

circuit to limit current flow. If the resistor is shorted, as shown in the illustration, the

current flow will increase and the lamp will become brighter. If the applied voltage

were high enough, the lamp would burn out, but in this case the fuse would protect the

lamp by opening first.

figure 3-5 A Shorted Resistor

Usually a short circuit will produce an open circuit by either blowing (opening)

the fuse or burning out a circuit component. But in some circuits, such as that illustrated

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in Figure 3-6, there may be additional resistors which will not allow one shorted resistor

to increase the current flow enough to blow the fuse or burn out a component.

With one resistor shorted out, the circuit will still function since the power

dissipated by the other resistors does not exceed the rating of the fuse.

Figure 3-6 A Short that Does Not Open the Circuit

To locate the shorted resistor while the circuit is functioning, a voltmeter could

be used. When it is connected across any of the un-shorted resistors, a portion of the

applied voltage will be indicated on the voltmeter scale. When it is connected across the

shorted resistor, the voltmeter will read zero.

3.1.1.5 Locating a Shorted Resistor in Series

The shorted resistor shown in Figure 3-7 below can be located with an

ohmmeter. First the switch is opened to isolate the circuit components.

Figure 3-7 Using an Ohmmeter to Locate a Shorted Resistor

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The circuit in Figure 3-7 is shown with an ohmmeter connected across each of the

resistors. Only the ohmmeter connected across the shorted resistor shows a zero

reading, indicating that this resistor is shorted.

3.1.2 Insulation Testing

Insulation testing is not simply a matter of measuring the resistance, in ohms,

between two points that are reputedly in electrical contact. Under working conditions

the insulation of an electrical installation is subjected to electrical stress. This stress can

cause a reduction in effective resistance between the points under consideration. It is

important that comparable conditions of electrical stress should be established when

insulation resistance is being measured.

Any insulation tester must have an output voltage that is equal to (and for

preference appreciably higher than) the working voltage of the circuit under test.

Insulation tests should be carried out after circuit installation and where specified in the

Maintenance Manual. The test should be carried out with a 250 V tester. The output of

the tester should be controlled so that the testing voltage cannot exceed 300 V.

Figure 3-8 A typical handle-driven Insulation Tester

3.1.2.1 Insulation Testing Procedure

Before beginning an insulation test the following preparations should be made:

All switches in the circuit concerned should be 'ON'.

All items of ancillary equipment, which are supplied by the system, should be

disconnected. Filaments should be removed.

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Components such as cutouts and relays, which are normally open, should have

their terminals bridged.

Remove the appropriate fuse or trip the appropriate circuit breaker.

One lead of the tester should be connected to earth and the other to the terminal

on the circuit side of the fuse holder or CB.

The insulation resistance values are likely to vary with changes in the

temperature and humidity of the local atmosphere.,.Results of tests and the

weather conditions at the time should be recorded.

3.1.2.2 After Testing

Immediately after an insulation test, functioning checks should be made on all

the services subjected to the test. If the insulation test or subsequent functioning tests

should reveal a fault, the fault should be rectified and the insulation and functioning

tests should be repeated in that sequence on the affected circuits.

Also remember to test the insulation tester before and after the test. Here are the tests:

Turn the handle with test leads touching and the reading should be - no

resistance.

Turn the handle with the leads apart - the needle will move towards infinity.

There is another type of insulation tester, which is not hand wound. This is the

battery insulation tester.

3.1.3 Bonding Testing

Bonding is the electrical interconnection of metallic parts of an aircraft,

normally at earth potential, for the safe distribution of electrical charges and currents.

It provides a means of protection against charges as a result of the build-up of

precipitation static and electrostatic induction resulting from lightning strikes, so that

the safety of the aircraft, or its occupants, is not endangered. The means provided are

such as to:

• minimise damage to the aircraft structure or components,

• prevent the passage of such electrical currents as would cause dangerous

malfunctioning of the aircraft or its equipment,

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• prevent the production of high potential differences within the aircraft.

Bonding also reduces the possibility of electric shock from the electrical supply system,

reduces interference with the functioning of essential services (e.g. radio

communications and navigational aids) and provides a low resistance electrical return

path for ·electric current in earth-return systems.

3.1.3.1 Primary and Secondary Conductors

Primary conductors are the conductors which are required to carry lightning

discharge current, secondary conductors are those provided for other forms of bonding.

3.1.3.2 Bonding of Aircraft of Metallic Construction

The skin of an all-metal aeroplane is considered adequate to ensure protection

against lightning discharge provided that the method of construction is such that it

produces satisfactory electrical contact at the joints.

NOTE: An electrical contact with a resistance less than 0.05 ohm is considered

satisfactory.

3.1.3.3 Bonding of Aircraft of Non-metallic Construction

With regard to aircraft of non-metallic or composite construction, a cage,

consisting of metallic conductors with surge carrying capabilities, must be provided to

form part of the aircraft structure. Metal parts of the aircraft should be bonded to this

cage.

Ground Discharge Methods

The earth system, which in the case of metallic construction is normally the

aircraft structure, must be automatically connected to ground on landing. This is

normally achieved through the nose wheel tyre, which is impregnated with an

electrically conducting compound.

NOTE: On some aircraft a static discharge whip, or similar device trailed from a

landing wheel assembly, is used to give ground contact on landing.

3.1.3.4 Electrostatic Charges

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The reduction or removal of electrostatic charges, which build up on such surfaces as

glass fibre reinforced plastic, can be achieved by the application of a special paint,

which produces a conductive surface.

3.1.3.5 Bonding Conductors

Solid bonding strip and braided bonding cord are selected by cross-sectional

area. Cords are usually made of braided copper, or aluminium, fitted at each end with

connecting tape or lugs. These should be used for bonding connections between moving

parts, or parts subjected to vibration. Cords are suitable for use as primary or secondary

conductors. Figure 3-9 shows a selection of bonding methods.

Figure 3-9 Bonding methods

3.1.3.6 Bonding Test

The bonding test is carried out using the test equipment described in this

Module. Since the length of a standard bonding tester lead is 60 feet, the measurement

between the extremities of larger aircraft may have to be done by selecting one or more

main earth points successively. In this case, the resistance value between the main earth

points chosen should be checked before proceeding to check the remote point.

The test lead should be used to check the resistance between selected points. The

values necessary are usually specified in the bonding test schedule, or the

manufacturer's publication for the aircraft concemed. When the pronged ends of the test

lead are brought into contact with the aircraft part, the test-meter will indicate, in ohms,

the resistance of the bond.

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To ensure good electrical contact at the test prongs it may be necessary to

penetrate or remove a small area of a non-conducting protective coating. After test, any

damage to the protective coating must be restored. If the resistance at a bond connection

is excessive, rectification action will depend on the-type of connection.

NOTE: Corrosion tends to form at bonding or earth connection and is often the

cause of excessive resistance.

3.1.3.7 Primary and Secondary Conductor Testing

Table 3-1 below gives an idea of values obtained during a bonding test.

Table 3-1 Pnmary and Secondary Conductor maximum bonding values

Note: Where readings are looked for beyond the range of the Bonding Tester, e.g. 0.5 megohm, the use of an instrument such as a 250 Volt Insulation Tester may be required, as stipulated by the bonding test schedule.

3.2 Use of Crimping Tools

3.2.1 PIDG Thin Wall Wire Terminals

There is a great variety of crimping tools at present on the market. The

following pages are extracts from the BAe 146 Wiring Manual. References are

made to appendices not contained herein but procedures and processes are

described for a variety of hand and hydraulic crimping tools.

Note: PIDG = Pre-Insulated Diamond Grip

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Table 3-2 lists the tools used with each wire size and terminal.

Table 3-2 PIDG Part numbers

a) Strip wires to S29-1 02 Section 3 to a length of 0.22 in. (5.53 mm) to

0.25 in. (6.35 mm).

b) Locate terminal in the crimping tool. Take up tool handle pressure until

terminal is held but not deformed. Insert the stripped wire so that the

conductor strands are just visible on the terminal palm before and after

crimping. Completely close the tool handles until the ratchet releases.

Open the tool handles and remove the crimped joint.

3.2.1.1 Insulation Support Crimping Adjustment

a) The insulation support crimping section of the tool has three positions: 1 = Tight, 2 = Medium, 3 = Loose.

b) Insert insulation adjustment pin in No 3 position.

c) Place terminal of the type to be used in the crimping dies.

d) Insert an unstripped cable of the type to be used into the insulation

support portion of the terminal. Complete the crimping cycle by closing

the tool handles until the ratchet releases. Remove the terminal and

check the insulation grip by bending the cable back and forth once. The

terminal insulation support sleeve should retain its grip on the cable

insulation. If the wire pulls out, set the adjustment pin to position 2 and

repeat the test. If necessary, fit adjustment pin in position 1 to achieve

the desired support. Do not use a tighter setting than required.

3.2.1.2 Colour and Dot Code

Tools and terminals are colour coded according to wire size for identification

purposes and in addition, the crimping tool leaves one or two raised dots on the terminal

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insulation as a check that the correct tool has been used for that particular cable size.

Table 3.2 shows the wire size, tool part number, handle colours, terminal insulation

colours and dot code which are to be complied with to ensure a correctly crimped joint.

3.2.1.3 Tool Maintenance

a) Check the die crimping areas for broken or chipped condition. Any tool

showing these signs must be withdrawn from use and returned to the

makers for rectification.

b) Lubricate all pins, pivot points and bearing surfaces with light machine

oil as follows:

Tools in full daily use: lubricate daily

Tools in occasional daily use: lubricate weekly

Tools used weekly or occasionally: lubricate monthly

In all cases, it is most important that before use excess oil is wiped from the tool,

especially in the crimping areas. Figure 3-10 shows two different types of crimping tool.

Figure 3-10 also shows a close up of the crimping jaws for the tool shown in Figure 3-

12.

figure 3-10 Typical Crimping Tools

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figure 3-12 Tooling for "Nylobond" PIOG Terminals (Size 24 -14)

Figure 3-13 Tooling for "Nylobond" PIDG Terminals (Size 24 -14)

3.2.1.4 “Certi-Crimp” Ratchet Inspection

The ratchet feature on AMP hand tools should be checked to ensure that the

ratchet does not release prematurely, allowing the dies to open before they have

bottomed.

To check the ratchet, proceed as follows:

a) Thoroughly clean the bottoming surfaces of the dies.

b) Make a test crimp using the maximum load, i.e. using maximum wire

size for the toolbeing used in the appropriate sized terminal. When the

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crimp is made, squeeze the handle until the ratchet is free, but do not

release the pressure on the tool handles.

c) If a 0.001 in shim can be inserted between the bottoming surfaces of the

dies, or there is noopening at all, the ratchet mechanism is satisfactory.

d) If the clearance between the bottoming surfaces of the dies is greater than

0.001, the dies are considered as not bottoming and the tool must be

withdrawn from use and returned to the makers (AMP Tool Repair

Department).

3.2.1.5 Die and Locator Clearance

The clearance between the dies, and locator and die face, should be checked

with feeler gauges and clearance should not exceed the dimension as shown on Figure

3-11. The clearance between the dies should be checked before the dies bottom.

If the clearances are exceeded then the tool must be withdrawn from use and

returned to the tool makers (AMP Tool Repair Department).

3.2.1.6 Tool Gauging for Conductor Crimp Jaws

Before commencing gauge checks ensure that the dies are clean and free from

particles.

To ensure dies are correctly positioned, a terminal of the correct type should be

crimped prior to gauging checks.

To gauge, close the tool handles until the ratchet is free but do not release the

pressure and the handles.

The "NO GO" plug gauge may partially enter the conductor crimp dies but not

pass through. The "GO" plug gauge should pass through the dies. Any tool failing

these checks must be withdrawn from use. Figure 3-14 shows the "GO/NO GO"

gauge.

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Figure 3-14 "GO/NO GO" Gauge

Before crimping: check for correct combination of tool', terminal and cable.

Check insulation support setting as given on previous page.

Open jaws fully and insert terminal, as shown in Figure 3-17 a, with the palm of

the terminal protruding through the locator and the barrel butting up to the locator. For

transverse crimping, place terminal palm through slot shown in Figure 3-17 b.

Close handles until terminal is just gripped. Insert stripped cable into barrel,

ensuring that approx. 0.03 of conductor protrudes on to the terminal palm. Hold wire

in position and close handles fully until ratchet releases.

NOTE: (1) It is most important that cable stripping is carried out correctly

Le. correct lengths stripped, no conductors cut or damaged, and

all cotton and glass cloth tails removed.

Figure 3-15 Terminal Crimp Barrels

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(2) It is most important that only bare conductor is in the terminal crimp

barrel. Do not force insulation into barrel when fitting cable to terminal.

See Figure 3-15 for correct position.

3.2.1.7 AMP Crimping Procedure

Crimping Instructions for AMP Tool No 59239, Size 12-10, Yellow

Strip cables to required length, ensure no strands are cut or severed and all

strands of insulation removed. To open tool handles, squeeze until ratchet

releases. Note that once ratchet engages, handles cannot be opened until

crimping action is completed. Figure 3-16 shows the crimping tool.

Figure 3-16 AMP Crimping Tool 59239 - Size 12-10 (Yellow)

Place terminal in crimping dies as shown in Figure 3-17a and close handles until

it is held firmly. Do not deform terminal. Insert stripped cable into terminal,

hold in position and complete crimping action until ratchet releases.

Correctly crimped joints will be as shown in Figure 3-17c.

For "in line" splice joints, position as shown in Figure 3-17b and crimp each half

of splice. This means two crimping operations.

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Figure 3-17 AMP Tool No 59239

Figure 3-18 and 3-19 show other types of crimping tools available. Figure 3-20

shows the location of the splice in the crimping jaws.

Figure 3-18 Tooling for "Stratotherm" Terminals Uninsulated with Insulation Support Size 22 to 10

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Figure 3-19 AMP Tooling for "Stratotherm" Terminals Uninsulated with Insulation Support (Size 12-10) Crimping Tool (Part No 59461)

3.2.1.8 Crimping Operation

1. Strip cables to required length. Ensure no strands are damaged or severed

and all strands of insulation removed.

2. To open tool handles, squeeze until ratchet releases. Note that once

ratchet engages handles cannot be opened until crimping action is

completed.

3. Place terminal in crimping jaws as shown in Figure 3-18 close handles

until terminal is held. Do not deform. Insert stripped cable into crimp

barrel, hold in position and complete crimp by closing handles until

ratchet releases.

4. Insulation support pins are to be adjusted according to cable insulation

diameter.

a) Position 1 for small insulation diameters

b) Position 2 for medium insulation diameters

c) Position 3 for large insulation diameters.

5. Insulation support is correct if a break or fracture does not occur when

cable is bent at 90°.

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6. Before crimping check for correct combination of tool, terminal, cable

and Insulation support setting.

7. Open jaws fully and insert terminal in jaws, as shown in Figure 3-20, the

palm of the terminal protruding through the locator and the barrel butting

up to the locator.

8. Close tool handles until terminal is just gripped. Do not deform. Insert

stripped cable into barrel, ensuring that approximately 0.03" of conductor

protrudes on to terminal palm. Hold cable in position and close tool

handles until ratchet releases.

NOTES: It is important that stripping is carried out correctly, i.e. correct

stripping length, no conductors cut or damaged, and all cotton

and glass "Tails" of insulation removed.

When fitting stripped cable to terminal, only bared conductor is to be in crimp

barrel. Do not force insulation into crimp barrel. For "inline" splice joints, position

the splice in tool jaw and crimp each half separately (two operations). Joint to be

insulated after crimping.

Figure 3-20 In-line splice joints

3.2.2 Sealed In-Line Crimping

This Specification describes the tools and methods used to provide an immersion

resistant sealed in-line crimp splice for single wire splicing.

3.2.2.1 Tool and Equipment

All tools shall be checked and tested in accordance with BS G178 and carry the

current tool check identification marks.

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3.2.2.2 Preparation

Personnel who have been instructed and tested in the correct use of crimping

tools shall only carry out crimping of splices. Only tools, which carry the current tool,

check identification marks shall be used. The crimping surfaces and moving parts of

crimping tools shall be kept clean and free of particles of metal, etc.

Moving parts shall be lubricated with light machine oil as necessary. Operators

shall make no adjustments, or alterations, to crimping tools.

3.2.2.3 Operation

Check that correct inline splice barrel, sealing sleeve, and marker sleeve have

been provided. Strip wires to a length of 8mm to 9mm (5/16 in approximately). When

splicing Fenwal supplied Fire Zone Wire, strip the outer cover a further 6mm to 7mm

(14 in approximately) as shown Figure 3-21.

Figure 3-21 Splicing Fenwall

Stripping to be in accordance with 20-41-08 Crimping Tool Raychem AD-

1377S.

1. Lightly twist together wires entering splice from the same side.

2. Slide sealing and marker sleeves onto one of the wires to be spliced.

3. Squeeze crimping tool handles together until ratchet releases.

4. Place barrel of crimp splice in correct colour crimp location of tool.

5. Insert wires into crimp barrel and crimp.

6. Do not allow wire insulation to enter the crimp barrel. Slide sealing

sleeve centrally over the crimped splice.

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7. Shrink sealing sleeve to 20-41-07. Heat should be applied until insert

melts and flows axially along the wire. Remove from heat source

immediately this occurs.

3.2.2.4 Inspection

All crimped joints shall be visually inspected for:

Correct combination of cable, tool and terminal.

Correct form and location of crimp.

Adequate insertion of conductor strands in crimp barrel.

Freedom from fracture, roughs and sharp edges and flash.

Absence of damage to the conductor or insulation.

3.2.2.5 Rules of In-line Splicing

When using in-line crimps certain points should be noted: Each barrel must

carry only one cable unless specifically permitted by the airworthiness authority.

The crimp must be fitted horizontally or positioned so that the ingress of

moisture is not possible.

Additional sleeving is not permitted to achieve the above.

Ensure operating temperatures are not exceeded.

Specific approval must be obtained from the appropriate airworthiness authority

before using in:

Screened cables

Coaxial cables

Multi cored cables.

Cables greater than size 10.

Thermocouple cables.

HV cables (above 250M RMS).

Fire resistance cables in protective zones.

Totally enclosed cables, that cannot be inspected.

Use of in-line is currently restricted to size 10 (35A) or smaller.

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Low temperature connectors must not be crimped on size 12 or larger EFGLAS.

Repair schemes are restricted to:

Minimum distance between joints in one cable is 12ft.

No more than 2 joints permitted in 10ft.

Maximum joints; runs of 20ft - 3, runs of 200ft - 5, runs over 200ft - 8

On installation wherever possible observe the following:

All joints must be accessible for visual inspection.

Joints should be positioned so as not to touch:

One another.

Ducting.

Straps.

Other features.

Joints must if possible be positioned on the outside of the loom.

All fixing attachments must be approved.

Joints must be staggered. If this is not possible then positive separation must be

carried out using insulation or cable clips.

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Figure 3-22 Examples of crimping in cable looms

3.3 Testing of Crimped Joints

3.3.1 Tensile Test

Typically, tensile and voltage drop tests are made on not less than two

specimens of each and every combination of crimp barrel, conductor, tool, die, locator

or positioned.

Each sample shall be tested in a suitable tensile testing machine in which an

axial pull is applied and in which the jaws separate at a steady rate of between one and

two inches per minute. Each specimen shall be tested to destruction and shall not fail

below a minimum load. Examples of loads (and milli-volt drop values) are given in the

table below.

The test samples shall have any insulation grip (if applicable) rendered

inoperative by removing the cable insulation. Pull off loads, test currents and voltage

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drop values for copper conductor crimps when using Milli-volts Specification

cable strandings. Ref: MIL-T-7928E. Table 7.6 details the pull off loads, test currents

and voltage drop values for copper conductor crimps.

Table III-3 Crimped Joint test table

3.3.2 Voltage Drop Test

The appropriate test current, given on the table on the previous page at an open

circuit voltage of not more than 30V, is passed through the specimen. Milli-volt drop

checks are carried out using test probes between a point adjacent to the forward end of

the crimp barrel and a point on the conductor immediately behind the crimp barrel. The

milli-volt drop must not exceed the figures in the table of examples on the previous

page. Figure 3-23 shows a test arrangement.

Figure III-23 Voltage Drop Test

3.3.2.1 Routine Inspection of Crimped Joints

Every crimped joint must be visually inspected for the following:

Correct combination of cable, tool termination and correct die marks, if

applicable.

Correct form and location of crimp.

Adequate insertion of conductor strands in crimp barrel.

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Absence of insulation or other non-conducting material in the crimp barrel.

Freedom from fracture, flash, rough or sharp edges.

Absence of damage to the conductor or insulation.

Insulation properly gripped by insulation crimp if applicable.

3.4 Connector Pin Removal and Insertion

3.4.1 Types of Removal–Insertion Tool

There is a vast range of electrical connectors used in aircraft electrical/avionics

systems. This section describes a range of plastic removal/insertion tools used to

remove or insert the pins of some connectors.

Figure 3-24 Typical pins and associated insertion tools

A typical insertion/extraction tool is shown at Figure 3-25.

Figure 3-105 Insertion/Extraction Tool

Plastic insertion and extraction tools were introduced to prevent damage to

contact retaining clips and insert materials, and are colour coded for contact size, i.e.

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Red, size 20; Blue, 16 and Yellow 12 and 22. In composite tools the extractor is always

White.

3.4.2 Plastic Tool

Figure 3-26 Use of plastic insertion tool

3.4.2.1 Installing (Coloured – End)

Figure 3-27a - Hold the insertion half of the tool (coloured) between the thumb

and forefinger and lay the wire to be inserted along the slot, leaving about ½' protruding

from the end of the tool to the crimp barrel of the contact.

Figure 3-27b - Squeeze the wire hard into the tool at the tip, between the thumb

and forefinger, and at the same time, quickly pull the protruding wire with the other

hand away from the tool.

Figure 3-27c - The wire will now have snapped into place. Pull it back through

the tool until the tip seats on the back end of the crimp barrel. Figure 3-27d - Holding

the connector with the rear seal facing you slowly push the contact straight into the

connector seal. Figure 3-27e - A firm stop will be evident when the contact positively

seats in the connector.

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Figure 3-27 Installing (coloured-end)

3.4.2.2 Removal (White-End)

Figure 3-28a - With the rear of the connector facing you, lay the wire of the contact to be removed along the slot of the removal half (White) of the tool, leaving about W' from the end of the tool to the rear of the connector.

Figure 3-28b - Squeeze the wire hard into the tool between the thumb and forefinger about %" From the tip and at the same time quickly pull the connector away from the tool with the other hand.

Figure 3-28c - The wire will now have snapped into place. Slide the tool down over the wire and into the rear seal and push it slowly into the connector until a positive resistance is felt. At this time the contact retaining clip is in the unlock position.

Figure 3-28d - Press the wire of the contact to be removed against the serrations of the plastic tool and pull both the tool and the contact-wire assembly out of the connector.

Figure 3-28 Removing white-end

Caution: Do not tip, spread or rotate the tool while it is in the connector.

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Figure 3-29A, B shows a comparison of front release and rear release contacts.

Figure 3-29 Front/Rear release Contacts

Figure 3-29a shows the front release system and 3-29b shows the front release system.

Figure 3-30 Rear/Front release System

3.4.3 Tweezer Type Insert/Extract Tools

Figure 3-31 Tweezer type extractor tool use

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Figure 3-32 Tweezer Type Insert/Extract Tools

3.4.3.1 Installation/Removal Instructions

To Install Contacts:

Open the tool tips by squeezing the handles and the tips around the wire

insulation. Slide the tool along the wire until the tip end butts against the

shoulder on the contact.

Carefully push the contact forward and directly in line with the grommet hole

until the contact is felt to snap into position.

Slide the tool back along the wire insulation until it clears the grommet and

remove the tool from the wire.

To Remove Contacts:

Open the tool tips sufficiently to place around the wire insulation. Slide the tool

down the wire until the tool tips enter the grommet and come to a positive stop

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(see Figure 3-33). A slight increase in resistance will be noticed just before

contact.

Holding the tool tips firmly against the positive stop on the contact, grip the wire

and simultaneously remove the tool, contact and wire.

Caution: The tips on the installing and removal tools used on small contacts

have very thin wall sections. This causes them to have sharp edges which

can cut the wire installation or connector sealing grommet. Do not squeeze,

spread, tip or rotate the tweezers while entering the connector grommet.

Figure 3-33 Insert/Extract Tool Operation

3.4.4 Soldering

General

Soldering is the process of joining metallic surfaces through the use of solder

without direct fusion of the base metals. Solder is a non-ferrous fusible tin alloy, which

melts when sufficient heat is applied to it and becomes solid when the heat is removed.

Definitions

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Wetting - Adhesion of liquid solder to a solid surface.

De-Wetting - The condition is a soldered area in which the liquid solder has not

closely adhered.

Cold Solder Connection - Referred to as a "Dry Joint". It is the unsatisfactory

connection resulting from de-Wetting and exhibiting an abrupt rise of solder

from the surface being soldered.

Disturbed Solder Connection - Unsatisfactory connection as a result of

movement of the conductor during the hardening of the solder.

Flux - A substance, such as "Borax" or "Rosin", used to help metals fuse

together by preventing oxidation during soldering.

Resin Soldered Connection - Unsatisfactory connection that has trapped flux.

Overheated Joint - An unsatisfactory connection, characterized by a rough

solder surface.

Heat sink - A thermal shunt with good heat dissipation characteristics, used to

conduct heat away from the component being soldered.

Tinning - The coating of a surface with a uniform layer of solder.

Wicking - A method in which a piece of copper braid is applied to the melted

solder. The copper braid acts as a wick to absorb and remove the solder.

Figure 3-34 shows some soldered joints in cross-section.

Figure 3-34 Soldered Joints

3.4.4.1 Soldering irons

For soldering irons, the following specifications must be met:

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1. Use a constant temperature soldering iron that has an insulation

transformer in its power supply. Magnetic fields can damage electrical

components.

2. If ESDS are to be soldered, a potential free solder station with

temperature regulator must be used. Also the ground of the soldering

station must connected to the ground of the PCB. This ensures that the

component and the soldering iron have the same ground potential.

3. Use a solder bit, which is appropriate for the degree of heat required.

Improper choice of a solder bit can result in severe damage to electronic

components or the PCB itself.

4. A sponge for cleaning the tip of the solder bit must be kept wet and

regularly cleaned. Use only distilled water.

Figure 3-35 Soldering Iron

3.4.4.2 Heatsinks

Semi-conductors, fine resistors, capacitors etc. can be damaged by heat during

soldering. The proper use of heat sinks can, to large extent, prevent the transference of

heat from the connection, which is being soldered to the component. Heat sinks are

made from a heat conducting material, such as copper, and are pinched to the conductor,

between the component and the end being soldered. In some cases the tip of a pair of

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long nose pliers will be sufficient, but this method must only be used if the heat sinks

are not available.

Figure 3-36 Heat Disposal Clips

3.4.4.3 Anti-Wicking Pliers

Anti-wicking pliers are used to prevent thermal damage of the insulation, and

flow of solder under the insulation. Figure 3-37 shows Anti-Wicking pliers.

Figure 3-37 Anti-Wicking Pliers

3.4.4.4 Solder Removal Guns

When soldered components are to be replaced, especially on printed circuit

boards, all solder is removed from the connection. To this so a "Solder Removal Gun"

can be used. The solder removal gun consists of a pump type plunger in a sealed tube.

The plunger can be pushed down against a spring where a spring-loaded release knob

holds the plunger in the down position. When the release knob is pressed, the plunger is

free to move upwards. When the plunger is released, suction is created at the tip of the

gun, sucking the heated solder into the gun. Figure 3-38 shows a solder removal gun.

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Figure 3-38 Solder Removal Gun

To remove solder from a connection proceed as follows:

(a) Push the plunger to the down position

(b) Heat the connection until the solder becomes liquid. Do not overheat.

(c) Hold the tip of the solder removal gun very close to the melted solder

and press the release knob.

(d) Repeat steps (a) to (c) until the solder is removed.

Figure 3-39 Solder Removal Gun Operation

3.4.4.5 Tinning

Strip the wire, then heat the wire until the solder runs freely. Remove the

soldering iron heat immediately to avoid possible damage to the insulation. When

tinned, the strands of the wire must be clearly visible and follow their original normal

cable routing. The solder must not extend beyond this area.

Figure 3-40 Tinning of a Wire

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3.4.4.6 Soldering preparation

(a) Tinning and soldering must be done in a clean area, this ensures that

components do not become contaminated.

(b) Take care that there is sufficient ventilation during the soldering.

Harmful vapours (metal vapour) are produced during soldering

operations.

(c) Tools and equipment must be free from oil, grease and other impurities.

(d) Install the correct solder bit into the soldering iron.

(e) Switch the soldering iron on. Do not use until it has reached its

operating temperature.

(f) When the surface to be soldered has gained the correct temperature, add

a small amount of solder and allow it to distribute itself regularly over

the parts to be connected.

(g) Take away the soldering iron from the connection and allow the solder

to cool slowly without moving the parts. Never force the cooling

process.

(h) Too much solder can be removed by using the solder removal gun.

(i) Clean the connections as soon as possible.

3.4.4.7 Soldering of connectors

(a) Push a suitable shrink sleeve over the wire.

(b) Heat the solder cup of the connector to be soldered and fill it with solder

(see Figure 3-42). Start at the bottom of the connector and work up as

shown in Figure 3-41

(c) Strip the wire to the required length.

(d) Tin the wire as described in above.

(e) Heat the solder cup until the solder melts, tilting the soldering iron bit to

allow entry of the wire and slowly insert the wire.

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(f) Hold the wire firmly in place and remove the soldering iron bit from the

solder pot.

(g) Make sure that the finished connection is correct (see figure 4-18).

(h) Push the heat shrink sleeve (if fitted) over the soldered connection and

shrink the sleeve.

Figure 3-41 shows the connection order for soldering connectors.

Figure 3-41 Connection Order

Figure 3-42 Filling the Solder Pot

3.4.4.8 Soldered Connections

The construction of the pins and sockets in a MIL or other type of connections

may be designed for the solder connections to the electric wires. At the end of the pin or

socket is a small solder pocket. Figure 3-43 shows a typical solder MS connector.

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Figure 3-43 MS Soldered Methods

When using the soldering method you must remove enough insulation so that

none extends into the solder pocket. With the wire in the pocket, solder is applied with a

small pointed soldering iron or soldering gun. The solder should be of the resin core

type and should be applied to the pocket as it is heated with the soldering iron.

As soon as the solder starts to flow smoothly into the pocket and penetrates the

wire, the soldering iron should be removed to avoid the possibility of buming the

insulation of either the wire being inserted or adjacent wires. Only enough solder should

be applied to fill the pocket, and all small drops of solder should be removed from

between the pins.

After each pin is soldered, a plastic sleeve insulator should be pushed down over

the soldered joint and metal pin to prevent the possibility of short circuiting. The

insulating sleeves should be tied or clamped to prevent them from slipping off the pins.

Note:

(a) The flux used for soldering is corrosive and can weaken the connections

over a period of time.

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(b) Errors such as too much heat, too much solder, not enough heat, and a

lack of connection cleanliness are difficult to eliminate.

(c) The soldering process can destroy gold-plated contacts.

(d) Solder wicking into the wire strands can create additional stress in the

wire.

3.5 Coaxial Cables, Wiring Protection Technics

3.5.1 Description

Antennas are connected to most of the radio receivers and transmitters with a

special type of shielded wire called "Coaxial Cable". Coaxial cables contain two or

more separate conductors. The inner most conductor may be solid or stranded copper

wire, and may be plain, tinned, silver plated or even gold plated. The remaining

conductors are in the form of tubes, usually of fine braid. The insulation is usually

Teflon or polyethylene. Outer coverings or jackets serve to weatherproof the cables and

protect them from fluids, and mechanical and electrical damage. Figure 3-44 shows a

typical coaxial cable.

Figure 3-44 Coaxial Cable

Coaxial cables have several advantages over standard cables. Firstly, they are

shielded against electrostatic and magnetic fields. An electrostatic field does not extend

beyond the outer conductor and the magnetic fields due to current flow in the inner and

outer conductors cancel each other out. Secondly, since coaxial cables do not radiate,

then likewise they will not pick up any energy or be influenced by magnetic fields.

Thirdly, coaxial cables have specific values of; impedance, capacitance per unit length

and attenuation per unit length

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3.5.2 Coaxial Stripping Procedures

Outer Jacket- Once the outer jacket has been removed, the following should be checked:

The outer jacket must not be chafed or incised.

The outer jacket must have been cut off flat all round and at right angles to the

longitudinal direction of the cable.

The outer jacket must not be frayed.

The strands of the underlying shield must not be notched or cut off.

Shield- After stripping the shield the following must be checked:

The shield must have been cut off evenly all round.

The braiding of the shield must not be damaged.

The underlying dielectric must not be chafed, compressed or incised.

Dielectric - After stripping the dielectric the following must be checked:

The dielectric must not be chafed, incised or compressed.

The dielectric must have been cut off flat all round the cable.

The dielectric must not be frayed.

The core wires must not be notched or cut off.

Figure 3-45 shows the process of stripping a coaxial cable.

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Figure 3-45 Coaxial Cable Stripping

There are a number of sizes and types of coaxial cable used for electronic

installation, and each type must be terminated in a way specified by the manufacturer of

the connectors. BNC connectors are perhaps the most widely used type. Figure 3-46

shows the method used in their installation.

Figure 3-46 BNC Coaxial Connector Installation

Referring to Figure 3-46:

(a) Fit the nut over the cable and cut the ends of the cable square.

(b) Remove one half inch of the outer jacket.

(c) Push the braid back and remove one-eighth inch of the insulation.

(d) Taper the braid over the end of the insulation.

(e) Slide the sleeve over the end of the cable, fit the inner shoulder of the

sleeve square against the end of the jacket.

(f) Comb the braid back over the taper of the sleeve.

(g) Remove the insulation from the conductor leaving one-eighth inch of the

insulation sticking out beyond the sleeve and one-eighth inch of the

conductor sticking out of the insulation.

(h) Solder the contact to the conductor.

(i) Push the body of the conductor over the contact and the end of the cable.

(j) Hold the cable and the body and screw the nut into the body

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3.5.3 Coalxial Cable Testing

The relationship to the length of a coaxial cable and its impedance is critical. If

the impedance of the line does not match the load impedance, not all the energy fed

down the line flows into the load. Some of the energy is reflected back to the source,

forming standing waves on the line. Every half wave along the line, high V and Low I

points appear, also between these points will be Low V and High I. The ratio of the

voltage across the line at the High V points to that at the Low V points is known as the

"Voltage Standing-Wave Ratio" (VSWR).

If a coaxial cable is damaged (either crushed, pinched or cut), it will affect the

impedance of the cable; this in tum will result in low power transmissions. Measuring

the VSWR on the line will identify the position of the damage. To measure the VSWR a

"Time Domain Meter" (TOM) is used.

3.5.4 Made Up Cabling

Cable looms and cabling made up on the bench must be inspected before

installation in the aircraft to verify the following.

That all cables, fittings, etc have been obtained from an approved source, have

been satisfactorily tested and have not deteriorated in storage or been damaged in

handling.

That all crimped joints and soldered joints have been made in accordance with

the relevant drawings, are clean and sound and insulating materials have not been

damaged by heat etc.

That all connectors and cable looms conform to drawing requirements in respect

of materials, terminations, length, angle of outlets, orientation of contact assemblies,

identification and protection of connections.

That cable-loom binding is secure. That continuity, resistance and insulation

tests are carried out in accordance with drawing requirements.

3.5.5 Installation of Electrical Wiring

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The following paragraphs describe how cables and cable bundles must be

installed and protected. This is a general description and the maintenance manual of the

aircraft you are fitting cables/cable bundles to must be used

3.5.5.1 General

The cable bundles must be fixed to the structure with cable clamps without extra

protection.

Note: Gas and fluid lines are not part of the structure.

Use conduits only as a protection for cable bundles.

Install and protect cable bundles in such a way that they are accessible for

inspection and maintenance.

Install and protect cable bundles in such a way as to prevent any form of

damage, such as caused by:

(a) Touching.

(b) Chafing.

(c) Hammering.

(d) Sliding.

(e) Kinking.

(f) High ambient Temperatures.

Cable bundles, including the means for fastening and protection, must be

resistant to the circumstances and substances, which exist in their surroundings.

3.5.5.2 Installation of Cable Bundle

Assemble the cable to bundles with bundle ties or bundle lacing tape.

To prevent damage tothe cable bundles, sufficient space must be kept between

the bundles and the surrounding parts.

Install cable bundles with a minimum clearance of 1cm (O.4inch) to prevent

chafing against sharp edges.

At least 15cm (6inch) separation is required between cables and lines carrying

fuel or oxygen.

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At least 7.5cm (3inch separation is required between cables and control cables.

Maintain a minimum 13mm (0.5inch) separation between cables and water lines,

pitot static lines, etc.

Note; where mechanical support is provided which will prevent the actual

contact, the minimum distance can be reduced to less than 13mm.

Maintain a 5cm (2inch) minimum separation between cables and insulated bleed

air ducts. Provide a mechanical support to prevent any possible contact between

the cable bundle and the insulated bleed-air duct.

3.5.6 Cable Clamps

Metal cable clamps must have a flexible rubber cushion.

Plastic cable clamps must only be used inside the pressure cabin in places where

the load on the clamp is minimal, as in cable trays, panels and Electrical Power

Centre (EPC) areas.

3.5.6.1 Installation

Make sure that mounting the ends of the flexible rubber cushion are linked

together. This is necessary to prevent the metal of the cable clamps damaging

the cables.

Install the mounting bolt on the top of the clamp.

Make sure that after mounting, the cable clamp fully encloses the cable bundle.

This is necessary to prevent the bundle from sliding in the cable clamp.

Make sure that the maximum outer diameter of the cable bundle does not exceed

the inner diameter of the cable bundle.

Mount the cables at the correct angles.

Lay the cables parallel and tightly together inside the cable clamp.

Ensure the correct distance between clamps is used.

Note: The distance between two cable clamps can vary between 10 to 30 cm (4

- 12 inch) inside the pressure cabin. This depends on the routing,

thickness and stiffness of the bundle. Figure 3-47 shows the required

distance of the cable clamps.

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Figure 3-47 Cable Clamp Spacing

3.5.7 Conduit (Metallic and PVC)

When using conduit for cable bundle protection they must be installed in such a

way that they can not be used as a hand hold, or as a foot rest by passengers or

maintenance personnel. The use of plastic conduit can only be used if the use of

metallic conduits is impossible.

The inner diameter of the conduit must be 25% larger than the maximum outer

diameter of the cable bundle. To prevent damage to the cables, the ends of the plastic

conduits must be provided with adapters. The end of the metallic conduit must be flared

and smooth. Figure 3-48 shows both metallic and PVC conduit in use on modern

aircraft.

Figure 3-48 Metallic & PVC Conduit

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3.5.7.1 Conduit Drainage

Where tubing is used, a drainage hole 1/8 inch diameter should be made at the

lowest point in the tubing, after this is established. This prevents condensed moisture

from running along the cables and finding its way into the electrical apparatus. When

this is not possible, the cable should incorporate a downward loop immediately after

leaving the apparatus. Where conduits, tubes or ducts are used, they should be installed

so that any moisture accumulating in them will drain away harmlessly, and the cables

used in them should be capable of withstanding such moisture as may be encountered.

Figure 3-49 shows a drainage hole in the cable conduit.

Figure 11-49 Conduit Drain Hole

3.5.8 Interference

Cables should be installed so as to reduce electrical interference to a minimum

and to avoid confusion between circuits on different types of services. The spacing

between any aircraft unscreened cable and unscreened radio aerial lead should normally

be not less than 18 inches.

3.5.9 Protection of Cabling

The cables must be protected from abrasion, mechanical strain and excessive

heat and against fuel, oil etc, water in either liquid or vapour form and from the weather.

Cables should be spaced from the skin of the aircraft by at least half-inch to avoid

damage from the high skin temperatures likely to be reached in the tropics. The cables

should not be run near a hot engine, or other components, unless a cooled air space or a

heat barrier is interposed. Figures 3-50 and 3-51 show different methods of protecting

cables.

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Figure 3-50 Cable Protection (Bulkhead Hole)

Figure 3-51 Support Of Cabling

Cables must not be supported on, nor must they be allowed to bear on, sharp

edges such as screw heads or ends, the edges of panels, metal fittings, bulkheads, etc.

Where cables are led through metal fittings or bulkheads, the edges of the holes

through which they pass must be radiused and smoothed and fitted with an insulating

bush or sleeve. Cables which are drawn through holes or tubes must be an easy fit

requiring only a moderate, steady pull, care being taken to keep the cables parallel to

each other and to avoid the formation of kinks which may fracture the conductor.

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Conduits, ducts and trays used for carrying cabling should have smooth internal

surfaces. Rigid ducts, etc should be adequately flared at the outlets or bushed with

insulating material.

3.5.10 Support Cabling

The cabling must be adequately supported throughout its length, and a sufficient

number of clips or supports must be provided for each run of cables to ensure that the

unsupported lengths will not vibrate unduly, leading to fracture of the conductors, or

failure of the insulation or covering.

Cables must be fitted and clipped so that no tension is applied in any

circumstances of flight, adjustment or maintenance. Loops or slackness will not occur in

any position where they might be caught and strained by normal movement of persons

in the aircraft, or during normal flying, maintenance or adjustment. Figures 3-52 and 3-

53 show methods of support.


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3.5.11 Tywraps

These are used to tie cable looms and are made of Nylon or PVC. Once wrapped

tight around the loom, they will stay locked, but cannot be unlocked. The inner surfaces

are designed to grip the cable loom and stay in place without slipping. They are also

available in colours for loom identification and coding. Fewer ties are needed in

comparison with normal whipping methods, because of the wide gripping surface of

Tywraps.

Note: The locking device in a Tywrap is a metal insert integral with the Tywrap

itself. Experience has shown on some aircraft that over tightening can

cause the metal insert to damage the cable upon which the Tywrap is

being used. Some cables damaged in this way are likely to give rise to

dangerous conditions. This being the case, all plastic Tywraps are being

used in some areas.

3.5.12 Cable Conduits (Superflexit)

Conduits made from PTFE are suitable for use where the operating temperature

range is from -70ºC to +240ºC. Conduits made from PVC are suitabl e for use where the

operating temperature range is from -20ºC to +70ºC.

The conduit is normally supplied plain but it is available with intemal or extemal

tinned copper braid. (Stainless steel full or partial extemal braided conduits are not

suitable for this type of attachment.)

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The method of attaching the conduit to the connector/end termination is by a

stainless steel Isoclip. A Viton sleeve is fitted over the end termination spigot before

fitting the conduit to prevent cutting into the conduit and to obtain a seal up to (30 PSI)

207 KN/m2.

The method of attaching the conduit with extemal tinned copper braid is as for

plain conduit.

The method of attaching intemal tinned copper braid depends on whether

continuity is required between fittings or not.

To obtain continuity through the braid to shell of connector/end assembly, the

braiding is fitted onto the spigot, the Viton inner sleeve fitted over the braid, the conduit

pushed over the Viton sleeve and all parts retained by Isoclip.

For insulating the tinned copper braid from the connector/end assembly, the

braid is fitted onto the spigot after the Viton sleeve has been fitted.

3.5.12.1 Procedure

1. Cut the conduit cleanly and squarely.

2. Using the approved Superflexit tool, and the Tool spigots deconvolute 14.5 mm (.57 in) ± 1 mm (.040 in) of the conduit. This is the required length of deconvoluted conduit for fitting over the Viton sleeve and the connector end termination.

3. Push the Viton sleeve onto the spigot of the end fitting, allowing approximately 1.5 mm (.062 in) to protrude beyond the end of the spigot. This sleeve is used as a resilient member between the deconvoluted end of the conduit and the metal spigot to prevent cutting the conduit and to obtain a seal.

4. Place the outer Viton sleeve over the conduit/conduit braid and slide back enough to make space for the Isoclip.

5. Place the Isoclip onto the conduit and then push the deconvoluted portion over the Viton sleeve and bring the Isoclip forward so that it is positioned as shown.

6. Place the Isoclip assembly tool over the Isoclip with the tensioner in the twisted end of the Isoclip

7. Wind the knob until a gap appears in the shaft, the edges of the gap lining up with any pair of markings on either side of the shaft window to ensure that the correct tension is put an the clip.

8. Compress the lever on the tool to cut off the excess wire on the Isoclip and bend back the wire with the top of the tool. Ensure that the cut ends are dressed down so as not to protrude.

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9. Slip the outer Viton sleeve forward over the Isoclip and deconvoluted end of the conduit.

3.5.13 Heat Shrink Tubing

Heat shrink tubing is available in a variety of sizes and temperature ranges. The

example shown is an extract from the BAe 146 Wiring Manual. This is general purpose,

flexible, heat shrinkable sleeving.

General Characteristics:

Self extinguishing (Raychem Type 1) (Hellermann Type 1) Spec: MIL I 23053B/5

Temperature Range: 55ºC to + 135ºC continuous. U p to 300ºC for short

duration (1 hour).

Shrinking Temperature: Min 121ºC, Recommended tem p 250ºC - 300ºC.

Shrink Factor: 50% of supplied diameter.Fluid Resistance: Skydrol 500.

Kerosene, Hydraulic fluid.

Colours: Yellow, Black, Red, White, and Blue.

Length: Four feet lengths.

Notes:

1. Select the largest size, which will snugly fit the item to be covered.

2. Wall thickness will be less if recovery is restricted during shrinking.

Figure 3-54 Heatshrink codes

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3.5.14 Heat Shrinkable Sleeves

Always use the smallest possible size that slides easily over the assembly. The

sleeves are to be cut to a length, so that, when shrunk, they completely cover the

connections and extend approximately 10mm (OAinch) over the insulation. Figure 3-55

shows insulation sleeve before and after shrinking.

Figure 3-55 Heat Shrink Sleeving

3.5.15 Cable Performance

The definition of cable performance has increased in complexity and precision

with the reduction of insulation thickness and weight. Some of the cables now used for

airframe wiring have no more than 0.006" of insulation thickness and thus there is little

margin for error in manufacture or in an aircraft installation. The operating temperature

dictates to a large extent the materials and constructions used, but installation

requirements need to be satisfied by defining properties such as resistance to insulation

"cut-through" and abrasion. It follows that cables need to be selected with care and the

factors detailed below should be considered in relation to any intended duty.

3.5.15.1 Temperature

The temperature rating of a cable must be defined to permit comparison with the

worst case requirements of the application. It follows that the location of a cable,

relative to hot air ducts and local hot spots such as power transformers and some

filament lighting, must be known. Cables have a specified maximum continuous

operating temperature, and for many types, this may be achieved by any combination of

ambient temperature plus temperature rise due to 12R losses. However, it should be

noted, that in general, it is undesirable to contribute more than a 40'C rise by electrical

heating and that operating temperature and installed life are directly related. The

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temperature rating of an airframe cable is determined by its construction as noted in

paragraph 6, and will be classified at one of the following temperatures:

105'C (obsolescent cable types), 135ºC, 150ºC, 210ºC and 260ºC.

Clearly this temperature rating has to be known when evaluating any design

application.

3.5.15.2 Cable Size

Cable is usually identified by a size number, which approximates to the A.w.G.

(American Wire Gauge) size of the conductor. However, some cables enjoy a number,

which refers to the square millimetres of a conductor cross section, which is a system

used extensively for commercial cables. The size of cable is the primary determinate of

the electrical protection level set by the circuit breaker or fuse, and should never be

reduced below the level established by proper co-ordination data. Manufacturers publish

rating data for single cables in free air, and for bundles of three cables in free air. By

study of the short term and continuous ratings for a given cable type and size, the

correct protection can be. Current rating data usually relates to a temperature rise of

40°C above ambient as stated above and due allowance must be made for such electrical

heating. Manufacturers' data will normally include conductor resistance in ohms per km

at 20°C and a temperature correction may be necessary if accurate voltage drop

calculations are necessary.

It should be noted that cable 'size' relates only to the conductor and thus the

overall diameter and surface finish for a given size may vary slightly between cable

types. Such differences in overall diameter may have an effect on cable sealing in

connectors and pressure bungs, and also the selection of pre-insulated terminal ends

where a dielectric crimp provided.

3.5.15.3 Voltage Rating

All cables have a rated voltage and some, such as equipment wires, may be

specified by Voltage. Particular reference should be made to the specified voltage of

any cable where higher than normal potentials may be used, examples being discharge

lamp circuits and windscreen heating.

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3.5.15.4 Flammability and Toxicity

All cables are required to have a defined level of resistance to burning when

exposed to standard flame tests. In addition to the requirements of flammability, there

exists within BCARs, EASAs and FARs, general requirements relating to the hazards of

smoke and toxicity. In recent years, greater emphasis has been placed upon these

characteristics and whilst they are not yet defined in many civil cable specifications, it is

generally true that new cable types have been more thoroughly investigated.

3.5.15.5 Mechanical Properties

The assessment of cable insulations includes the ability to withstand the pressure

of a sharp edge (cut-through), and for the ability to withstand scraping with a defined

blade. It is these tests which figure significantly in assessing airframe cable and which

are the controlled methods of replacing assessment by scraping with the thumb nail. As

noted earlier, differing constructions result in marked changes in handling properties

especially with regard to stiffness and 'springiness'. Installation of looms of thin wall

hard dielectric cable has to have regard to the reluctance of such looms to be 'set' in

position, especially if the supporting structure is flimsy. It must not, however, be

assumed that this apparent strength is translated into the ability to withstand physical

abuse.

3.5.15.6 Fluid Contamination

Cables are required to display a defined level of resistance to the effects of

commonly used aircraft fluids but this is not to say that cables can withstand continuous

contamination, which should be avoided. A related hazard is that presented by sealing

compounds because these may contain agents which are aggressive to cable insulation.

It follows that where a new cable type is introduced, the compatibility with such

compounds should be checked. Equally, the use of a new fluid on an aircraft, e.g. new

types of hydraulic fluid, should be considered in relation to the ability of cables to

withstand contamination.

3.5.16 Cable Construction

3.5.16.1 Conductors

For equipment interconnection and airframe cables, the conductors are normally

of the stranded type and are usually made from plated copper. However, size 24 and

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smaller sizes of conductor will be of copper alloy having a higher tensile strength. Fire

resistant cables may also be of copper alloy or copper conductors throughout all

applicable sizes.

The total conductor consists of plated strands which are circular in section and

which are laid up into one of a number of stranded forms. Aluminium conductors are

also available for cables of size 8 and larger but such cables have not been without any

problems. Any modification which involves conversion from copper to aluminium

should be classed as 'major' and thoroughly investigated, especially in regard to

termination techniques. Obviously, 'aluminium cables' will need to be significantly

larger in cross section than copper for a given electrical load, because of the higher

electrical resistance of aluminium.

3.5.16.2 Conductor Plating

Plating is employed on copper, copper alloy and aluminium conductors to

improve resistance to corrosion and to assist termination techniques. Very often it is the

plating which will determine the temperature rating of a given cable and the figures

given below are those widely recognised within the UK.

Tin plated copper maximum continuous temperature - 135°C.

Silver plated copper maximum continuous temperature - 200°C.

Nickel plated copper maximum continuous temperature - 260°C.

Nickel Clad plated copper maximum continuous temperature - 260°C.

Nickel clad copper is used instead of nickel plate on fire resistant cable to

provide a thicker nickel element.

The temperature figures quoted above may have to be varied downwards

because of limitations imposed by the cable insulation. Higher figures, notably 150°C

for tin plating, are sometimes quoted in the USA but performance at such temperatures,

especially in regard to stable crimp resistance and solderability is the subject of debate,

if not dispute. It should be noted that the plating used on crimped terminal ends must be

compatible with the conductor plating of the cable, and information should be sought

from termination manufacturers.

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3.5.17 Dielectric Materials/Cable Types

It is not practicable to review, in this Leaflet, the performance of all of the many

types of cable constructions available except in general terms. Extensive studies have

been made, especially in the USA, in an attempt to determine an optimum cable type.

The conclusion drawn is that there is not an overall best cable and that all the materials

studied have advantages and disadvantages. This is little help to a user who is seeking to

resolve the conflicting guidance and advice offered by organisations which have a keen

commercial interest in the decisions of an intending purchaser. This Information Leaflet

is intended to alert staff to the difficulty of making a sound judgement in what has

traditionally been considered to be a simple subject.

Insulation material is applied to conductors by one of two basic methods,

extrusion and wrapping. In general terms, extrudable materials are 'heat meltable' and

are not employed for higher temperature applications. It follows that towards the upper

limit of their operating temperature, their mechanical strength, when measured by

abrasion or cut through, can be significantly less than that measured at room

temperature. Airframe categories of cable usually have a double extrusion, which are

not always of the same material. A double extrusion is also claimed to impart 'crack

stopping' qualities. Radiation cross linking of processed material is employed on high

performance cables and this eliminates melting, increases strength and allows for

thinner wall thickness. Cables employing such construction perform well on the British

Standard test for wet arc tracking.

The most commonly used wrapped insulation material is Kapton*, which is the

registered trade name to an aromatic polyimide produced by Dupont. Many cable

manufacturers world-wide use Kapton, either singly or in combination with other

materials to give a so called hybrid construction. Single or double tapes are spirally

wound over the conductor to a defined overlap to give the required tape thicknesses at

anyone point. Kapton is naturally copper coloured and it is usual to apply a top coat to

provide a coloured surface which will accept print and also give added protection to the

cable. It follows that it is totally incorrect to talk of Kapton cables without further

definition. Some constructions, notably cables made in the USA to MIL-W-81381/11,

have been the subject of adverse comment and it is possible that the use of this

particular type will be discontinued in some environments. This would not reflect

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general rejection of cables containing Kapton because most constructions provide good

overall performance including excellent mechanical strength, especially the newer

higher hybrid types.

*Kapton is a Du-Pont trademark

The process of wrapping insulation provides good control of insulation wall

thickness and there are now cable types which employ only 4 layers of 'Kapton', giving

a total wall thickness of approximately 0.006 inches, and these are being employed

throughout the airframe of some recently certified aircraft types. The CAA has not

granted an Accessory Approval as 'Airframe' types to such cables, these having been

accepted on a 'Component' basis.

A previous paragraph reviewed the special case of PVC insulated cables such as

Minyvin (BSG221) and all PVC cables are now classed as 'Obsolescent - unsuitable for

new designs'.

3.5.18 Cable Failures

The following types of failure and quality faults are amongst those seen in recent

years

3.5.18.1 Arc Tracking

Electrical wet-wire arc tracking is a phenomenon that has been known for many

years. This can occur when leakage currents on a wet insulation surface are great

enough to vaporize the moisture, resulting in the formation of dry spots. These dry spots

offer a high amount of resistance to current flow. In turn, an induced voltage will

develop across these spots and result in the occurrence of small surface discharges.

Initially, these discharges will appear as scintillations at the insulation surface. These

discharges produce highly localized temperatures on the order of 1 OOOºC.

Temperatures of this magnitude will cause thermal degradation of the insulation

material, the nature of which depends on the insulation material used. The FAA

conducted a series of bench scale tests which demonstrated that the ability of an aircraft

wire to resist wet arc tracking and possible flashover is highly dependent on the

composition of the wire insulation. In addition, the conductivity level of the electrolyte

may influence the time and type of failure (arc track or open circuit) that can occur.

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Figure 3-56 An example of the damage caused by Wet Arc Tracking

The actions of cable manufacturers should resolve the problem, but the greatest

need is to ensure that hot stamp printing is properly controlled. 'Inter-connect' and

'Equipment Wires' should not be hot stamp printed.

Minyvin -Some batches of Minyvin have in the past shown a tendency to shed

the outer nylon sheath because of splitting along a flow line inadvertently introduced

during manufacture. In dry areas of aircraft, replacement of such cable is not a matter of

urgency but if moisture, especially hydraulic fluid, is present then cable must be

replaced. In areas which are exposed and prone to fluid contamination, such as

undercarriage bays, modifications to introduce a more suitable cable have been raised

on some aircraft types.

BMS 13-28 - Larger sizes of this mineral-filled PTFE cable, especially those

used on Boeing 707, 727 and 737 aircraft, tend to experience complete insulation failure

due to a longitudinal splitting of the total dielectric. Replacement by BMS13-58 or

EFGLAS to BS G222 under modification action is desirable.

3.5.18.2 Abrasion

Some types of cable have shown a tendency to 'wear through' the insulation at a

point where the cable rubs on cable or cable rubs on structure. Areas of high vibration

induce this failure mechanism and it may be supposed that the stiffer construction of

some cables tends to produce a greater contact force and transmit vibration where

previously it was damped. Careful cable loom tying and clipping is necessary to

alleviate this.

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Figure 3-57 A typical source of abrasion

3.5.18.3 Conductor “Knuckling Through”

Some earlier cable constructions tended to exhibit 'knuckling of conductors

which could be severe enough to penetrate the insulation. This was induced by applying

excessive pull through forces and care should be taken not to put cables under tension.

FEPSIL to BS G206, which is now 'obsolescent', requires particular care in manufacture

and installation to avoid this defect.

3.5.18.4 Red Plague

Cables with silver plated conductors can exhibit the aptly named 'Red Plague' if

the plating has been damaged and then exposed to moisture. Consequently, silver plated

conductors are generally unsuitable for use in unpressurised areas.

3.5.18.5 Glycol Fire

It is known that should de-icing fluid contaminate silver plated conductors, an

electrical fire can result. Accordingly, silver plated conductors should not be employed

in areas where de-icing fluid can be present.

3.5.18.6 Poor Solderability

It should be recognised that the quantity of free tin or plated conductors rapidly

reduces with time. The replacement of soldered connections during aircraft maintenance

will probably require that conductors are 'tinned' as part of the process. The loss of free

tin starts as the cable is manufactured and thus prolonged storage should be avoided.

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BAB IV PIPES AND HOSES

Pipes and hoses can be called upon to carry a wide variety of different fluids

within an aircraft, including fuel, hydraulic and engine oils, de-icing fluids, pitot and

static air.

The pressure within these pipes can vary from ambient to 300 M Pa (300 bar or

4000 PSI). All pipes and hoses must be manufactured, installed and connected so that

no leaks occur in service, because a leak in a very low-pressure pitot air tube can be just

as dangerous as a leak in an extremely high- pressure hydraulic line.

Rigid pipelines are, generally, made from stainless steel, Tungum (Trade name

for a hightensile, copper alloy) and aluminium alloy. Replacement pipelines are,

usually, supplied by the manufacturer, ready for installation, with the pipe bent to the

correct curvature and the pipe ends flared and provided with the appropriate end

fittings.

In certain circumstances, it may be permissible to manufacture new pipelines

from lengths of pipe. A new pipeline will be made, by cutting the basic pipe to the

correct length, attaching the correct couplings and expanding the ends by the use of a

flaring tool.

Requests for the basic pipe material will require details of the:

Metal specification (OTO, BS, AN etc.)

Outside diameter (00)

Gauge of the wall thickness (SWG)

Length of pipe required.

Flexible hoses are obtained from the aircraft manufacturer using the aircraft's

Illustrated Parts Catalogue (lPC). It is possible that, in certain circumstances, a

replacement hose can be manufactured in a workshop or hose bay. Approval to

manufacture the replacement hose must be sought from the aircraft's manufacturer.

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4.1 Bending and Flaring Aircraft Pipes

4.1.1 Bending Pipes

To lessen the possibility of the pipe wall kinking when it is being bent, it may be

filled with a special alloy, which can be removed after the bending operation. These

alloys are known as 'fusible alloys', some of which melt below 100ºC and can, therefore,

be melted out by immersion in boiling water.

The pipe is oiled first, to prevent the alloy adhering to the tube wall. It is next

plugged at one end, pre-heated and then filled with the melted alloy. Once cooled, the

pipe can then be bent as required.

After bending, the pipe should be unloaded, by immersing it in boiling water

until all the alloy has run out. The pipe must then be cleaned internally to ensure that

any alloy adhering to the walls of the pipe is removed. This is accomplished by using a

'pull through' with the pipe immersed in boiling water or by using a steam cleaner.

The complete removal of the fusible alloy from the pipe is extremely important

as its presence may lead to blockages or corrosion and, in steel tubes, which may be

subsequently heattreated, the presence of any alloy would cause inter-crystalline

cracking

4.1.1.1 Simple Bending Jigs

A simple bending jig (refer to Figure 4-1) is supplied with a range of rollers and

stops and the pipe is bent using the correct combination of components checking the

new pipe against either a template or the old pipe.

Figure 4-1 Bending jig

4.1.1.2 Bending Machines

Various sizes of tube bending machine are available, including Type K1A which

is suitable for tubes up to ½, in. old, and Type K2B for tubes 1/2 in. to 1 in. old. They

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can be secured by bolts to a bench, or the underside of the base plate may be gripped in

a vice. Each machine is provided with a set of interchangeable circular formers which

are grooved to receive, different diameters of tubing. The tube is forced round the

former; a range of guides is provided to correspond with the range of formers.

Each former provides for a mean radius of bend equal approximately to four

times the diameter of the corresponding tube; this radius should be regarded in each

instance as the safe minimum for the diameter of the tube concerned. The former can be

removed and exchanged by withdrawing the centre pin and sliding the former out; after

replacing a former, ensure that the centre pin is pushed right home before commencing

bending operations.

The guide is inserted between the former and the roller after the tube to be bent

has been laid in the groove of the former. The roller is mounted on the slideable

crosshead on the bending lever arm; the position of the roller can be adjusted by rotating

the knurled screw at the end of the lever. This adjustment determines the point at which

pressure will be applied to the tube. A sliding pressure indicator, mounted in the

bending lever arm, indicates the position of the roller in relation to the guide, tube, and

former in use. An adjustable stop is provided to withstand the thrust transmitted to the

tube by the operation of bending. The position of the stop is varied to suit the class of

bend required.

Figure 4-2 Pipe bending machine

4.1.2 Pipe Flaring Tools

Flaring can be achieved only when the end of the pipe has been accurately

squared off and cleaned out. Once a flare has been formed correctly, it should remain

completely fluid tight at all normal pressures

Pipe flaring tools, come in a variety of sizes, with a range of pipe sizes that can

be flared by each particular tool. A typical flaring tool (refer to Figure 4-3), is used to

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flare tubes in the range 12 mm to 25 mm (½ in to 1 in). Sets of half-bushes or dies cover

the range of tube sizes for each machine. The flaring tool is usually mounted in a hand

vice or some other rigid mounting.

Once the half-bushes have been installed, the union-nut and collar are placed

onto the tube and the tube is then clamped into the bushes, with the tube end flush with

the end of the dies or half bushes.

Figure 4-3 Pipe flaring tool

The threaded sleeve is slowly fed into the end of the tube whilst simultaneously

turning the expander cone via the rotation handle. This spreads the end of the tube until

it contacts the inner face of the bushes. A correctly finished flare should leave

prescribed amount of the tube projecting from the collar.

The finished flared end with the union nut and collar can be connected to a

variety of other end fittings. These can include other pipes, and both internal and

external adapters fitted to a number of different components.

4.1.2.1 Standard Flared Pipe Couplings

Various types of standard flared pipe couplings (refer to Figure 4-5), are

available in aircraft fluid systems. These couplings have different angles and whilst they

may look similar, they are not interchangeable. The AGS system uses a 32° flare whilst

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the AN system uses flares of 74° included angle. Care must be taken to ensure that the

correct couplings are fitted when manufacturing these pipes.

Table 4-1 Projection tolerances

Figure 4-4 Pipe flare projection

Figure 4-5 Standard aircraft couplings

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4.1.2.2 Tests After Installations

All pipes will have been pressure tested following manufacture, but it is usually

necessary to carry out pressure and flow tests after installation of a pipe, to ensure that

there are no leaks from the pipe and its connections and that, where essential to the

correct operation of the associated system, the required flow rate is obtained.

Power for carrying out the tests may be provided by the aircraft engine-driven

pumps or by an external test rig suitable for the system concerned. The tests

should be carried out strictly in accordance with the relevant Maintenance

Manual. Special note should also be taken of any precautions specified for safety

of personnel or the prevention of damage to the aircraft or its systems.

While the associated system is pressurised, and while the services are being

operated, the pipelines should be inspected for flexing or displacement to ensure

that the required clearances are maintained. The pipe supports should be

checked for security of attachment and the pipes for local distortion at the

clamping points.

Leakage from pipes in liquid systems (e.g. hydraulic systems) can usually be

detected by careful visual inspection, and leakage from gas systems (e.g.

pneumatic systems) can usually be detected aurally, or, after painting the pipes

and connections with a solution of water and acid-free soap, be detected by the

appearance of bubbles. If the soap solution is used it should be washed off

immediately after the test.

If leakage from a connection is apparent, the connection may be tightened, but

should not be over-tightened in an attempt to cure the leak. Leaks are often

caused by solid particles at the mating faces of a joint, by misalignment of a

nipple, or by damage to one of the components in the joint. Loosening and re-

tightening of a coupling will often cure a leak but if it does not do so, the

coupling should be disconnected and the cause of the leakage ascertained.

After all tests have been completed satisfactorily, it is important to ensure that

any liquid which may have leaked or been spilled on the airframe structure or

components, is removed. In addition to any fire hazard, aircraft liquids may also

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have deleterious effects on some of the alloys and compounds with which they

come into contact.

When the work of installing and testing a pipe is complete, the connections

should, where applicable, be locked in the appropriate manner.

After a pipe coupling has been assembled for the first time, it should be

disassembled and the extension of the flared end beyond the collar checked.

With the collar as far as it will go towards the flared end of the pipe, the

projection of the pipe beyond the collar face must be measured. The tolerances

given in the table below are permissible.

4.1.2.3 Flareless-Tube Fittings

The MS (Military Standard) flareless-tube fittings are finding wide application

in aircraft plumbing systems. Using this type fitting eliminates all tube flaring, yet

provides a safe, strong, dependable tube connection. The fitting consists of three parts; a

body, a sleeve, and a nut. The body has a counterbored shoulder, against which the end

of the tube rests. The angle of the counterbore causes the cutting edge of the sleeve to

cut into the outside of the tube when the two are joined. Installation of flareless-tube

fitting is discussed later in this chapter.

Figure 4-6 Flareless tube fitting

4.1.2.4 Flareless-Tube Assemblies

Although the use of flareless-tube fittings eliminates all tube flaring, another

operation, referred to as pre-setting is necessary prior to installation of a new flareless-

tube assembly. The presetting operation is performed as follows:

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Step 1: Cut the tube to the correct length, with the ends perfectly square. Debur

the inside and outside of the tube. Slip the nut, then the sleeve, over the tube.

Step 2: Lubricate the threads of the fitting and nut with hydraulic fluid. Place the

fitting in a vice, and hold the tubing firmly and squarely on the seat in the fitting.

(Tube must bottom firmly in the fitting). Tighten the nut until the cutting edge of

the sleeve grips the tube. This point is determined by slowly turning the tube

back and forth while tightening the nut. When the tube no longer turns, the nut is

ready for final tightening. Final tightening depends upon the tubing. For

aluminium alloy tubing up to and including ½ inch outside diameter, tighten the

nut from 1 to 1 1/16 turns. For steel tubing and aluminium alloy tubing over 1/2

inch outside diameter, tighten from 1 1/16 to 1 1/2 turns.

Step 3: After presetting the sleeve, disconnect the tubing from the fitting and

check the following points. The tube should extend 3/32" to 1/8" beyond the

sleeve pilot; otherwise blow off may occur, The sleeve pilot should contact the

tube or have a maximum clearance of 0.005 inch for aluminium alloy tubing or

0.015 inch for steel tubing. A slight collapse of the tube at the sleeve cut is

permissible. No movement of the sleeve pilot, except rotation, is permissible.

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Figure 4-7 Steps in fitting f1areless pipe fittings

4.1.2.5 Flareless-Tube Installation

Tighten the nut by hand until an increase in resistance to turning is encountered.

Should it be impossible to run the nut down with the fingers, use a wrench, but be alert

for the first signs of bottoming. It is important that the final tightening commence at the

point where the nut just begins to bottom. With a wrench, turn the nut 1/6 turn (one flat

on a hex nut) Use a wrench on the connector to prevent it from turning while tightening

the nut. After the tube assembly is installed, the system should be pressure tested.

Should a connection leak it is permissible to tighten the nut an additional 1/6 turn

(making a total of 1/3 turn). If, after tightening the nut a total of 1/3 turn, leakage still

exists, the assembly should be removed and the components of the assembly inspected

for scores, cracks, presence of foreign material, or damage from overt tightening.

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NOTE- Over tightening a flareless-tube nut drives the cutting edge of the sleeve

deeply into the tube, causing the tube to be weakened to the point where normal in-

flight vibration could cause the tube to shear. After inspection (if no discrepancies are

found), reassemble the connections and repeat the pressure test procedures.

CAUTION: Do not in any case tighten the nut beyond 1/3 turn (two flats on the

hex nut); this is the maximum the fitting may be tightened without the possibility of

permanently damaging the sleeve and nut.

Common faults are:

Flare distorted into nut threads

Sleeve cracked

Flare cracked or split

Flare out of round

Inside of flare rough or scratched

Fitting cone rough or scratched

Threads of nut or union dirty, damaged or broken

4.2 Inspection and Testing of Pipes and Hoses 4.2.1 Inspection Points

Before any inspections can be done, it must be ensured that the components are

scrupulously clean and that all critical areas are visible if the inspection is done while

the component is in its normal, installed location (in situ).

Rigid pipes should be inspected for signs of:

Chafing

Corrosion - both externally and internally where possible

Cracking of flared ends where appropriate

Deformation and Dents (no dents are allowed in the heel of a bend)

Deterioration in condition of end fittings and their threads.

Hose assemblies should be inspected for such defects as:

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Blistering - Both externally and internally where possible

Burn damage or discolouration

Chafing, circumferential cracking or crazing of the outer cover

Date of manufacture - to ensure that it is within its prescribed life, and that it

will remain so until the next inspection

Deterioration in condition of end fittings and their threads

Flattening, kinking or twisting.

The relevant maintenance manual will state the intervals of inspections and the

criteria which must be met before rigid pipes or hose assemblies may be considered fit

for further service.

4.2.2 Bore Testing of Pipes

Pipes should be tested to ensure that the bore is clear and dimensionally correct

after forming. One method of satisfying this requirement is to pass a steel ball, or bullet,

with a diameter of 80% of the internal diameter of the pipe, through the pipe in both

directions. When the design or size of the pipe and end fittings, makes this test

impractical or when a more searching test is required, the drawing will normally require

a flow test to be performed.

Figure 4-8 Bore test of pipe

4.2.3 Hydraulic Pressure Testing of Pipes

Hydraulic pressure testing consists of firstly carrying out a flow test. This means

a full bore flow by pumping fluid through the pipe and checking the flow at the open

end. If this check is satisfactory, the open end should be suitably blanked.

Once the flow test has been carried out, the oil pressure should then be built up

to that prescribed on the drawing, usually 1½:, times the maximum working pressure.

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The duration of the test must give the pipe a chance to show any leaks or other

problems.

4.2.4 Pneumatic and Oxygen Pressure Testing of Pipes

These pipes are usually given an initial hydraulic pressure test, using water as

the test medium, followed by a compressed air test that is limited to maximum system

pressure. Using highpressure air during the test is very dangerous and the pipers) under

test should be placed behind a protective screen and/or submerged in water.

4.2.5 Cleaning After Test

After a pipe has been tested, it should normally be flushed out using a suitable

solvent, dried out using a jet of clean, dry air and blanked off, using the approved

blanks.

Pipes that will be used in high-pressure air and gaseous or liquid oxygen systems

must be scrupulously clean and free from any possible contamination by oil or grease. It

is normal to recommend that pipes for use in these systems are flushed with

Trichloroethane or some other suitable solvent, blown through with double filtered air

and blanked-off, with the approved blanks immediately afterwards.

4.2.6 Pneumatic and Oxygen Pressure Testing of Pipes

Once the manufactured hose has been checked for satisfactory physical

condition, the hose must be flow and pressure tested. The flow test will verify whether

the hose inner lining is secure and not acting as a form of non-return valve. This is

achieved by passing the fluid through the hose assembly both ways to confirm that there

is an equal and free flow.

Where a replacement hose has been manufactured in a local hose bay, a bore test

may be done, in the same manner as that with rigid pipes, by use of a ball bearing being

rolled in bothdirections through the hose. In this instance, however, the diameter of the

ball should be 90% of the internal diameter of the hose's end fittings.

The hose should then be 'proof-tested' by capping one end of the hose and

applying the test pressure, usually 1½ times the maximum working pressure to it for

between one and five minutes.

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4.3 Installation and Clamping of Pipes

4.3.1 Pipe Supports

Prior to installation, the pipe should be checked to establish that it is of the

correct type and that there is evidence of prior inspection and testing. This may involve

checking the inspector's stamp and part number. Once the pipe has been checked for

signs of damage, dirt or corrosion, and found serviceable, it must then be immediately

installed.

When transporting lengths of pipe, especially long lengths, great care must be

taken not to kink or otherwise damage the pipe prior to installation. Once in position,

the pipes should be loosely placed into position in the supporting clamps, and adjusted

so that the connections align correctly. The connections can then be tightened up, the

clamps fastened and any bonding leads attached.

In some instances, packing will be installed between the pipe and the clamping

material. This will usually be to reduce vibration or to insulate the pipe and clamp

material, if they are likely to suffer from electrolytic corrosion.

Individual pipe clamping is usually achieved using 'P' clips. These are light alloy

loops with a rubber sleeve, which wrap around the pipe and are held by a single bolt to

the aircraft structure.

To avoid the risk of fretting occurring between the pipe and various parts of the

aircraft, minimum dimensions must be observed between these components, which can

be found in the AMM. The CAAIPs list these dimensions as 6 mm (0.25 in) from fixed

structure, 18 mm (0.7 in) from control rods and 25 mm (1 in) from control cables, but

the AMM must always take precedent.

4.3.2 Connection of Pipes

When connecting pipes with brazed, flared or flareless couplings, there are a

number of points to be considered.

Union nuts must be free to rotate and can be slid back from the end of the pipe

without fouling.

All loose items such as nipples and washers, are of the correct type and correctly

located.

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All pipe ends align correctly without any undue pressure on the pipe. (Pipes

should never be forced into position, neither should they ever be pulled-up into

position by their union nuts).

4.3.3 Maintenance of Pipes and Hoses

The correct methods of installing pipes and hoses (refer to Figure 4-9) must be

followed if damage (and possibly disaster) is not to result. Pipes attached to the airframe

structure, are often shielded and will not usually be liable to accidental damage. Other

pipes may be located in exposed positions, where they may be susceptible to damage or

corrosion.

Pipes located in wheel bays or attached to an undercarriage leg could easily be

damaged by stones and mud or corroded by thrown-up water. Some pipes may be badly

sited and may be subject to abuse from carelessly performed and unrelated servicing

activities.

Chafing can occur in many places, such as clamps and clips, so care must be

shown to eliminate or at least reduce the chances of this happening. Cracking of pipes

can occur when pulsations are present and/or the pipe has sharp bends. This risk must

also be considered when inspecting pipe runs.

Liquid leaks can be found by the presence of fluid, or at least dampness, on the

pipe or clamps. Gaseous leaks must be searched for using one of the proprietary leak-

detecting fluids.

The relevant AMM will give details on how a particular hose is installed in the

aircraft, but, in general, a hose should be at least 3% longer than the maximum distance

between end fittings. Consideration should also be given to the orientation of a hose

and, once correctly installed, the witness lines, marked on the hose, should be straight.

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Figure IV-9 Correct and incorrect installations

4.3.4 Pipe Identification Labels

Once a pipe has been fitted to the aircraft, it should have system identification

label attached to enable engineers to identify which system each pipe belongs to. The

label comes in rolls of about 25 mm wide and uses colours, symbols and letters to

differentiate between different pipes. A small length of the label is wound around the

pipe at convenient points.

Figure IV-10 Pipe code identification codes

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BAB V

CONTROL CABLES

The majority of aircraft control cables have swaged end fittings. Splicing is

seldom used on modern aircraft.

Swaging is an operation in which a metal end fitting is secured to the end of the

cable by plastic deformation of the hollow shank of the fitting.

The end of the cable is inserted into the hollow shank of the end fitting, and the

shank is then squeezes in a swaging machine, so that it grips the cable. This is the most

satisfactory method attaching an end fitting to a cable, and it can be expected to provide

a cable assembly at least as strong as the cable itself.

5.1 Swaging of End Fitting

Most transport aircraft and a large number of light aircraft, use control cables

manufactured in this way.

Manufacturers of cable assemblies normally swage with rotary machines. In

these machines the shank of the end fitting is placed between suitable dies and is

subjected to a series of forming blows, which reduce the shank diameter and lock the

fitting to the cable.

Swaging may also be carried out on a portable swaging machine, which

squeezes the shank of the end fitting between dies.

A range of swaged end fittings is covered by BS specifications, but some older

types of aircraft may be fitted with cable assemblies containing components complying

with SBAC AS specifications which are not obsolete. When it is necessary to make up

control cables for these aircraft, approval may be granted for the use of equivalent BS

parts, but the complete cable control run may have to be changed. BS specifications

provide a range of fittings which prevent incorrect assembly of control cables. Turn-

barrels and tension rods are designed to connect to screwed end and tapped end swaged

fittings respectively. For each size of cable two alternative sizes of end fittings are

available and each size is provided with either a left or right hand thread. Swaged

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fittings can thus be arranged to ensure that a control run cannot be incorrectly

assembled.

Figure 5-1 Portable swaging machine

5.1.1 Swaging Procedures

The procedure outlined below is applicable for portable machine, which in all

cases should be in accordance with the manufacturer's instructions. Where use of a

different type of machine is authorized, the procedure is similar, except for the setting

and operation of the machine, which in all cases should be in accordance with the

manufacturers instructions.

a) Ensure that the new cable is the correct size, by using a suitable gauge.

b) Cut the cable to the length specified on the drawing, and ensure that the

ends are clean and square.

NOTE: Swaging elongates the end fitting and an allowance for this must be

made when cutting the cable. The allowance to be made should be stated on the

appropriate drawing or specification.

c) Select the appropriate end fitting and clean it by immersing it in solvent;

then shake and wipe dry.

d) Assemble the end fitting to drawing requirements. With drilled-through

fittings, the cable end must pass the inspection hole, but be clear of the

locking wire hole. For fittings with a blind hole, the cable must bottom in

the hole. Bottoming may be checked by marking the cable with paint, at a

distance from the end equal to the depth of the hole and ensuring that the

paint mark reaches the fitting when the cable is inserted. When the cable

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and the fitting are correctly assembled, they should both be lightly

lubricated.

e) Fit the dies for the particular end fitting in the swaging machine, open the

handles of the machine and unscrew the adjuster until the end fitting can

be placed in the dies. With the end fitting centred in the die recess, close

the handles fully and screw in the adjuster until the dies grip the fitting.

Open the handles and tighten the adjuster by the amount of squeeze

required for the particular end fitting; normally this should be

approximately 0.18 mm (0.007 in).

f) Place the fitting in the position so as to swage to within approximately

1.2 mm (0.050 in) from the inspection hole, Check that the cable is in the

correct position and operate the handles to squeeze the fitting.

g) Release the handles and rotate the fitting through approximately 50°.

Repeat the squeezing and rotating until the fitting has been moved one

full turn.

h) Withdraw the end fitting from the dies 1.6 mm (0,0625 in) and repeat the

cycle of squeezing and turning.

i) Continue operation until the whole shank is swaged, Check the diameter

of the shank and if it has not been reduced to the size required by the

appropriate drawing or specification, re-set the adjusting screw and

repeat the swaging operation.

j) When the shank of the end fitting has been reduced to the correct

diameter, remove and inspect the fitting.

k) Fit the identification device as prescribed in the drawing and mark it with

the cable part number in the prescribed manner (in some cases the part

number may be etched directly onto the end fitting), The identification

may be in the form of a wired-on tag, or a cylindrical sleeve lightly

swaged onto the shank of the end fitting.

l) Assemble any fittings, such as cable stops, on the cable and swage on the

opposite end fitting.

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m) Dip the end fittings in lanolin, to prevent corrosion resulting from

damaged plating and to exclude moisture.

Inspection of Swaged Fittings

a) On completion of the swaging operations, the following inspection

should be carried out.

b) Check that the correct combination of cable and fittings has been used.

c) Re-check the diameter of the swaged shank, using a GO-NOT GO gauge

or a micrometer. If the diameter of the fitting is too small, it has been

over-swaged and as such the cable and the fitting must be rejected.

Excessive work hardening of the fitting will cause it to crack and may

also damage the cable,

d) Check, by means of the inspection hole or paint mark, that the cable is

correctly engaged in the end fitting.

e) Check that the swaging operation has not disturbed the lay of the cable,

where the cable enters the end fitting,

f) Ensure that the shank: is smooth, parallel and in line with the head of the

fitting and that the swaged shank length is correct.

g) Proof load the completed cable assembly in accordance with the

appropriate.

h) Inspect the fittings for cracks using a lens of 10 x magnification, or carry

out a crack detection test, using magnetic or dye processes, as

appropriate,

i) Check that the cable assembly is the correct length and ensure that any

required identification marking, including evidence of proof loading, has

been carried out and that any specified protective treatment has been

applied,

NOTE: The first swaged fitting in a production batch is usually sectioned after

proof loading, so that the interior surface can be examined for cracks. If this check is

satisfactory, the settings on the swaging machine should be noted and used for

completion of the batch.

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5.1.2 Swaged Splices

A number of proprietary methods are used to secure cable in the form of a loop,

which may then be used to attach the cable to a terminal fitting or turnbuckle. The

'Talurit' swaged splice is approved for use on some British aircraft control cables and is

also widely used on ground equipment. The process provides a cable assembly which,

when used with cable to BS W9 and WII, has a strength equal to approximately 90% of

the breaking strength of the cable. It may only be used to replace cables employing the

same type of splice, or hand splices and must not be used where swaged end fittings

were used previously.

Figure 5-2 Fitting of Talurit Ferrule

To make this type of splice, the end of the cable is threaded through a ferrule of

the appropriate size, looped and passed back through the ferrule. A thimble is fitted in

the loop and the ferrule is squeezed between swages (dies) in a hand-operated or power-

operated press. The metal of the ferrule is extruded between the two parallel lengths of

cable and around the cable strands firmly locking the cable without disturbing its lay.

5.2 Handling, Inspection and Testing of Ctrl Cable&Associated Hardware

5.2.1 Handling of Cable To ensure maximum life of cable it is essential that they are handled with care.

If a cable is kinked it must be rejected. To obviate kinking, cable is supplied on drums.

When removing cable from drums a rod should be passed through the centre of

the drum and the cable fed off as the drum is rotated. The minimum diameter for drums

is forty times the diameter of the cable.

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Long cables are often loose coiled for ease of handling and in this case the

minimum diameter of the coil should be fifty times the diameter of the cable or six

inches whichever is the larger. If during handling a cable has to be passed through the

hand, the hand should be protected with a pad of cloth wrapped around the cable.

Cable may be permanently damaged, or its working life may be considerably

curtailed, by careless handling and unwinding. Care is necessary to prevent the cable

from forming itself into a loop, which, if pulled tight, could produce a kink. A kink is

shown by the core strand leaving the centre of the rope and lying between the outer

strands or protruding in the form of a small loop.

Cable should always be stored on suitably designed reels. The diameter of the

reel barrel should be at least forty times the cable diameter. British Standards stipulate

that reels should be made from a wood which will not corrode the cable and that interior

surfaces should be lined with an inert waterproof material. Precautions should also be

taken to protect the cable from grit and moisture and from damage in transit.

To remove cable from a reel, a spindle should be placed through the centre of

the reel and supported in a suitable stand. Cable may then be removed by pulling the

free end in line with the reel, allowing the reel to rotate. Cable should not be unwound

by paying off loose coils, or by pulling the cable away from a stationary reel laid on its

side.

When a long length of cable has been cut from a reel and it is necessary to coil

the cut piece, the coil diameter should be at least 50 times the cable diameter, with a

minimum diameter of 150 mm (6 in).

Care must be taken to prevent dust, grit and moisture, from coming into contact

with the coiled cable.

The ends of stored cable are whipped to prevent fraying and if a length has been

cut from the reel, the remaining free end should be whipped.

When a coil is being unwound, the coil should be rotated so that the cable is paid

out in a straight line.

5.2.2 Cutting of Cables Cable should always be cut using mechanical methods. Cable cutters or heavy

duty pliers should normally be used, alternatively, the cable may be laid on an anvil and

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cut with a sharp chisel and hammer blows. Cable should not be cut by flame. If a non-

preformed cable is being cut, it should be whipped with waxed cord on both sides of the

cut, prior to being cut. With a preformed cable it will normally only be necessary to

bind the cable temporarily with masking tape.

5.2.3 Cleaning of Cables Cables should not be immersed in grease solvent for cleaning purposes, the

correct way to clean a cable is to moisten a cloth in grease solvent and remove surface

dirt etc. If a cable is immersed in grease solvent, the solvent will wash out the grease

which is put in the cable during manufacture. The outcome would be that the life of the

cable will be reduced.

5.2.4 Corrosion of Cables No corrosion is allowed on cables in aircraft. If corrosion is found on a cable the cable must be rejected

5.2.5 Cable Wear Critical areas for strand breakage are where the cable passes over pulleys or

through fairleads. Examination of cables will normally involve passing a cloth along the

length of the cable, which will both clean any dirt from it and detect broken strands if

the cloth 'snags' on the projecting wires.

There will be limits, published by the manufacturer, which say how many

strands per unit length can be broken. Removed cables can be bent through a gentle

radius, which may show up broken internal strands that would not be visible when

installed and tensioned.

External wear will extend along the cable, equal to the distance the cable moves

at that location and may occur on one side of the cable or over its entire circumference.

The limits of permitted wear will be found in the AMM.

Figure 5-3 Cable wear

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Internal wear occurs in similar places in the wire to external wear, around

pulleys and fairleads and is much more difficult to detect. Separating the strands, after

removing the cable, is the only way to detect internal wear and this only permits limited

inspection.

Generally any signs of internal wear within a cable will mean its replacement.

Broken strands on a cable at a location not adjacent to a pulley or fairlead, could

be an indication that the breakage was due to corrosion.

Inspection for broken wires is carried out by bending the cable as shown (see

Figure 5-4) if possible, or by running the full length of cable through the protected hand,

if necessary in short stages. You will feel the broken wires snag the cloth.

Figure 5-4 Inspecting for broken wires

The inspection of a cable for internal corrosion should be done off aircraft, and

will involve rejection of the cable if corrosion is found.

The maintenance carried out on cable runs usually involves both regular

inspections and preservative measures. With the majority of cables being steel-based, it

is vital that cables, passing through high risk areas such as battery bays, toilets and

galleys, receive regular rust preventative treatments in addition to visual inspections.

Most cables have external corrosion preventative compounds applied in varying

amounts, whilst internally they will have been soaked in some form of thin grease or

low-temperature oil to resist the formation of the difficult to detect internal corrosion.

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Normally in dry and desert atmospheres, the application of certain compounds to

cables is not permitted. This is because the adhesive properties of these compounds will

cause the sand and dust to stick to the cable and, thus, cause extremely high rates of

wear.

All controls will be monitored, by the flight deck crew, on a day-to-day basis

but, during maintenance, more subjective tests must be completed. The tension of the

cables will be measured, as will the rigging of the complete runs, to ensure that the

controls remain accurate and precise in their operation.

Whilst it is not usual to find faults on the cable end fittings, these should all be

checked for any signs of damage, corrosion and stressing of the cable at the end fitting.

Items checked will include turnbuckles and ball end fittings, to ensure that the cable is

operating at the designed angle, tension and over the correct range.

5.2.6 Checking the Tension of Installed Cables The correct tension for a control cable is specified in the Manufacturers

Maintenance Manual. It is checked by the use of a Tensiometer and adjusted on the

turnbuckles. The British tensiometer consists of a system of pulleys, two fixed and one

connected to a pointer an spring loaded. The deflection of the pointer indicates the

tension on a scale appropriate to the size of cable. It is essential to use the correct type

of tensiometer for the cable size.

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Figure 5-5 A cable tensiometer

Figure 5-6 Operation of the Pacific T5 tensiometer

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Figure 5-7 Reading the Pacific tensiometer

5.2.7 Inspection of Control of Cable Pulleys Pulleys are fitted to change the direction of a cable run. They are made from

Tufnol or Micarta. An integral sealed ball bearing is provided. Cable guards are

provided to prevent the cable coming off the pulley.

When inspecting cables for the previously mentioned wear and breakages, the

complete cable runs must be examined for incorrect routing, fraying, twisting or wear at

fairleads, pulleys and guards.

Pulleys must be inspected for wear, to detect indications of seizure, flat spots,

embedded foreign material and excessive tension. Any signs of contact with adjacent

structure, pipe-work, wiring and other controls must also be thoroughly investigated.

Figure 5-8 Pulley wear patterns

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Figure 5-9 Pulley assembly and cable alignment

5.2.8 Fairleads To prevent chafing of the cables, fairleads are fitted to the aircraft structure

where the cables pass through, e.g. bulkheads and frames. They are made of Tufnol,

Micarta or Nylon, and are normally of two halves bolted together. The cable runs

through a hole in the fairlead.

Fairleads must not be lubricated as they will collect dust and dirt.

Figure 5-10 A typical fairlead

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Figure 5-11 A sealed fairlead assembly

5.2.9 Turnbuckle & Locking System

Turnbuckles are the usual device for tensioning a cable system. A turnbuckle

assembly consists of a left hand threaded fitting swaged on to one cable end, and a right

hand threaded fitting swaged to the other cable end, and a barrel, tapped left and right

hand between them.

Figure 5-12 Components ofa turnbuckle

Turnbuckles are in safety when:

British types - A hardened steel pin will not pass through the safety

inspection hole.

American types - All of the fitting thread is engaged in the barrel.

It is common practice for the left hand threaded end of the barrel to be identified

with a grooved machined on the outer surface.

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Figure 5-13 Turnbuckle assembly

Most turnbuckles currently are locked using spring clips which are passed down

grooves cut in the threads of the fittings and the barrel. The clip is positively located

when the locking tongue is located under the lip of the barrel centre hole.

When use of a clip is not possible, or wire locking is specified, this should be

done in accordance with the aircraft manufacturer's requirements, usually to the FAA or

CAA standards as appropriate.

Figure 5-14 A turnbuckle locked with a clip

Figure 5-15 Turnbuckle wire locking procedure

5.3 Bowden and Teleflex Cable Systems

5.3.1 Bowden Cable Systems

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A typical Bowden cable control might be a brake lever on the control column

operating a remote brake control valve.

Maintenance of Bowden cable systems is usually restricted to cleaning and

lubrication of the inner cable at regular intervals and adjustment of the outer conduit

(e.g. if the brakes needed adjustment). The lubrication would keep moisture out of the

cable to prevent it freezing at low temperatures

5.3.1.1 Servicing

a) Inspect the cable ends for fraying and corrosion

b) Inspect the conduit for kinks and signs of wear

c) Adjust the cable for slackness by adjuster (screw out, i.e. increase the

length of conduit to take up the slackness in cable) Check for adequate

locking.

d) Lubricate, on assembly, with recommended grease.

Figure 5-16 A Bowden cable assembly

Figure 5-17 Bowden cable connections

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5.3.2 Teleflex Cable Systems The Teleflex cable system is more complex than the Bowden cable system in

that the operating cable, within the conduit, is actually a number of spirally wound

cables which surround a core tension cable, giving it support. This allows the cable to

transmit a push force as easily as a pull force, doing away with the need for any form of

retum spring.

A typical use of a Teleflex system might be a throttle lever to engine fuel control

system connection.

The Teleflex cable system is a snug fit within the conduit and, because there

might be the chance of it becoming seized, due to foreign objects, dirt or freezing, it is

vital that the inner cables are regularly removed, cleaned and lubricated with low

temperature grease. It is also important that the conduits are thoroughly cleaned using a

form of 'pull-through', prior to the inner cable being installed.

At longer intervals, it might become necessary to inspect the outer conduit for

signs of damage or kinking; which can cause the control to become tight or 'notchy'.

5.3.2.1 Attachment of Teleflex End Fittings

a) Box Unit i. Tuck the cable into the slot in the pinion and ensure that the cable helix

engages with the pinion teeth to give a wrap of at least 40 degrees ("single

entry" units). On double entry units the cable should engage with the pinion

to give a wrap of 180 degrees, the cable projecting through the lead-out hole

throughout the travel of the control. Ensure that the cable end does not foul

the blanked end of the conduit when fully extended. All box units should be

packed with recommended grease.

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Figure 5-18 Method of Attaching to Box Type Unit

ii. Sliding end fittings (fork end type). Unscrew the threaded hexagon plug

from the body, screw the lock nut right back, and pass the cable through

the plug. Screw the lock spring on to the end of the cable so that 3/16-in.

of cable projects.

iii. Insert the cable end, with its lock spring, into the bore of the body of the

end fitting, and screw the hexagon plug tight down, preventing the body

from rotating. Check that the free end of the cable is beyond the

inspection hole, but not beyond the fork gap (for a fork end fitting).

Tighten the lock nut and turn up the tat washer. Check that the distance

from the face of the body to the end of the sliding tube does not exceed

0.45 in. (0.35-in. old type, without tab washer). This ensures that the lock

spring is tightly compressed.

Figure 5-19 Teleflex end fitting assembly

In assembling, the body of the end fitting must not be screwed on to the hexagon

plug. The plug should be screwed into the fork, not fork into plug. Failure to apply this

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rule will result in the lock spring unscrewing. The same method should be used when

removing the fork, and care should be taken not to jam the spring and foul up the wire

wrap.

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BAB VI WELDING, BRAZING, SOLDERING, AND BONDING

Welding, Soldering and Bonding are methods of creating permanent joints

between materials and use is made of all three methods in the aerospace industry,

primarily at the production stage and to a more limited degree during maintenance and

overhaul stages.

Within the aircraft industry welding is considered to be a specialist skill and only

suitably approved and authorised personnel can undertake welding procedures.

Approved welders must satisfy the CAA of their competency, by submitting

several 'test pieces' of their typical work for testing and they are subjected to similar re-

tests every 12 months in order to retain their approvals.

Maintenance technicians may, however, be called upon to do some soldering and

bonding procedures so, with these facts in mind, only the basic methods of welding will

be discussed in this topic, while greater emphasis is placed upon procedures involving

soldering and bonding.

6.1 Welding

6.1.1 Methods of Welding Welding may be defined as the permanent joining, by fusion, of two pieces of

material (usually metals), by the progressive melting and subsequent solidification of

the materials at the site of the joint.

The basic principle, of fusion welding of metals, is the same for all processes, in

that the surfaces, or edges, of the metal to be joined, are brought to a molten state and

allowed, or caused, to intermix (with or without the addition of a filler metal), so that

the parent metal and filler metal (if used) form a homogeneous molten pool which,

when cooled, forms the complete weld. Welds require the application of sufficient heat

energy to melt the metals involved in the joint and the high temperatures are achieved

by various methods.

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6.1.2 Oxy-acetylene Welding The cutting of steel sections and plate material may be done by means of a flame

torch, using a mixture of oxygen, with one of the appropriate fuel gases (acetylene,

hydrogen, natural gas or propane).

For welding, however, only an oxygen and acetylene mixture will provide a

sufficiently, high heat input, needed for the welding process. The temperature of the

oxy-acetylene flame is approximately 3150°C.

The oxy-acetylene method can be used for welding ferrous or nonferrous metals

but, when welding non-ferrous metals, it is necessary that an additional material (a flux)

be used, usually with a filler metal, to assist in the fusion process. The purpose of the

flux is to prevent oxidation of the joint site so that the molten metals can fuse together

more easily and, thus, eliminate brittleness in the joint.

Figure 6-1 Oxy-acetylene welding

6.1.3 Manual Metal Arc Welding (MMAW) This welding process uses an electric arc as the heat source. The arc is

established between a flux-coated, filler metal rod and the workpiece, which are

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connected to an electrical power source so that they are the anode and cathode

electrodes of the circuit. When the power is switched on, the heat, generated by the

resulting arc, melts the flux-coated electrode and the edges of the parent material to

form a weld pool. The temperature of the arc is approximately 4000°C to 4500°C.

Figure 6-2 Manual Metal Arc Welding

6.1.4 Metal Inert Gas Welding (MIG) In this semi-automatic welding process the heat source is also an electric arc, but

the electrode is a bare wire, which is consumable and is supplied, from a reel, to the

welding gun, by a wire feed unit. A shielding gas is employed; in place of a flux

material, to protect the weld pool. The type of shielding gas, used, will vary with the

application. Some of the gases and gas mixtures used are:

Argon

Carbon dioxide

Argon/carbon dioxide

Argon/oxygen

Argon/nitrogen

Helium.

Note: This process may also be referred to according to the type of shielding gas

(or mixture of gases) which is being used and whether those gases are inert or active.

The two types of this process are:

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Metal Inert Gas (MIG) welding: where the shielding is provided by a shroud of

inert gas.

Metal Active Gas (MAG) welding: where the shielding is provided by a shroud

of active, or non-inert, gas or mixture of gases.

Figure 6-3 MIG Welding

6.1.5 Tungsten Inert Gas Welding (TIG) This process also uses an electric arc as the heat source, but here a tungsten non-

consumable electrode is used to form the arc with the workpiece. An inert shielding gas

(argon) is required to protect both the weld pool and the tungsten electrode from the

oxygen and moisture in the atmosphere.

For this reason the process is sometimes called argon arc welding. A filler rod is

usually required to give reinforcement to the weld .

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Figure 6-4 TIG Welding

6.1.6 Flash Butt Welding The components to be joined are set up as opposite poles in an electric circuit

and, when the current is switched on, the components are moved into and out of contact

with one another. This action causes an arc to be struck and, when welding temperature

is reached, a force is applied to both components, so that their molten surfaces are fused

together.

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Figure 6-5 Flash Butt Welding

6.1.7 Resistance Spot Welding A method used to join comparatively thin sheets of metal, spot welding is a form

of resistance welding. The sheets of metal are sandwiched between two, pointed

electrodes on which force is exerted as the current is applied. The heat is generated at a

local spot where the resistance to the flow of the electricity is at its highest and the

metal fuses at these spots. The pointed electrodes are made from copper alloy and are

usually water-cooled.

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Figure 6-6 Resistance Spot Welding

6.1.8 Resistance Seam Welding The principle of seam welding is similar to that of spot welding (namely

resistance to the flow of electricity). The main difference is that in place of the pointed

electrodes, this method uses discs or wheels, which are moved along the length of the

weld. The supply of current is intermittent, so causing a spot weld to overlap its

neighbour and, thereby, form a continuous seam weld.

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Figure 6-7 Resistance Seam Welding

6.1.9 Inspection and Testing of Welds The wide use of welding in industry has resulted in an increasing demand for

standards relating to welded constructions in various branches of engineering. These

standards generally include requirements for certain welding tests to be conducted,

primarily for the qualification of welding procedures and operators.

Sophisticated methods of non-destructive testing of welds include the use of

Radiographic, Ultrasonic and Magnetic Particle testing procedures, all of which are

done by specially trained, and approved, personnel. Specimen welds are also

destructively tested, by fracturing or sectioning, to test the integrity of a specific

welding procedure.

These methods are beyond the scope of unqualified personnel, so that aircraft

maintenance technicians are, usually, constrained solely to the visual inspection of

welds (following thorough cleaning of the relevant areas). It may, however, be possible

that, after suitable training, some technicians can be granted approval to conduct limited

Dye Penetrant inspection procedures on certain welds, which will be specified in the

appropriate servicing manual.

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Figure 6-8 Welding defects

Figure 6-9 Weld nomenclature

Figure 6-10 Weld seam dimensions

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6.2 Soldering & Brazing

6.2.1 Method of Soldering Soldering differs from welding in that it is done at considerably lower

temperatures so that the parent metals do not melt and fuse together. Instead, a fusible

and, usually, non-ferrous alloy (with a lower melting point) is applied between the

heated metals of the joint, such that the fusible alloy forms a metallic bond with the

parent metals and, on cooling, creates a solid joint.

The word 'solder' does, in fact, come from the same stem as the word 'solid' (as

does the American term, which is pronounced 'sodder', for the same process).

Soldering can be divided into two basic methods, one of which uses higher

temperature ranges than the other, but both of which are conducted at temperatures

below the melting points of the parent metals of the intended joint.

The two basic methods of soldering are:

Hard Soldering: done at temperatures in excess of 500°C and which include the

processes of Brazing and Silver Soldering

Soft Soldering: done at temperatures within the range of 180°C to 330°C, which,

consequently, create joints of lower strength (but less expense) than those

achieved by the hard soldering methods.

Note: The hard soldering processes are, normally, beyond the remit of the

aircraft servicing technician, so only brief consideration is given to them here, with

more attention being given to the soft soldering method

6.2.2 Hard Soldering (Brazing and Silver Soldering) Brazing, as the name implies, uses a Copper/Zinc (Brass) alloy, as the filler

metal (spelter) between the parent metals of the joint. The degree of alloying will dictate

the temperature at which the process is done but the melting point of the brazing alloys

can be as high as 880°C.

Brazing is a process of joining in which, during, or after heating, the molten

filler metal is drawn into, or retained in, the space between closely adjacent surfaces of

the parts to be joined, by capillary attraction.

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In general applications, workshops and small factories, a flame, directed onto

the joint area, is the source of heat. However, in the more sophisticated applications,

used in industry, heating for hard soldering may be provided by a:

Gas, oil or electrically heated, closed furnace

High-frequency (HF) induction coil.

As with welding, it is necessary to employ the use of a flux material to assist the

fusion of the filler with the parent metals and to prevent oxidation of the joint.

The flux mostly used for brazing processes is borax, which is based on Sodium

Borate powder, mixed with water, to a thin paste before being applied, by brush or

swab, to the site of the joint. Other fluxes are also available where required.

Silver Soldering entails the use of a Copper/Zinc/Silver or Nickel/Silver alloy as

the joining metal and (again depending on the alloy employed), can be done at

temperatures of between 650°C to 700°C. Brass, copper, monel metal and stainless steel

are typical metals on which silver soldering processes can be used.

6.2.3 Soft Soldering

Soft Soldering involves the use of a LeadlTin alloy (with traces of Bismuth and

Antimony added when required) as the filler metal, which melts at temperatures

between approximately 180°C to 330°C, depending on the composition of the alloy.

The lower temperature requirement, of the soft soldering process, allows the use of

indirect heat.

In earlier times, the heat was provided by the application of an implement with a

wooden handle and a smooth, flat, base or 'bit' (originally made of iron). The 'iron' was

directly heated in a flame, then quickly cleaned, before being applied to the solder joint,

where the transference of its heat would facilitate the melting of the filler metal. This

process possibly needed repeating several times (as the iron tended to lose its heat fairly

quickly) before a large task could be completed.

It was found that copper is a better heat conductor than iron, is less prone to

corrosion and is, therefore, easier to keep clean. Copper, consequently, became the

metal most preferred for use as the soldering 'bit', though the implement retained its

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name of the soldering 'iron'. While needing re-heating less frequently, it remains

necessary to regularly reheat the copper bit of the directly heated soldering irons.

The advent of electrically heated (and thermostatically controlled) soldering

irons has overcome the re-heating problem, associated with directly heated irons, and

consideration is given here only to the method of soft soldering with the use of

electrically (or indirectly) heated soldering irons.

While the method described is the most commonly used in small workshops (or

in DIY applications), there are, however, three further methods which are used in

industrial applications. Those methods involve:

Applying a naked flame to the joint

Dip soldering

Heating by non-contact techniques.

6.2.4 Using Indirectly Heated (Electric) Soldering Iron Electric soldering irons are available in a variety of sizes and weights with bits

shaped to suit the particular application. Typically, the 25 watt, electric soldering iron

(refer to figure 6-11), is widely used for making joints in electric circuitry. The heating

element contained in the barrel of the iron is supplied directly from the mains electrical

supply.

Figure 6-11 Using an electrically heated soldering iron

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Larger, 40 watt (or as large as 125 waUl irons, with proportionately larger bits,

may be used when it is required to create overlapping joints (lap joints) of sheet metals

(though this is a task, not normally done by aircraft maintenance technicians).

Before any soldering operation is attempted, the joint surfaces (and the soldering

iron) must be properly prepared. It is of paramount importance that the joint surfaces be

absolutely free of dirt and grease (and surface oxides), so that the solder will be able to

satisfactorily form intermetallic compounds and, thus, bond completely with the parent

metals.

To ensure this, the approved cleaning methods must be used for the relevant

metals (abrasives, etchants de-greasants etc.) and, finally, an appropriate flux is applied

to the cleaned surfaces, to prevent oxidation at the jOint and to assist in the flow and

fusion of the solder.

Note: Some solders have a flux included in their hollow core, while others,

require the application of a separate flux material.

After the surfaces have been carefully prepared, the electric soldering iron can

be switched on and allowed to reach its operating temperature. This is, usually,

indicated by a small, integral warning lamp but may be deduced by applying a piece of

solder to the bit and seeing the solder melt when the temperature is adequate.

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Figure 6-12 Tinning the joint surface

The heated bit of the soldering iron must, next, be 'tinned', This is achieved by,

firstly, ensuring that the bit is thoroughly cleaned then dipping the bit in flux (if a

separate flux is being used) and applying solder to the bit until a thin film of solder

completely covers the working area of the soldering bit. It is important that the tinning

of the bit is done correctly, otherwise problems will be experienced with the soldering

operation,

Each surface of the prepared joint must also be carefully tinned in a similar

manner, so that a thin film of solder covers the total area of the joint surfaces.

Care must be taken, when applying solder to the joint surfaces, to ensure that it

is as thin and as smooth as possible and that the heat is maintained, to allow the inter-

metallic compound between the parent metal and the layer of solder to form.

This compound is an important factor and contributes greatly to the strength of

the joint, as it is, actually, stronger than the solder.

When the two surfaces of the joint are correctly tinned, they are placed together

and the hot iron is applied to an outer surface of the joint. The heat is transmitted

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through the metal and melts the solder interfaces so that they fuse together and a typical

soldered lap joint of the metals is completed.

Figure 6-13 Soldered Lap joint

Note: Even when making electrical connections, using soft solder, a type of lap

joint must be made, since an end-to-end joint in wire would be impracticable.

6.2.5 Active and Passive Fluxes Metal surfaces become more reactive to oxygen when they are heated and, as

previously discussed, to prevent this oxidation, during the soldering process, a suitable

flux is applied to the surfaces being joined.

The flux should possess certain characteristics in that it:

Forms a liquid film over the joint and excludes the gases in the atmosphere

Prevents any further oxidation during the heating cycle

Assists in dissolving the oxide film on the metal surface and the solder

Is displaced from the joint by liquid filler metal.

Fluxes for soft soldering are often classified into two groups, which are the:

Active group: which are corrosive or acid fluxes

Passive group: which are non-corrosive fluxes.

The flux can be applied separately, or as a constituent within the solder. Fluxes may take the form of a liquid, paste or solid, and the application, for which they are being used, will govern the type selected.

Active (corrosive) fluxes are used where conditions require a rapidly working and highly active flux. The common active fluxes are listed below.

WARNING: THESE FLUXES CAN CAUSE BURNS TO FLESH AND CLOTHING. PROTECT THE EYES WITH GOGGLES AND WEAR RUBBER GLOVES AND APRON WHEN USING A CORROSIVE FLUX.

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Zinc Chloride (ZnCI): commonly called 'killed spirits'. This used on general

sheet-metal work and may be obtained commercially under its trade name of

'Baker's Soldering Fluid'

Ammonium Chloride (NH4CI): commonly called sal ammoniac. This used, in

block form, for cleaning the face of the soldering bit before tinning, or in

powdered form, with Zinc Chloride, for tinning cast iron

Hydrochloric Acid (HCI): used in the raw state for pickling the surfaces of the

metal and rendering them clean. As a flux it is extremely active and is suitable

for soldering zinc and galvanised mild steel

Phosphoric Acid: used, primarily, on stainless steels.

Note: Flux residues of acid fluxes remain active after soldering and will cause

corrosion unless removed by thorough cleansing, - first in a weak solution of

caustic soda - and then in water.

Passive (Non-Corrosive) fluxes are divided into three types, which are:

Natural resin: dissolved in suitable organic solvents, it is the closest

approximation to a non-corrosive flux and is particularly suitable for use in the

electrical industry

Tallow: used by plumbers, for the jointing of lead sheet and pipes. Similar to

resin, it is only slightly active when heated to the temperature of the soldering

process

Olive Oil: used for soldering pewter items. 6.2.5.1 Flux Removal

It is essential that all flux residues be removed, since they can present a

corrosion hazard. The method of removal will be determined by the type of flux used,

but will entail the use of one, or a combination of, the following:

A solution of caustic soda

A solution of sulphuric acid

A supply of warm water

Physical abrasion.

6.2.6 Inspecting and Testing of Soldered Joints

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The inspection of soldered joints is done mainly by visual means, though, in

some applications, tensile testing is recommended.

Electrical contacts, using soft soldering methods, may be tested by gently pulling

on the wires to confirm the security of the joint. These joints may also be tested for

electrical continuity and resistance, using appropriate instruments.

6.3 Bonding

6.3.1 The Mechanic of Joints Adhesive bonding has been used on an ever-increasing scale and particularly in

the aerospace industry. Adhesives are used for constructional tasks varying from aircraft

fuselages, flight control surfaces, to propellers and helicopter rotor blades.

The actual adhesive bond may be achieved in two ways:

Mechanical: - here the adhesive penetrates into the surface and forms a

mechanical lock, by keying into the surface. It also forms re-entrants, where the

adhesive penetrates behind parts of the structure, and becomes an integral part of

the component to be joined .

Chemical (Specific): - in this method of bonding, the adhesive is spread over

the surfaces to be joined and forms a chemical bond with the surface.

In practice, most adhesives use both ways of bonding to form a joint.

6.3.2 Stress on Bonded Joints Adhesive joints are liable to experience four main types of stress

Joint stress is at a maximum when the adhesive is in shear (refer to figure 6-14).

Adhesives should not be used if significant stresses will be carried in tension or peel.

Lap joints are the types more, generally favoured, as the strength of the adhesive bond is

proportional to the area bonded.

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Figure 6-14 Stresses on Bonded Joints

Tensile

Where the two surfaces are pulled directly apart.

Shear

Where the two surfaces tend to slide across each other.

Cleavage

Where two edges are pulled apart.

Peel

Where one surface is stripped back from the other

6.3.3 Procedure for Bonding To achieve optimum bonding, performance, and life in service, from adhesives

and sealants, it is absolutely crucial to follow carefully planned processes and

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procedures and to pay the utmost attention to quality at every stage. In fact, the major

criticisms, levelled against the use of adhesives, are:

Absolute cleanliness at all stages is essential. Surface preparation of the

component is also crucial. To ensure consistent results on structural components,

a purpose-built 'clean room' is required, in order to reduce contamination to a

minimum.

Pressure and heat may be required. Sophisticated equipment is required to

produce pressure over the components in areas where adhesives are applied.

This will often entail vacuum bags, purpose-built ovens, or pressurised curing

ovens (autoclaves).

Inspection of the bonded joint is difficult. Special inspection techniques and test

pieces are necessary to check the integrity of the bond. Prior to preparing the

mating surfaces for 'gluing', it is necessary to carry out a 'dry' lay-up i.e. a trial

assembly of all related parts to check and adjust the fit if necessary. This

procedure is essential, to enable the final assembly 'wet' lay-up to proceed

without delay, and without the risk of generating swarf or of contaminating

specially prepared surfaces.

6.3.3.1 Surface Preparation

Grease, oil, or other contaminants, must be removed by suitable solvents.

An optimum surface roughness must be produced.

Once pre-treated, a surface must be protected from harmful contamination until

the bonding process is complete.

Surfaces to be bonded are normally thoroughly cleaned/degreased in a suitable

solvent. This may be followed by a chemical etch or light blasting treatment,

followed by a water wash and subsequent drying.

6.3.3.2 Final Assembly

The adhesive is applied (usually within a specified time, otherwise re-processing

may be necessary), and the assembly suitably clamped, or put in a nylon vacuum bag,

and heated in an autoclave. The curing process then takes place under carefully

controlled temperature and pressure conditions.

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When cool, the component is inspected, visually for positioning and for a

satisfactory spew line. The glue-line thickness is also checked, with a calibrated

electronic probe, and specimen test pieces are tested for shear and peel properties.

Following a satisfactory inspection, the component is finally given appropriate

corrosion protection (usually over-painting).

Note: After commencing the final (wet) lay-up, curing of the adhesive must be

carried out within a specified time (usually 12 hours). If this period is exceeded by a

few hours it is necessary to increase the temperature and pressure levels during curing

(and to obtain an official 'concession' cover for this discrepancy).

If the permissible time between wet lay-up and curing is greatly exceeded (e.g. a

full shift or day), it will be necessary to dismantle and not only re-commence the wet

lay-up, but also to, possibly, repeat some of the preliminary surface preparation

treatments (such as etching).

6.3.3.3 Typical (Abreviated) Process

Dry lay-up (i.e. 'dummy run')

Prepare faces to be bonded (alumina blast, etch (pickle) anodise, etc).

Water wash and dry.

Apply adhesive in clean room and clamp or apply vacuum bag.

Cure in press/oven or autoclave (typically 120°C - 170°C)

Release autoclave pressure when cool.

Inspect:

Positioning, uniform, continuous glue-line etc.

Glue-line thickness (electronic probe).

Specimen test-piece results (shear and peel).

Carry out final post-cure surface treatments. (e.g. over-painting of primer,

sealant or top coat of solvent-resistant paint)

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BAB VII DISASSEMBLY, INSPECTION, REPAIR AND ASSEMBLY

TECHNIQUES

An operational aircraft can suffer from many defects and these can be defined as

any event or occurrence, which reduces the serviceability of the aircraft below 100%.

The manufacturer should specify the inspection areas and the faults, which are

expected to be found. In most instances the inspector is looking for indications of

abnormality in the item being inspected.

7.1 Types of Defect

Typical examples are:

Metal Parts as applicable to all metal parts, bodies or casings of units in systems

and in electrical, instrument and radio installations, metal pipes, ducting, tubes,

rods and levers. These would be inspected for:

o Cleanliness and external evidence of damage

o Leaks and discharge

o Overheating

o Fluid ingress

o Obstruction of drainage or vent holes or overflow pipe orifices

o Correct seating of panels and fairings and serviceability of fasteners

o Distortion, dents, scores, and chafing

o Pulled or missing fasteners, rivets, bolts or screws

o Evidence of cracks or wear

o Separation of adhesive bonding

o Failures of welds or spot welds

o Deterioration of protective treatment and corrosion

o Security of attachments, fasteners, connections, locking and bonding.

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Rubber, Fabric, Glass Fibre and Plastic Parts: such as coverings, ducting,

flexible mountings, seals, insulation of electrical cables, windows. These parts

would, typically, be inspected for:

o Cleanliness

o Cracks, cuts, chafing, kinking, twisting, crushing, contraction - sufficient

free length

o Deterioration, crazing, loss of flexibility

o Overheating

o Fluid soakage

o Security of attachment, correct connections and locking.

Control System Components: cables, chains, pulleys, rods and tubes would be

inspected for:

o Correct alignment - no fouling

o Free movement, distortion, evidence of bowing

o Scores, chafing, fraying, kinking

o Evidence of wear, flattening

o Cracks, loose rivets, deterioration of protective treatment and corrosion

o Electrical bonding correctly positioned, undamaged and secure

o Attachments, end connections and locking secure.

Electrical Components: actuators, alternators and generators, motors, relays,

solenoids and contactors. Such items would be inspected for:

o Cleanliness, obvious damage

o Evidence of overheating

o Corrosion and security of attachments and connections

o Cleanliness, scoring and worn brushes, adequate spring tension after

removal of protective covers

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o Overheating and fluid ingress

o Cleanliness, burning and pitting of contacts

o Evidence of overheating and security of contacts after removal of

protective covers

7.1.1 External Damage Damage to the outside of the airframe can occur by interference between

moving parts such as flying controls and flaps, although this is quite rare. The most

common reasons for airframe damage is by being struck by ground equipment or severe

hail in flight.

During ground servicing many vehicles need to be manoeuvred close to the

airframe and some have to be in light contact with it to work properly. Contact with the

airframe by any of these vehicles can cause dents or puncturing of the pressure hull,

resulting in a time-consuming repair

7.1.2 Inlet and Exhausts Any inlet or exhaust can be a potential nest site for wildlife. The damage done

by these birds, rodents and insects can be very expensive to rectify. Other items that

have been known to block access holes include branches, leaves and polythene bags.

A careful check of all inlets and exhausts, during inspections, must be made, to

ensure that there is nothing blocking them. A blocked duct can result in the overheating

of equipment, or major damage to the internal working parts of the engine.

7.1.3 Liquid Systems Liquid systems usually have gauges to ascertain the quantity in that particular

system. A physical quantity check is often done in addition to using the gauges, as the

gauges are not always reliable.

These systems usually include oil tanks for the engine, APU and Integrated

Drive Generators (lDG), and also the hydraulics, fuel and potable water tanks.

The cause of a lower-than-expected level should be immediately investigated,

bearing in mind, that some systems consume specific amounts of fluids during normal

operation. The consumption rate must be calculated before instigating any trouble-

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shooting. A low hydraulic system should not be replenished without first investigating

the cause of the leak.

External leaks of oil and fuel systems are normally easy to locate. The

rectification of an external leak is usually achieved by simply replacing the component,

seal or pipe work at fault, and completing any tests required by the AMM.

If the leak is internal, then a much more thorough inspection of the component

must be made, as the problem is more difficult to find. The symptoms are usually

signalled by a slower movement of the services or by the erratic operation of services,

due to the return line being pressurised.

Some hydraulic oils, especially the phosphate ester based fluids, are very toxic

and require personnel protection when working on and replenishing their systems. Some

oils used are slightly toxic so care must be taken if there is a large leak.

Potable water tanks are often permanently pressurised, so that a leak that starts

somewhere between the tank and the services will continue, even if the aircraft is not

flying. Once the pressure is removed, the leak can be investigated, cured and the tank

re-filled.

The physical signs of water inside the aircraft or dripping from the hull should

be the signs of a leak that requires investigation. The unpredictable passenger

consumption of water means that the tank level is no indication of a leak in the system.

Windscreen de-icers are usually in the form of a pressurised container, which

supplies fluid on demand to the spray nozzles. If the fluid leaks onto the flight deck it

will give off a distinctive odour in the enclosed space. As the containers are replaced

when low, it is more likely that the pipe work will be the likely cause of the leak.

7.1.4 Gaseous Systems These include gases such as oxygen, nitrogen and air. If the gas is to be used

from a system during flight, a leak will be very hard to confirm unless a physical check

is carried out using a leak detector such as 'Snoop' or 'Sherlock'.

A leak from an oxygen system is extremely dangerous, due to the chances of an

explosion, if it comes into contact with oil or grease. Once the leak has been cured, the

system can be recharged and leak tested.

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Nitrogen, used in hydraulic accumulators, can leak into the liquid part of the

hydraulic system. This will make the hydraulic system feel spongy and reduce the

response of the operating actuators.

If the gas leaks into the atmosphere, the system will not function correctly and

the efficiency of the system may be reduced. The main cause of accumulators leaking

externally is due to faulty seals or gauges.

Accumulators assist the hydraulic system as an emergency backup, which only

works correctly if it is charged to the correct pressure.

Pneumatic systems contain high-pressure air of a stated pressure, and should

have the same pressure at the end of the flight as at the start. If the pressure is low at the

end of the flight, then the compressor could be suspected.

If the pressure falls between flights, it is probably due to a slow leak in the

storage system, and this can be investigated using leak-detecting fluids.

7.2 Visual Inspections

7.2.1 Dimensions There are a number of places where checking the measurement of a component

can establish its serviceability. Landing gear oleo shock struts can be checked for

correct inflation, by measuring their extension. If the dimension is less than quoted in

the manual, then it may be low on pressure and further checks will be required. These

checks are usually only done during line maintenance, with checking of the pressure

being required for trouble shooting or hangar maintenance.

Combined hydraulic and spring dampers, fitted to some landing gears, often

have one or more engraved lines on the sliding portion of the unit. This can indicate

whether the hydraulic precharge is correct or requires replenishment.

7.2.2 Tyres

Tyres have their serviceability indicated by the depth of the groove in the tyre

tread. The AMM gives information of what constitutes a worn or damaged tyre.

Apart from normal wear, other defects, that can affect a tyre, are cuts, blisters,

creep and low pressure.

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Most tyres can be re-treaded a number of times after they have reached their

wear limits, but the retread can only be completed if the complete tyre has not been

damaged badly.

Creep is the movement of a cover around the rim, in very small movements, due

to heavy braking action. This movement is dangerous if the tyre is fitted with a tube, as

the movement can tear the charging valve out of the tube, causing a rapid loss of

pressure.

To provide an indicator, small white marks are painted across the wheel rim and

the tyre side wall cover so, if creep takes place, the marks will split in half and indicate

clearly that the tyre cover has moved in relation to the wheel rim.

The installation of tubeless covers has reduced the problem of creep, as the

valve is permanently fitted to the wheel. It is still possible for tyres to creep a small

amount, but the air remains in the tyre as the seal remains secure.

Tyre-inflation devices usually consist of high-pressure bottles fitted with a

pressure-reducing valve or a simple air compressor. The pressure a tyre should be

inflated to depends on various factors such as the weight of the aircraft.

The correct pressure for a specific aircraft is given in the relevant AMM for the

aircraft in question. It is possible for a tyre to lose a small amount of pressure overnight.

A pressure drop of less than 10% of the recommended pressure is not unusual, but the

exact figures are given in theAMM.

If a tyre is completely deflated with the weight of the aircraft on it, or is one of a

pair on a single landing gear leg, which has run without pressure, all the tyres concerned

must be replaced due to the possible, unseen damage within the cover. Again the AMM

will dictate the conditions.

7.2.3 Wheels

Defects to aircraft wheels are usually due to impact damage from heavy landings

or from items on the runway hitting the wheel rim. Other problems can arise from

corrosion starting as a result of the impact damage and the shearing of wheel bolts,

which hold the two halves of a split wheel together. Wheels are usually inspected

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thoroughly during tyre replacement and it is very unusual for serious defects to be found

during normal inspections of a wheel.

7.2.4 Brakes Brake units are normally attached onto the axle of an undercarriage leg, and

located inside the well of the main wheels. During braking operation they absorb large

amounts of energy as heat. This results in the brake rotors and stators wearing away

and, if they become too hot, the stator material may break up.

Figure 7-1 The Boeing 737 brake, showing wear-pins

Inspection of brake units between flights is essential, to check for signs of

excessive heating and to ensure that they have not worn beyond their limits.

Wear results in the total thickness of the brake pack being reduced, which means

that by measuring either the thickness of the pack, the amount of wear can be

monitored. Once the amount of wear reaches a set figure, the brakepack will be

overhauled.

If the pads are breaking up there will be signs of debris, excessive amounts of

powder and, in extreme cases, scoring of the discs. This will require immediate

replacement of the complete brake unit.

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A rejected take-off at maximum weight will produce the maximum possible

amount of heat and wear. It is usual to replace all brake units and main wheels after this

has happened, but again the AMM will give the required information on what must be

changed and when.

7.2.5 Landing Gear Locks These items are normally fitted to the aircraft's undercarriage as a safety device

to prevent them inadvertently collapsing. They are usually fitted when the aircraft is to

stay on the ground for some time, and removed before the next flight. The most likely

defects will be damage to the locking pin ball bearing device or the loss of the high

visibility warning flags. These flags will, hopefully, attract attention to themselves to

ensure that they are not left in position when the aircraft next goes flying.

Figure 7-2 The Boeing 737 APU extinguisher bottle indicators

7.2.6 Indicators The most common type of indicator is the 'blow- out' disc used in fire

extinguishing and oxygen systems. This shows that a high-pressure gas bottle has

discharged its contents overboard, blowing the disc from its flush housing in the

aircraft's skin.

The reason for the ruptured disc could be due to a fire extinguisher having been

operated or the extinguishant having been discharged due to an excessive pressure being

reached.

7.2.7 External Probes

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There are several different types of probe, projecting into the airflow, to send

information to the flight deck. These can include the pitot/static probes and the angle-of

attack (AOA) probes.

To prevent these from freezing they have electrical heating elements built into

them and, occasionally, they can become overheated. Usually this is when they are left

switched 'on' on the ground with a faulty weigh-on-wheels (WOW) switch.

Figure 7-3 The Boeing 737 pitot probes and alpha vane

This switch is designed to reduce or remove power to the probes when on the

ground, and to increase or restore it in flight On smaller aircraft there is no WOW

switch and it is up to the pilot to turn them off after landing. If the elements overheat

they can burn out and the probes will show this by discoloration.

Probes are designed to project out from the aircraft skin, and this makes them

vulnerable to physical damage. Probes need to be regularly inspected for signs of

physical damage or discoloration.

7.2.8 Handles and Latches Handles and latches usually wear through constant use. The handles and latches

of cargo bays and baggage holds, which are operated every time the aircraft lands, are

particularly prone to wear. Technicians have to be aware that all panel fasteners will

wear slowly and these panels must be secured in flight

Most fasteners have a 'positive' form of closing or locking, whilst the more

important installations use an indication system (such as painted lines and flush fitting

catches) to ensure correct closure. These must be regularly checked and, when found

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worn, they should be repaired or replaced. Losing a panel in flight is dangerous enough,

but may be more so if it is drawn into one of the engines, and causes its destruction.

7.2.9 Panels and Doors

These items can be of any size and can be faulty for several reasons. They can

be damaged by excessive use and their frames can become damaged where items have

to be passed through them (such as with baggage hold doors).

If the latches are poorly designed or badly adjusted, they may have been

operated with incorrect tools during service and may have been damaged.

7.2.10 Emergency System Indication Some systems use protective covers, to prevent inadvertent operation of a

switch. These covers are usually held closed by some form of frangible device that will

indicate the system has been operated when it is broken. Thin copper wire is,

sometimes, used to hold the protective cover closed on fire extinguisher switches. A

broken wire will indicate that the cover has been lifted and the system may have been

operated. Any indication like this must be thoroughly investigated.

Figure 7-4 Copper 'tell-tale' wire on a hand-held fire extinguisher

7.2.11 Other Inspections Moving parts - proper lubrication, security of attachment, binding, excessive

wear, proper safety wiring, proper operation and adjustment, proper installation,

correct travel, cracked fittings, security of hinges, defective bearings,

cleanliness, corrosion, deformation, and sealing and tension.

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Fluid lines and hoses - proper hose or rigid tubing material, proper fittings,

correct fitting torque, leaks, tears, cracks, dents, kinks, chafing, proper bend

radius, security, corrosion, deterioration, obstructions and foreign matter, and

proper installation.

Wiring - proper type and gauge, security, chafing, burning, defective insulation,

loose or broken terminals, heat deterioration, corroded terminals, and proper

installation.

Bolts - Correct torque, elongation of bearing surfaces, deformation, shear

damage, tension damage, proper installation, proper size and type and corrosion.

Filters, screens, and fluids - cleanliness, contamination, replacement times,

proper types, and proper installation.

Powerplant - engine mount security, mount bolt torque, spark plug security,

ignition harness security, oil leaks, exhaust leaks, muffler cracks and wear,

security of all engine accessories, engine case cracks, oil breather obstructions,

firewall condition, and proper operation of mechanical controls.

Propellers - nicks, dent cracks, cleanliness, lubrication, gouges, proper blade

angles, blade tracking, proper dimensions, governor leaks and operation and

control linkages for proper tension and installation. Nicks on the leading edge of

the blade are important items to inspect for: they produce stress concentrations

that need to be removed immediately upon discovery in order to prevent the

blade separating at the nick.

Ground runs - engine temperatures and pressures, static RPM, magneto drop,

engine· response to the changes of power, unusual engine noises, ignition switch

operation, fuel shutoff/selector valves, idle speed and mixture settings, suction

gauge, fuel flow indictor operation.

7.2.12 Lifed Items There are a number of items on the aircraft that have a specific length of time in

service (known as a 'life'). They would be major airframe and engine components with

finite fatigue lives. The company technical department monitors these and they will be

replaced during major servicing.

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The components which can become unserviceable due to life expiry may

include, engine fire bottles, cabin fire extinguishers, first aid kits, portable oxygen

bottles and emergency oxygen generators

7.2.13 Light Bulbs These have to be checked regularly, to ensure they remain serviceable at all

times. Most bulbs with important functions like fire warning lights and undercarriage

indication will be duplicated. This can be achieved either by using two separate bulbs or

by a single, twin-filament type. The bulb covers can also be damaged, leading to broken

glass or plastic on the flight deck, with its subsequent foreign object damage (FOD)

hazard.

7.2.14 Permited Defects All aircraft have a list of permitted defects that do not have to be immediately

corrected. These defects can be left outstanding by the operator until a more convenient

time can be found to rectify them.

7.3 Corrosion Removal Assessment and Reprotection

7.3.1 Locations of Corrosion in Aircraft Certain locations in aircraft are more prone to corrosion than others. The rate of

deterioration varies widely with aircraft design, build, operational use and

environment. External surfaces are open to inspection and are usually protected

by paint. Magnesium and aluminium alloy surfaces are particularly susceptible

to corrosion along rivet lines, lap joints, fasteners, faying surfaces and where

protective coatings have been damaged or neglected.

Exhaust Areas - Fairings, located in the path of the exhaust gases of gas turbine

and piston engines, are subject to highly corrosive influences. This is

particularly so where exhaust deposits may be trapped in fissures, crevices,

seams or hinges. Such deposits are difficult to remove by ordinary cleaning

methods.

During maintenance, the fairings in critical areas should be removed for

cleaning and examination. All fairings, in other exhaust areas, should also be

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thoroughly cleaned and inspected. In some situations, a chemical barrier can be

applied to critical areas, to facilitate easier removal of deposits at a later date,

and to reduce the corrosive effects of these deposits.

Engine Intakes and Cooling Air Vents - The protective finish, on engine frontal

areas, is abraded by dust and eroded by rain. Heat-exchanger cores and cooling

fins may also be vulnerable to corrosion.

Special attention should be given, particularly in a corrosive environment, to

obstructions and crevices in the path of cooling air. These must be treated, as

soon as is practical.

Landing Gear - Landing gear bays are exposed to flying debris, such as water

and gravel, and require frequent cleaning and touching-up. Careful inspection

should be made of crevices, ribs and lower-skin surfaces, where debris can

lodge. Landing gear assemblies should be examined, paying particular attention

to magnesium alloy wheels, paint-work, bearings, exposed switches and

electrical equipment.

Frequent cleaning, water-dispersing treatment and re-Iubrication will be

required, whilst ensuring that bearings are not contaminated, either with the

cleaning water or with the waterdispersing fluids, used when re-Iubricating.

Bilge and Water Entrapment Area - Although specifications call for drains

wherever water is likely to collect, these drains can become blocked by debris,

such as sealant or grease. Inspection of these drains must be frequent. Any areas

beneath galleys and toileUwash-rooms must be very carefully inspected for

corrosion, as these are usually the worst places in the whole airframe for severe

corrosion. The protection in these areas must also be carefully inspected and

renewed if necessary.

Recesses in Flaps and Hinges - Potential corrosion areas are found at flap and

speed brake recesses, where water and dirt may collect and go unnoticed,

because the moveable parts are normally in the 'closed' position. If these items

are left 'open', when the aircraft is parked, they may collect salt, from the

atmosphere, or debris, which may be blowing about on the airfield. Thorough

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inspection of the components and their associated stowage bays, is required at

regular intervals.

The hinges, in these areas, are also vulnerable to dissimilar metal corrosion,

between the steel pins and the aluminium tangs. Seizure can also occur, at the

hinges of access doors and panels that are seldom used.

Magnesium Alloy Skins - These, give little trouble, providing the protective

surface finishes are undamaged and well maintained. Following maintenance

work, such as riveting and drilling, it is impossible to completely protect the skin

to the original specification. All magnesium alloy skin areas must be thoroughly

and regularly inspected, with special emphasis on edge locations, fasteners and

paint finishes.

Aluminium Alloy Skins - The most vulnerable skins are those which have been

integrally machined, usually in main-plane structures. Due to the alloys and to

the manufacturing processes used, they can be susceptible to intergranular and

exfoliation corrosion.

Small bumps or raised areas under the paint sometimes indicate exfoliation of

the actual metal. Treatment requires removal of all exfoliated metal followed by

blending and restoration of the finish.

Spot-Welded Skins and Sandwich Constructions - Corrosive agents may

become trapped between the metal layers of spot-welded skins and moisture,

entering the seams, may set up electrolytic corrosion that eventually corrodes the

spot-welds, or causes the skin to bulge, Generally, spot-welding is not

considered good practice on aircraft structures.

Cavities, gaps, punctures or damaged places in honeycomb sandwich panels

should be sealed to exclude water or dirt. Water should not be permitted to

accumulate in the structure adjacent to sandwich panels. Inspection of

honeycomb sandwich panels and box structures is difficult and generally

requires that the structure be dismantled.

Electrical Equipment - Sealing, venting and protective paint cannot wholly

obviate the corrosion in battery compartments. Spray, from electrolyte, spreads

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to adjacent cavities and causes rapid attack on unprotected surfaces. Inspection

should also be extended to all vent systems associated with battery bays.

Circuit-breakers, contacts and switches are extremely sensitive to the effects of

corrosion and need close inspection.

Control Cables - Loss of protective coatings, on carbon steel control cables can,

over a period of time, lead to mechanical problems and system failure.

Corrosion-resistant cables can also be affected by corrosive, marine

environments.

Any corrosion found on the outside of a control cable should result in a thorough

inspection of the internal strands and, if any damage is found, the cable should

be rejected.

Cables should be carefully inspected, in the vicinity of bell-cranks, sheaves and

in other places where the cables flex as there is more chance of corrosion getting

inside the cables when the strands are moving around (or being moved by) these

items.

7.3.2 Prevention of Corrosion Due to the high cost of modern aircraft, operators are expecting them to last

much longer than perhaps even the manufacturer anticipated. As a result, the

manufacturers have taken more care in the design of the aircraft, to improve the

corrosion-resistance of aircraft. This improvement includes the use of new materials and

improved surface treatments and protective finishes. The use of preventative

maintenance has also been emphasised more than previously.

Preventative maintenance, relative to corrosion control, should include the:

Adequate and regular cleaning of the aircraft

Periodic lubrication (often after the cleaning) of moving parts

Regular and detailed inspection for corrosion and failure of protective treatments

Prompt treatment of corrosion and touch-up of damaged paint

Keeping of drain holes clear

Draining of fuel cell sumps

Daily wiping down of most critical areas

Sealing of aircraft during foul weather and ventilation on sunny days

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Use of protective covers and blanks.

7.3.3 Corrosion Removal General treatments for corrosion removal include:

Cleaning and stripping of the protective coating in the corroded area.

Removal of as much of the corrosion products as possible.

Neutralisation of the remaining residue.

Checking if damage is within limits

Restoration of protective surface films

Application of temporary or permanent coatings or paint finishes.

7.3.4 Cleaning and Paint Removal

It is essential that the complete suspect area be cleaned of all grease, dirt or

preservatives. This will aid in determining the extent of corrosive spread. The selection

of cleaning materials will depend on the type of matter to be removed.

Solvents such as trichloroethylene (example trade name 'Genklene') may be used

for oil, grease or soft compounds, while heavy-duty removal of thick or dried

compounds may need solvent/emulsion-type cleaners.

General purpose, water-removable stripper is recommended for most paint

stripping. Adequate ventilation should be provided and synthetic rubber surfaces such

as tyres, fabric and acrylics should be protected (remover will also soften sealants).

Rubber gloves, acid-repellent aprons and goggles, should be worn by personnel

involved with paint removal operations. The following is the general paint stripping

procedure:

Brush the area with stripper, to a depth of approximately 0.8 mm - 1.6 mm (0.03

in - 0.06 in). Ensure that the brush is only used for paint stripping.

Allow stripper to remain on the surface long enough for the paint to wrinkle.

This may take from 10 minutes to several hours.

Re-apply the stripper to those areas which have not stripped. Non-metallic

scrapers may be used.

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Remove the loosened paint and residual stripper by washing and scrubbing the

surface with water and a broom or brush. Water spray may assist, or the use of

steam cleaning equipment may be necessary.

Note: Strippers can damage composite resins and plastics, so every effort

should be made to 'mask' these vulnerable areas.

7.3.5 Ferrous Metals Atmospheric oxidation of iron or steel surfaces causes ferrous oxide rust to be

deposited. Some metal oxides protect the underlying base metal, but rust promotes

additional attack by attracting moisture and must be removed.

Rust shows on bolt heads, nuts or any un-protected hardware. Its presence is not

immediately dangerous, but it will indicate a need for maintenance and will suggest

possible further corrosive attack on more critical areas. The most practical means of

controlling the corrosion of steel is the complete removal of corrosion products by

mechanical means.

Abrasive papers, power buffers, wire brushes and steel wool are all acceptable

methods of removing rust on lightly stressed areas. Residual rust usually remains in pits

and crevices. Some (dilute) phosphoric acid solutions may be used to neutralise

oxidation and to convert active rust to phosphates, but they are not particularly effective

on installed components.

7.3.6 High-stressed Steel Components Corrosion on these components may be dangerous and should be removed

carefully with mild abrasive papers or fine buffing compounds. Care should be taken

not to overheat parts during corrosion removal. Protective finishes should be re-applied

immediately.

7.3.7 Aluminium and Aluminium Alloys

Corrosion attack, on aluminium surfaces, gives obvious indications, since the

products are white and voluminous. Even in its early stages, aluminium corrosion is

evident as general etching, pitting or roughness.

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Aluminium alloys form a smooth surface oxidation, which provides a hard shell,

that, in turn, may form a barrier to corrosive elements. This must not be confused with

the more serious forms of corrosion.

General surface attack penetrates slowly, but is speeded up in the presence of

dissolved salts. Considerable attack can take place before serious loss of strength

occurs. Three forms of attack, which are particularly serious, are:

Penetrating pit-type corrosion through the walls of tubing.

Stress corrosion cracking under sustained stress.

Intergranular attack characteristic of certain improperly heat treated alloys.

Treatment involves mechanical or chemical removal of as much of the corrosion

products as possible and the inhibition of residual materials by chemical means. This,

again, should be followed by restoration of permanent surface coatings.

7.3.8 Alclad Obviously great care must be taken, not to remove too much of the protective

aluminium layer by mechanical methods, as the core alloy metal may be exposed,

therefore, where heavy corrosion is found, on clad aluminium alloys, it must be

removed by chemical methods wherever possible.

Corrosion-free areas must be masked off and the appropriate remover (usually a

phosphoricacid based fluid) applied, normally with the use of a stiff bristled brush, to

the corroded surface, until all corrosion products have been removed.

Figure 7-5 Caustic soda test and its neutralisation

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Copious amounts of clean water should, next, be used to flood the area and

remove all traces of the acid, then the surface should be dried thoroughly.

Note: A method of checking that the protective aluminium coating remains

intact is by the application of one drop of diluted caustic soda to the cleaned area. If the

Alclad has been removed, the aluminium alloy core will show as a black stain, whereas,

if the cladding is intact, the caustic soda will cause a white stain.

Since the caustic soda is corrosive, and will promote corrosion, it must be

neutralised with a solution of water and chromic anhydride.

The surface must be protected by applying a chromate conversion coating (such

as Alocrom 1200 or similar) to the surface.

Further surface protection may be given by a coat of suitable primer, followed

by the approved top coat of paint.

7.3.9 Magnesium Alloys The corrosion products are removed from magnesium alloys by the use of

chromic/sulphuric acid solutions (not the phosphoric acid types), brushed well into the

affected areas. Clean, cold water is employed to flush the solution away and the dried

area can, again, be protected, by the use of Alocrom 1200 or a similar, approved,

compound.

7.3.10 Acid Spillage

An acid spillage, on aircraft components, can cause severe damage. Acids will

corrode most metals used in the construction of aircraft. They will also destroy wood

and most other fabrics. Correct Health and Safety procedures must be followed when

working with such spillages.

Aircraft batteries, of the lead/acid type, give off acidic fumes and battery bays

should be well ventilated, while surfaces in the area should be treated with anti-acid

paint. Vigilance is required of everyone working in the vicinity of batteries, to detect (as

early as possible) the signs of acid spillage. The correct procedure to be taken, in the

event of an acid spillage, is as follows:

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Mop up as much of the spilled acid using wet rags or paper wipes. Try not to

spread the acid.

If possible, flood the area with large quantities of clean water, taking care that

electrical equipment is suitably protected from the water.

If flooding is not practical, neutralise the area with a 10% (by weight) solution

of bicarbonate of soda (sodium bicarbonate) with water.

Wash the area using this mixture and rinse with cold water.

Test the area, using universal indicating paper (or litmus paper),to check if acid

has been cleaned up.

Dry the area completely and examine the area for signs of damaged paint or

plated finish and signs of corrosion, especially where the paint may have been

damaged.

Remove corrosion, repair damage and restore surface protection as appropriate.

7.3.11 Alkali Spillage This is most likely to occur from the alternative Nickel-Cadmium (Ni-Cd) or

Nickel-Iron (Ni-Fe) type of batteries, containing an electrolyte of Potassium Hydroxide

(or Potassium Hydrate). The compartments of these batteries should also be painted

with anti-corrosive paint and adequate ventilation is as important as with the lead/acid

type of batteries. Proper Health and Safety procedures are, again, imperative.

Removal of the alkali spillage, and subsequent protective treatment, follows the

same basic steps as outlined in acid spillage, with the exception that the alkali is

neutralised with a solution of 5% (by weight) of chromic acid crystals in water.

7.3.12 Mercury Spillage

WARNING: MERCURY (AND ITS VAPOUR) IS EXTREMELY

TOXIC. INSTANCES OF MERCURY POISONING MUST, BY

LAW, BE REPORTED TO THE HEALTH AND SAFETY

EXECUTIVE. ALL SAFETY PRECAUTIONS RELATING TO

THE SAFE HANDLING OF MERCURY MUST BE STRICTLY

FOLLOWED.

Mercury contamination is far more serious than any of the battery spillages and

prompt action is required to ensure the integrity of the aircraft structure.

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While contamination from mercury is extremely rare on passenger aircraft,

sources of mercury spillage result from the breakage of (or leakage from) containers,

instruments, switches and certain test equipment. The spilled mercury can, quickly,

separate into small globules, which have the capability of flowing (hence its name

'Quick Silver') into the tiniest of crevices, to create damage.

Mercury can rapidly attack bare light alloys (it forms an amalgam with metals),

causing intergranular penetration and embrittlement which can start cracks and

accelerate powder propagation, resulting in a potentially catastrophic weakening of the

aircraft structure.

Signs of mercury attack on aluminium alloys are greyish powder, whiskery

growths, or fuzzy deposits. If mercury corrosion is found, or suspected, then it must be

assumed that intergranular penetration has occurred and the structural strength is

impaired. The metal in that area should be removed and the area repaired in accordance

with manufacturer's instructions.

Ensure that toxic vapour precautions are observed at all times during the

following operation:

Do not move aircraft after finding spillage. This may prevent spreading.

Remove spillage carefully by one of the following mechanical methods:

Capillary brush method (using nickel-plated carbon fibre brushes).

Heavy-duty vacuum with collector trap.

Adhesive tape, pressed (carefully) onto globules may pick them up

Foam collector pads (also pressed, carefully, onto globules).

Alternative, chemical methods, of mercury recovery entail the use of:

Calcium polysulphide paste.

Brushes, made from bare strands of fine copper wire

Neutralise the spillage area, using 'Flowers of Sulphur'.

Try to remove evidence of corrosion.

The area should be further checked, using radiography, to establish that all

globules have been removed and to check extent of corrosion damage.

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Examine area for corrosion using a magnifier. Any parts found contaminated

should be removed and replaced.

Note: Twist drills (which may be used to separate riveted panels, in an attempt

to clean contaminated surfaces) must be discarded after use.

Further, periodic checks, using radiography, will be necessary on any airframe

that has suffered mercury contamination.

7.3.13 Anti-Corrosion Treatments These are intended to remain intact throughout the life of the component, as

distinct from coatings, which may be renewed as a routine servicing operation. They

give better adhesion for paint and most resist corrosive attack better than the metal to

which they are applied

7.3.14 Electroplating There are two categories of electroplating, which consist of:

Figure 7-6 An example of the electroplating process. A spoon being silver plated

Coatings less noble than the basic metal. Here the coating is anodic and so, if

base metal is exposed, the coating will corrode in preference to the base metal.

Commonly called sacrificial protection, an example is found in the cadmium (or

zinc) plating of steel.

Coatings more noble (e.g. nickel or chromium on steel) than the base metal. The

nobler metals do not corrode easily in air or water and are resistant to acid

attack. If, however, the basic metal is exposed, it will corrode locally through

electrolytic action. The attack may result in pitting corrosion of the base metal or

the corrosion may spread beneath the coating.

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7.3.15 Spread Metal Coatings Most metal coatings can be applied by spraying, but only aluminium and zinc

are used on aircraft. Aluminium, sprayed on steel, is frequently used for

hightemperature areas. The process (aluminising), produces a film about 0.1 mm (0.004

in) thick, which prevents oxidation of the underlying metal.

Figure 7-7 Aluminium spray coating of a steel part

7.3.16 Cladding The hot rolling of pure aluminium onto aluminium alloy (Alclad) has already

been discussed, as has the problem associated with the cladding becoming damaged,

exposing the core, and the resulting corrosion of the core alloy

Figure 7-8 The cladding process

7.3.17 Surface Conversion Coatings These are produced by chemical action. The treatment changes the immediate

surface layer into a film of metal oxide, which has better corrosion resistance than the

metal.

Among those widely used on aircraft are:

Anodising of aluminium alloys, by an electrolytic process, which thickens the

natural, oxide film on the aluminium. The film is hard and inert.

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Chromating of magnesium alloys, to produce a brown to black surface film of

chromates, which form a protective layer. Some trade names include Alocrom

100, Alocrom 1200 (dip), Walterisation 'L' Process, Aloclene 300, Bonderite

710, Tridure 'AL' Process, Alchromate process, Kenvert 40.

Passivation of zinc and cadmium by immersion in a chromate solution.

Other surface conversion coatings are produced for special purposes, notably the

phosphating of steel. There are numerous proprietary processes, each known by

its trade name (e.g. Bonderising, Parkerising, or Walterising).

Figure 7-9 The surface conversion process

7.4 General Repair Method

7.4.1 Structure Classification Owing to the difficulty of formulating repair instructions for members or parts

of similar size designed to take different loads, the airframe structure has been divided

into three classifications:-

a) Primary Structure. These parts of the airframe are highly stressed and,

if damaged, may cause failure of the aircraft and loss of life of the

aircrew, e.g. spars, longerons, engine mounting, stressed skin, etc. They

are sometimes shown in RED (or white) in repair manuals and drawings.

b) Secondary Structure. These parts of the airframe are highly stressed

but, if damaged, will not

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cause failure of the aircraft or loss of life of the aircrew, e.g. flooring,

which is normally stronger than is necessary and if damaged locally

would not collapse. This classification also includes ancillary frames

designed to support components such as oxygen bottles, cameras, etc.

They are sometimes shown as YELLOW (or hatched) in repair manuals

and drawings.

c) Tertiary Structure. These are lightly stressed parts such as fairing,

wheel shields, minor component brackets, etc. They are often shown in

GREEN (or stippled) in repair manuals or drawings.

Figure 7-10 Primary, secondary and tertiary structure identifications

7.4.2 Identification of Structure The structure diagrams are coloured or specially shaded to represent the various

classifications. Primary structures are coloured red or shown white. Secondary

structures are coloured yellow or shown hatched. Tertiary structures are coloured green

or shown stippled. If there is any doubt whatsoever, as to whether a piece of structure is

primary or secondary, it is safest to assume it is primary.

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Figure 7-11 Repair to Z-section flanges

7.4.3 Application An example of the use of structure classification is that of repairing damaged Z-

section flanges. The Z-sections may be of similar size, but of different gauge material,

to suit the loads they are designed to bear, A flange classified as a tertiary structure

would be repaired, if damaged, by fitting a strap to overlap the damage by three rivets

either side at 4D pitch and 2D land, a secondary structure by four rivets either side at

4D pitch and 2D land, and a primary structure by five rivets either side at 4D pitch and

2D land. Thus, the repair to the primary structure would be longer in length and stronger

than the repair to a tertiary structure. The gauge of material of the repair patch and the

size and type of rivets to use in the repair is given on the repair instruction sheet, as

illustrated.

Another example of the use of structure classification is that of repairing holes in

sheet metal parts. The rivet pitch for the repair patch is modified in accordance with the

importance of the damaged structure. Rivet pitch for a tertiary structure may be 8D, a

secondary structure 6D, and primary structure 4D.

7.4.4 Repair Materials Repairs must conform to the original strength and shape of the structure,

therefore, the specification and gauge of the repair material, rivet size, pitch and type,

etc., must be similar to the adjacent structure some repair instructions stipulate thicker

gauge material and larger size rivets for the repair.

7.4.5 Support of Structure The support of the structure during repair, especially if it is a primary structure,

is very important; it is essential that the structure be suitably supported and braced

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before any member or part of a member is removed. Adequate support can be obtained

by the use of trestles, wedges, Gclamps and lengths of timber, so that they take the loads

which would normally come on the part to be removed. This prevents distortion of the

structure during repair or renewal of parts, and is known as "jury rigging

Figure 7-12 Jury Rigging

7.4.6 Damage Classification Damage, irrespective of the type of structure in which it occurs, is divided into

four classifications. These damage classifications are fully described in the Vol. 6 of the

aircraft handbook, and are as follows

a) Negligible damage.

b) Damage repairable by patching.

c) Damage repairable by insertion.

d) Damage needing renewal of parts

7.4.7 Repair Procedure Before starting to repair the structure, investigate the full extent of the damage.

Damage visible the point of impact is termed Primary damage, and can be easily seen,

but unseen damage, termed Secondary damage, may be present but remote from the

point of impact. Secondary damage, which may be more serious than the primary

damage, may occur a considerable distance inside the structure. It is caused by the

transmission of force from one member to another resulting in cracks, buckled plates,

drawn rivet or bolt holes, ply separation or collapse of glued joints. Secondary damage

may also occur due to fatigue failure following weakening of the structure by the

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primary damage. After inspection of the damage, the following repair procedure should

be adopted

a) Cleaning Up. With the exception of flat depressions which are classified

as negligible, all damage should be cleaned up before it is classified.

Cleaning up must be carefully done to ensure that material is not

unnecessarily removed, otherwise a complex repair in place of a simple

repair may be necessary.

b) Classification of Damage. After cleaning up, classify the damage into

negligibssle, repairable, or necessitating repair by renewal; when

classifying the damage also consider corrosion of the part or area. If the

damage is repairable, consult the Repair Manual to ascertain whether

there are any special considerations applicable to the repair.

c) Checking the Repair. The structure must be adequately supported. As

each stage of the repair is completed, check the work against the relevant

repair instructions. Ensure structure classification is correct. Patch

material and thickness, rivet type, diameter and pitch, correct dimensions

and positioning of reinforcing members, etc., should be compared with

the repair drawings and instructions. The alignment of the structure

should be frequently checked with trammels, plumb lines, clinometer,

etc., during the process of the repair, and any distortion eliminated.

d) Completion of Repair. Drainage holes must not be obstructed by repair

material. If this has happened the drainage hole should be continued

through the addition; if this is not permissible a new drainage hole should

be cut as near as possible to the original position. Bonding strips that

have been disconnected must be replaced; if broken, a new length of strip

should be soldered to the existing end, or the complete strip renewed.

Control surfaces that have been repaired must be checked for mass

balance. Protective treatment in the repair area that has been damaged

must be renewed.

7.4.8 Bowing Limits The maximum amount of bow in members that can be considered negligible is 1

in 600, or, as stated in the Repair Manual. When testing a member for bow, the test

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must be done on that part of the member in which the section is uniform. The following

methods may be used

Straightedge and Feeler Gauge. This method may only be used on members

that are free from fittings over the major portion of their length. Place the straightedge

on the member parallel with its axis and measure the distance between the member and

the straightedge with feeler gauges at the point of maximum bow, then calculate the

amount of bow from the formula,

Bow = 𝐃𝐃𝐃𝐃𝐃𝐃𝐃𝐃𝐃𝐃𝐃𝐃𝐃𝐃𝐃𝐃 𝐛𝐛𝐃𝐃𝐃𝐃𝐛𝐛𝐃𝐃𝐃𝐃𝐃𝐃 𝐦𝐦𝐃𝐃𝐦𝐦𝐛𝐛𝐃𝐃𝐦𝐦𝐃𝐃 𝐃𝐃𝐃𝐃𝐚𝐚 𝐃𝐃𝐃𝐃𝐦𝐦𝐃𝐃𝐃𝐃𝐬𝐬𝐬𝐬𝐃𝐃𝐃𝐃𝐚𝐚𝐬𝐬𝐃𝐃

𝐋𝐋𝐃𝐃𝐃𝐃𝐬𝐬𝐃𝐃𝐬𝐬 𝐨𝐨𝐨𝐨 𝐦𝐦𝐃𝐃𝐦𝐦𝐛𝐛𝐃𝐃𝐦𝐦 𝐮𝐮𝐃𝐃𝐚𝐚𝐃𝐃𝐦𝐦 𝐃𝐃𝐃𝐃𝐃𝐃𝐃𝐃

Bow Distance between member and straightedge Length of member under test

For example, if the length of straightedge used was 2 ft. long, and the maximum gap

was 0.080 in., the bow would be excessive because

Bow = 𝟎𝟎.𝟎𝟎𝟎𝟎𝟎𝟎𝐃𝐃𝐃𝐃𝟐𝟐𝟐𝟐.𝟎𝟎𝐃𝐃𝐃𝐃

= 𝟎𝟎

𝟐𝟐,𝟐𝟐𝟎𝟎𝟎𝟎 =

𝟏𝟏𝟑𝟑𝟎𝟎𝟎𝟎

= or 1 in 300

Three-point Trammel. If a member to be tested has protruding fittings, a three-

point trammel with adjustable points can be used. These can be set in line on a marking-

out table and the bow in the member measured with feelers between the member and the

centre point. Alternatively, the centre trammel point can be adjusted to touch the

member and the amount of bow measured on the marking-out table; this latter method

reproduces twice the errors (see illustration).

Figure 7-13 Testing for bow

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7.4.9 Dents Dents in tubes can be measured by comparing the diameter of the tube at the

dent with the overall diameter of the tube. Another method is to set a pair of outside

callipers to the outside diameter of the tube, at a point where the tube is not dented,

then, with the callipers set to this outside diameter, transfer the callipers to the dent

placing one leg of the callipers into the dent. Using feeler gauges, measure the distance

between the other point of the callipers and the tube, thus obtaining a direct reading of

the dent.

Figure 7-14 Measuring a dent

7.4.10 Abrupt Changes of Section In ali repair work, particularly on primary structures, abrupt changes of section

must be avoided as they may cause fatigue failure. For an acute bend in sheet metal,

always use correctly radiused bend bars. Punctures and irregularly shaped holes should

be cleaned up with the corners radiused as large as possible. Repair sleeves used for

joining tubes must have chamfered ends; some repair schemes stipulate that the edges of

repair patches are to be chamfered.

7.4.11 Repair of Control Surfaces To prevent vibration (flutter) of a control surface, within the speed range of an

aircraft, a weight, termed a mass balance weight, is fitted to the control surface in front

of the hinge line. When repairing a control surface, the additional repair material will

increase the weight of the control surface on one side of the hinge-line, therefore,

alteration to the balance weight must be made in order to maintain the effective mass

balance. Mass balance weights usually consist of lead or steel washers, bolted inside or

outside the leading edge of the control surface

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Figure 7-15 Mass balance calculating weight addition

The methods of correcting mass balance of control surfaces vary with different

types aircraft. One method is to check the weight of the addition repair material (W) as

the distance the centre of the repair is to the centre line of the control surface hinges (0),

and calculate the weight addition in relation to the distance of the mass balance (M).

(M) from the centre line of the control surface hinges(d). The weight which must be

added to the mass balance is calculated from the formula:

Weight Addition = 𝐖𝐖 𝐱𝐱 𝐃𝐃𝐚𝐚

In the example illustrated

Weight Addition = 𝟐𝟐 𝐱𝐱 𝟏𝟏𝟐𝟐𝟎𝟎

= 3 ozs

Repairs to the control surface, forward of the hinge line, would entail subtraction

of the weight addition from the balance weight, the alteration to the balance weight

must be within the limits stated in the Repair Manual. If the limits are exceeded, the

control surface must be renewed. Alteration to mass balance should be recorded on the

control surface modification plate.

7.4.12 Negligible Damage Slight damage to the airframe may be classified negligible and need not be

repaired, though whenever there is danger of the damage extending it must be treated.

The repair of all damage is governed by the structure classification. For instance, a

crack considered negligible in a tertiary structure, which only requires drilled holes at

either end of the crack to prevent it spreading, may necessitate repair by patching,

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insertion, or renewal of the part if present in a primary structure. Examples of negligible

damage are described and illustrated in the following paragraphs.

7.4.13 Other Damage 7.4.13.1 Dents

If possible, before assessing a large dent an attempt should be made to restore

the member to shape, care being taken to avoid cracking the member. The dent must be

free from cracks and abrasions, and is classified by depth and maximum diameter, e.g.,

a negligible dent in stressed skin may not exceed one gauge of the skin in depth and

0.75 in. maximum diameter. For two gauge depth, the maximum diameter is halved

(0.375 in.). Dents greater in depth or diameter would be classified as holes, and repaired

accordingly.

Figure 7-16 A negligible dent flanges

7.4.13.2 Nicks

The material on each side of a nick in the free edge of a member should be

removed in order to provide a gradual change of section. The maximum allowable depth

of nicks in the free flange of a Z-section may be 1/8 in., and not less than 5 in. apart.

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Figure 7-17 Nick in free flange of a Z-section

7.4.13.3 Cracks

With the exception of cracks in tubes and machined fittings, holes should be

drilled at the end of all cracks to prevent them from spreading. The maximum length of

a negligible crack may be restricted to the diameter of a negligible hole in that member.

Cracks in tubes and machined fittings are never classed as negligible; machined fittings

must be renewed, and tubes must be repaired or renewed.

Figure 7-18 Treatment of cracks

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7.4.13.4 Holes

Before assessment of any holes in the form of punctures in stressed skin, the

damage should be cleaned up, preferably by drilling, and all burrs removed with fine

emery cloth. The maximum diameter of negligible holes in the stressed skin may be 3/8

in., and not less than 5 in. apart.

Figure 7-19 Treatment of a puncture hole

7.4.13.5 Scores or Scratches

Before assessing the depth of a score or scratch, the material on either side of the

defect should be smoothed out to a gradual curvature. The scores or scratches are then

disregarded provided that they are not deeper than one-tenth of the gauge of the plate,

nor deeper than 0.010 in. in a machined fitting.

7.4.13.6 Damage to Tube

No damage in the middle third of the tube length can be classified negligible. In

the remaining two thirds the depth of dents in steel tubes must not exceed one fiftieth of

the outside diameter of the tube. Depth of scores and abrasions must not exceed one

eighth of the tube wall thickness. A score or abrasion must not exceed in length the

outside diameter of the tube, and the extent of the damage round the tube must not

exceed one third of the diameter.

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Figure 7-20 Damage to steel tubes

7.4.14 Repair Types 7.4.14.1 Damage repairable by Patching

Any damage greater than negligible should be repaired by patching, provided

that the extent of the damage does not exceed that specified in the Repair Manual Care

must be taken to ensure that the repair materials conform with the structure

classification. A structure which is difficult to assess, for example, whether it is a

secondary or primary structure, should be upgraded (primary). Examples of repair by

patching are described and illustrated in the following paragraphs.

7.4.14.2 Sheet Metal

The repair instructions for small cracks, punctures and deep scores usually

stipulate the following:

a) The repair patch must be of the same gauge and material as the existing

metal sheet. Unless otherwise stated, a repair patch must not be fitted on

the outside of the structure, e.g., on the outside of the stressed skin of

high speed aircraft a flush finish must be maintained.

b) Rivets used to secure the patch should be the same pitch, type, size and

material as those used in the nearest edge of the panel; if countersunk,

the same type of countersunk must be made, e.g., dimpled or cut

countersunk.

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c) The distance of the rivet centre from the edge of the repair patch (rivet

land, or landing), must be at least twice the diameter of the rivet.

d) All overlapping (faying) surfaces should be coated with jointing

compound, and the repair assembled and riveted while the jointing

compound is wet. Jointing compound helps to prevent corrosion and

ensures a weatherproof joint.

Procedure The repair illustrated consists of cleaning out a puncture to regular

shape, and preparing a patch to overlap the damage by 7 D.

Figure 7-21 A repair patch

7.5 NDT Technics

Nondestructive testing or Non-destructive testing (NDT) is a wide group of

analysis techniques used in science and industry to evaluate the properties of a material,

component or system without causing damage. The terms Nondestructive examination

(NDE), Nondestructive inspection (NDI), and Nondestructive evaluation (NDE) are

also commonly used to describe this technology. Because NDT does not permanently

alter the article being inspected, it is a highly valuable technique that can save both

money and time in product evaluation, troubleshooting, and research.

Common NDT methods include ultrasonic, magnetic-particle, liquid

penetrant, radiographic, remote visual inspection (RVI), eddy-current testing, and low

coherence interferometry. NDT is a commonly used in forensic engineering, mechanical

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engineering, electrical engineering, civil engineering, systems engineering, aeronautical

engineering, medicine, and art.

7.5.1 Applications 7.5.1.1 Weld Verification

In manufacturing, welds are commonly used to join two or more metal parts.

Because these connections may encounter loads and fatigue during product lifetime,

there is a chance that they may fail if not created to proper specification. For example,

the base metal must reach a certain temperature during the welding process, must cool

at a specific rate, and must be welded with compatible materials or the joint may not be

strong enough to hold the parts together, or cracks may form in the weld causing it to

fail. The typical welding defects (lack of fusion of the weld to the base metal, cracks or

porosity inside the weld, and variations in weld density) could cause a structure to break

or a pipeline to rupture.

Welds may be tested using NDT techniques such as industrial radiography

or industrial CT scanning using X-rays or gamma rays, ultrasonic testing, liquid

penetrant testing magnetic particle inspection or via eddy current. In a proper weld,

these tests would indicate a lack of cracks in the radiograph, show clear passage of

sound through the weld and back, or indicate a clear surface without penetrant captured

in cracks.

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Figure 7-22 A Liquid pnetrant testing

1. Section of material with a surface-breaking crack that is not visible to the naked eye. 2. Penetrant is applied to the surface. 3. Excess penetrant is removed. 4. Developer is applied, rendering the crack visible.

Welding techniques may also be actively monitored with acoustic emission

techniques before production to design the best set of parameters to use to properly join

two materials. In the case of high stress or safety critical welds, weld monitoring will be

employed to confirm the specified welding parameters (arc current,arc voltage, travel

speed, heat input etc) are being adhered to those stated in the welding procedure. This

verifies the weld as correct to procedure prior to nondestructive evaluation and

metalurgy tests.

7.5.1.2 Structural Mechanics

Structures can be complex systems that undergo different loads during their

lifetime. Some complex structures, such as the turbomachinery in a liquid-fuel rocket,

can also cost millions of dollars. Engineers will commonly model these structures as

coupled second-order systems, approximating dynamic structure components

with springs, masses, and dampers. These sets of differential equations can be used to

derive a transfer function that models the behavior of the system.

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In NDT, the structure undergoes a dynamic input, such as the tap of a hammer or

a controlled impulse. Key properties, such as displacement or acceleration at different

points of the structure, are measured as the corresponding output. This output is

recorded and compared to the corresponding output given by the transfer function and

the known input. Differences may indicate an inappropriate model (which may alert

engineers to unpredicted instabilities or performance outside of tolerances), failed

components, or an inadequate control system.

7.6 Disassembly and Re-assembly Technics

This unit is an example of the Aeroskills Structures Maintenance pathway. It covers the competencies required to disassemble and reassemble aircraft structure for the purpose of major repair or modification.

Table 7-1 Procedure of structural assy - desassy

Element Performance Criteria 1 . Interpret specifications and

organise materials 1. Specifications and

drawings or repair scheme documentation are interpreted to determine component and material requirements.

2. Procedure for assembly/disassembly of structure is determined in order to plan equipment use.

3. Appropriate jigs, fixtures or bracing requirements are determined to ensure maintenance of contour/structural integrity during disassembly/assembly operations.

4. All components and equipment are organised.

2 . Prepare aircraft or sub-assembly for structural disassembly

1. Structure is supported and prepared with appropriate jigs, fixtures or bracing as required.

2. Structural component removal is undertaken to

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provide access as required.

3 . Disassemble aircraft structure or sub-assembly

1. Aircraft standard practices are applied to the removal of structural hardware and fasteners.

2. Disassembled components are tagged to facilitate correct reassembly as required.

4 . Prepare components and tooling for assembly

1. Jigs and fixtures are set up to ensure accuracy of component assembly.

2. Component alignment is checked for conformance to specifications prior to fastener hole generation.

3. Hole location/relocation is carried out in accordance with specification procedures and standard practices.

4. Standard practices are followed in hole generation sequencing to ensure that assembly stress defects are not built in.

5. Components are disassembled, cleaned, deburred and surface treatments are applied prior to final assembly.

5 . Assemble aircraft structure or sub- assembly

1. Sealants and/or adhesives are selected and applied in accordance with assembly specifications or appropriate documentation.

2. Components are positioned and secured with appropriate temporary fastening devices for accurate assembly.

3. Fasteners are selected and installed in accordance with assembly

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specifications or appropriate manuals.

6 . Inspect completed assemblies 1. Assembled components are inspected to confirm dimensional accuracy and specifications are met.

2. Checking or testing equipment is used where appropriate to ensure requirements are met.

3. Required documentation is completed and processed in accordance with standard enterprise procedures.

Evidence of knowledge of aircraft construction principles, application of

applicable OH&S regulations, and use of approved maintenance documentation and

aircraft publications relating to aircraft structure is required. Specific evidence of

knowledge and skills for this unit is demonstrated by:

• Correctly interpreting repair scheme/modification drawings (including third

angle projection, isometric, sectional formats) and procedural

instructions/specifications.

• Correctly supporting the aircraft structure by jacking, trestling, bracing and/or

jigging methods.

• Describing the basic construction methods used to assembly:

o fuselage (pressure and non-pressurised)

o wings, vertical and horizontal stabilisers, rotary wing tail cones and

pylons

o engine nacelles/pylons

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o doors and windows, including seals, sealants and locking mechanisms.

• Identifying various aircraft metals/composite materials and their basic

metallurgy properties by interpretation of markings, numbering systems or

visual, chemical or mechanical means.

• Handling and storing aircraft metals and composite materials to industry

standards.

• Identifying aircraft structural assembly fasteners (metal and composite) by

interpretation of markings, numbering systems, size, shape and colour.

• Using appropriate hand tools and machines to remove and assemble aircraft

structural components, parts, sections and skin including riveting equipment,

drilling equipment, aligning tools, reamers and material fasteners (grip pins).

• Applying correct removal and installation techniques for general and close

tolerance fasteners (rivets, standard and oversize - hilocks) including hole

preparation and location techniques.

• Performing aircraft alignment and measurement checks.

• Restoring aircraft structure surface finishes.

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BAB VIII AIRCRAFT HANDLING AND STORAGE

If an aircraft requires moving and no pilot is available, then a tug and towing

arm must be used. This task will require a qualified tug driver, a supervisor, a 'brake

man' and other personnel to keep a lookout. A qualified pilot always does the taxiing of

larger aircraft, although engineers sometimes taxi light aircraft.

Each aircraft and its operator will have laid down rules regarding the way in

which each aircraft will be towed. These rules will include the number of people

needed, the type of tug, the radio calls if the aircraft is on the manoeuvring area, the

maximum towing speed and many other details. These must always be followed if

accidents are to be avoided.

8.1 Aircraft Towing and Taxiing

Aircraft, when moving, either under power or whilst being towed, are sources of

numerous risk areas. An airliner can be over 60 metres long and have a wing span

greater than 60 metres. This means that when manoeuvring in restricted spaces, there is

always the risk of part of the aircraft striking another object, due to a phenomenon

known as 'Swept Wing Growth' (refer to Figure 8-1).

Figure 8-1 Swept wing growth

It must be borne in mind that, when turning, the wing tips and tail of a large

aircraft can move considerable distances in the opposite direction to that of the nose.

This is why, whenever an aircraft is approaching its parking spot, there must be

personnel available to watch out for any potential conflicts.

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Driving in the vicinity of a parked aircraft must always be done with care, especially if

the driver is alone or visibility from the cab of the vehicle is limited.

8.1.1 Aircraft Taxiing Aeroplanes and helicopters are designed to fly, and movement on the ground is

often a rather awkward procedure. Because of this, only qualified persons authorized to

taxi aircraft may actually taxi an aircraft. Before starting an engine, be sure that the

areas in front and behind the aircraft are clear of people and equipment. A maintenance

technician should be checked out by a properly qualified instructor before taxiing a new

or different aircraft.

From the cockpit, it is difficult to assure that there is sufficient clearance

between the aircraft structure and any buildings or other aircraft. Therefore, it is a good

policy to station signalmen where they can watch the wings or rotor and any

obstructions. When this is done, it is important that all personnel use the same signals

and understand exactly what the signals mean to avoid misunderstanding at a crucial

time. The signalman has the responsibility of remaining in a position that is visible from

the cockpit. To ensure that you can be seen at all times, make sure that you can see the

pilots eyes while directing him.

When taxiing an aircraft at a tower-controlled airport, you typically must receive

a clearance from ground control before you begin taxiing. Once the aircraft is in motion,

immediately tap the brakes to insure they are working properly. After testing the brakes,

test the nose gear steering system to make sure it is operating.

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Figure 8-2 Standard hand signals allow ground personnel to direct the movement of aircraft.

Figure 8-3 Standard hand signals allow ground personnel to direct the movement of aircraft.

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8.1.2 Tailwheel Aircraft Tailwheel aircraft present certain difficulties during taxi because the tail is low

leaving the operators view over the nose obstructed. Because of this, an operator must

alternately turn the nose from side to side in a series of S-turns during taxi to avoid

objects or hazards in the aircrafts path.

The abrupt use of brakes should be avoided on a tailwheel aircraft since a hard

brake application could cause the aircraft to nose over. Furthermore, tailwheel aircraft

are difficult to taxi in windy conditions, especially crosswinds. The reason for this is

that tailwheel aircraft are designed much like a weathervane, with the pivot point at the

main landing gear. The vertical stabilizer on the tail and the fuselage behind the main

gear present a large surface to a crosswind.

Since there is so much more surface area behind the main gear than in front of it,

a crosswind creates a powerful turning force that pushes the aircrafts nose into the wind.

To avoid losing control of the aircraft, you must exercise extreme caution when taxiing

a tailwheel aircraft in crosswind conditions.

8.1.3 Light Signals Busy airports usually require radio contact between an aircraft and the control

tower when the aircraft moves on taxiways or runways. In the event that you must taxi

an aircraft that does not have a radio or in the event of radio failure, control towers are

equipped with highly directional light guns they can use to signal you.

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Table 8-1 Located in the cab of the control tower is a powerful light that controllers can use to direct light beams of various colors toward you aircraft. Each color or color combination has a specific meaning for an aircraft on the airport surface.

8.1.4 Aircraft Towing It is often necessary to move an aircraft without using its engines. This can be

accomplished by towing the aircraft. Large aircraft are towed with a tractor. or special

towing vehicle, and are connected to the vehicle with a special tow bar. Extreme care

must be used to avoid towing an aircraft too fast and to be sure that there is always

sufficient clearance between the wings and any obstructions.

When an aircraft is being towed. a qualified person should be in the cockpit to

operate the aircraft brakes when needed since the brakes on a towing vehicle are usually

insufficient to overcome a large aircrafts momentum. Extra personnel should be

assigned to watch the wing tips and tail for clearance between other objects.

The nose gear on most aircraft have a very definite limit to the amount it can be

turned and, when towing, it is easy to exceed these limits. If the turning radius is

exceeded, the nose gear strut and steering mechanism will be damaged. Damage can be

quite extensive, requiring replacement of the nose gear shock strut. Some aircraft have a

method of disconnecting a locking device so the nose wheel can be swiveled to

facilitate maneuvering. If this is the case, the locking device must always be

disconnected when an aircraft is towed. Furthermore, remember to reset the lock after

removing the tow-bar from the aircraft. Persons riding in the aircraft should not attempt

to steer the nosewheel when a towbar is attached to the aircraft.

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Figure 8-4 Nosewheel towing attachment

Although small aircraft can be moved by hand, substantial damage can result

from careless or improper handling procedures. For example, you should never move an

aeroplane by pulling on its propeller. The propeller is designed to move the aircraft

through the air, but the thrust it produces is uniform. Moving the aeroplane by pulling

on one blade puts an asymmetrical load on both the propeller and the engine.

When towing an aircraft, you should always use a tow bar. Most tow bars attach

to the nose wheel and are used both to move and steer the aeroplane. After an aircraft

has been towed with a tow bar and parked in the desired position, remove the tow bar

from the nose strut and place it beside the nose-wheel, or stow it away. If an engine is

started with a tow bar still attached to a nosegear, the tow bar, propeller. and aircraft

will typically sustain substantial damage.

When pushing an aircraft. be sure to push only at points that are specified by the

aircraft manufacturer. Never push on control surfaces, nor in the centre of a strut. NO

STEP and NO PUSH decals mean just that.

8.1.5 Helicopter Ground Handling Ground movement of helicopters is different than that for conventional aircraft.

The most common landing gear on helicopters consists of a set of skids which allows

operation from various surfaces. To move a helicopter on the ground. Small wheels are

attached to its skids and the helicopter is raised off the ground onto the wheels.

Some helicopters do utilize wheels as landing gear. In this situation. the nose

gear is typically free to swivel as the helicopter is taxied. These helicopters have tow

bars that attach to the nose gear, and are towed in much the same way as fixed-gear

aircraft.

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Figure 8-5 The attachment of small wheels to the skids allows for easier ground handling on most light helicopters.

8.1.6 Moving Methods Normal moving methods of moving aircraft on the ground are by means of:

Hand: by pushing and using a steering arm

Tractor: using a bridle and steering arm or with a purpose-made towing arm

Taxiing: moving the aircraft, using its own power.

When an aircraft has to be moved from one place to another, either by man-

handling, by the use of a tractor (also called a towing 'tug') or by taxiing, there

are a number of safety precautions which have to be applied every time.

8.1.7 Moving by Hand and Steering Arm This method is generally used for moving light aircraft small distances.

Figure 8-6 A steering arm

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Care should be exercised, during the move, to avoid damage to the structure,

particularly on aircraft constructed from wood and fabric. On aircraft, which have a

nose-wheel, a steering arm is attached to the wheel axle, in order to guide the aircraft,

while the moving force is applied to strong parts of the aircraft.

It is generally better to push the aircraft backwards, since the leading edges are

stronger than the trailing edges. It is also permitted to push at the undercarriage struts

and wing support struts. Areas to avoid include:

Flying Control Surfaces

Propellers

Wing and Tail-plane trailing edges.

On aircraft with steerable nose wheels, which are connected to the rudder pedals,

care should be taken not to exceed the towing limit, which may be marked on the

undercarriage leg. On this type of aircraft the rudder controls should not be locked

during towing. If the aircraft has a tail skid, in place of a wheel, it is customary to lift

the tail clear of the ground, ensuring first that the propeller is positioned horizontally, so

that it does not strike the ground.

8.1.8 Using a Bridle and Steering Arm This method is sometimes used, when the aircraft is to be moved over uneven or

boggy ground, because, if normal towing procedures were used, they would be likely to

cause an unnecessary strain on the nose undercarriage.

Using this alternative method, a special bridle (consisting of cables and

attaching shackles) is attached to specific points on each main undercarriage leg and a

steering arm is attached to the nose undercarriage for directional control.

The aircraft is normally towed backwards, using a tractor attached to the bridle.

It is normal to tow the aircraft backwards as this reduces the stress on the weaker nose

undercarriage.

If towing points are not available, then ropes may be passed round the legs, as

near to the top as possible, taking care not to foul on adjacent pipes or structure. A

separate tractor should then be connected to each main undercarriage and steering

control achieved by using the steering arm.

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8.1.9 Using a Purpose-Made Towing Arm This is the normal method used on large aircraft. The aircraft is normally towed

with a suitable tractor (or tug) and using the correct, purpose-made towing arm for the

specific aircraft. A person familiar with, and authorised to operate, the aircraft brake

system should be seated in the cockpit (or on the flight deck) to apply the brakes in an

emergency. The brakes should not normally be applied unless the aircraft is stationary.

The relevant maintenance manual will normally specify details of the towing

arm and any limitations on the towing procedure. On many aircraft with nose-wheel

steering, it is normal practice to disconnect or depressurise the aircraft steering system

before towing .

Figure 8-7 A purpose made towing arm

8.1.10 Precaution when Towing Aircraft Towing speed should be kept to a safe level at all times (walking pace is a safe

limit). A steering limit is often imposed, so that the radii of turns are kept within

specified limits, thus minimising tyre scrubbing and reducing the twisting loads on the

undercarriage. It is usual to tow the aircraft forwards in a straight line after executing a

turn, in order to relieve stresses built up in the turn. The steering limit is often shown by

marks painted on the fixed part of the nose leg, but may, sometimes, be overcome by

the disconnection of a pin, joining the torque links.

Suitably briefed personnel should be positioned at the wing tips and tail when

manoeuvring in or around confined spaces, so that obstructions may be avoided. One

person shall be supervising the aircraft movement (NOT the tractor driver) and should

be positioned so that all members of the team can be observed.

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Particular care should be given, when towing swept wing aircraft, to "wing tip

growth". This is the tendency of the swept wing to "grow" in a turn and was discussed

in 'Flight-Line Safety', which is contained in the early topic concerning Safety

Precautions.

Before commencing the towing operation, the brake system should be checked

and the brake accumulator charged as necessary. Brake pressure should be carefully

monitored during the move.

Large, multi-engined aircraft will usually be towed with special-purpose tug and

a suitable towing arm that includes a shear pin, designed to shear if a pre-determined

towing load is exceeded.

In an emergency it may be necessary to move an aircraft from the runway if it

has one or more deflated tyres. Provided there is one sound tyre on the axle the aircraft

may be towed to the maintenance area, but sharp turns must be avoided and towing

speed kept to a minimum.

If there are no sound tyres on an axle, the aircraft should only be moved the

shortest distance in order to clear an active runway and serviceable wheels should be

provided before towing. After any tyre failure, the associated wheel and other wheels on

the same axle should be inspected for signs of damage.

8.2 Aircraft Jacking, Chocking, Hoisting and Securing

Aircraft must often be raised from a hanger floor for weighing. Maintenance, or

repair. There are several methods of doing this, however, and you should follow the

aircraft manufacturers instructions. If an aircraft slips out of a hoist or fails off a jack,

the cost to repair the aircraft is usually quite high.

It is often necessary to lift only one wheel from the floor to change a tire or to

service a wheel or brake. For this type of jacking, some manufacturers have made

provisions on the strut for the placement of a short hydraulic jack. When using this

method, never place the jack under the brake housing or in any location that is not

specifically approved by the manufacturer. On aircraft with spring steel landing gear

legs. manufacturers typically provide a special jack pad that clamps to the gear leg,

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providing a jack point. It is usually recommended that both wheels NOT be lifted off

the floor at the same time when jacking from the landing gear struts.

When jacked from the struts, some aircraft have a tendency to move sideways

and tilt the jack as the weight is removed from the tire. If this should occur, lower the

jack and straighten it, and then raise the wheel again. To keep the aircraft from moving

while it is on the jack, the wheels that are not jacked should be securely chocked.

Most modern aircraft have jack pads located on their main wing spars. In

addition, many nosewheel- type aircraft have an attach point on the tail where a lack

stand is placed.

The most important consideration when jacking an aircraft is to follow the

manufacturers instructions in detail. Be sure to use the proper jacks so that the aircraft

remains level with no tendency for it to slip off of the jacks. Most higher-capacity jacks

have screw-type safety collars to prevent the jack from inadvertently retracting. Be sure

that these collars are screwed down as the aeroplane is raised. Jacks that do not have the

screw-type safety usually have holes drilled in the shaft so lock pins can be inserted to

guard against the jack retraction.

Figure 8-8 Main and Nose landing gear jack pads

Many light aircraft can be jacked from only the main spar position by securing a

weighted stand to the tail tied own ring. When using this method, make sure to place

enough weight in the stand, or tie the tail to a tied own ring embedded in the hangar

floor. Some aircraft can have their tail held clown by weights placed on the main spar of

the horizontal stabilizer. However, make sure this procedure is approved by the aircraft

manufacturer before attempting it.

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Guard against any movement within the aircraft when it is on jacks, since

shifting the weight behind the jack could cause the aircraft to tilt enough to fall off the

jack.

Before lowering the aircraft, be sure to remove work stands, ladders and other

equipment. Items placed under the aircraft while it was on jacks could cause damage

when the aircraft is lowered. Furthermore, be sure that the landing gear is down and

locked before the aircraft is lowered evenly.

Figure 8-9 A jacks safety collar should be used to keep the jack from collapsing if a seal fails.

It is possible for some landing gear to produce a side load on the jacks as the

weight is taken by the tires, and this must be watched to prevent this side load from

causing the jack to tip. Be sure that the oleo struts do not bind and hold the aircraft. If

they do bind enough to allow the jack to be lowered away from the wing and the strut

should suddenly collapse, it can drop the aeroplane back onto the jack and cause serious

damage.

Always use only the equipment and jacking methods approved by the

manufacturer. To do otherwise can cause serious personal injury or major damage to the

aircraft.

8.2.1 Special Considerations

Because of the position of the jacking points, the C.G. of some aircraft may be

well behind, or in front of, the main jacking points. It may be necessary to add ballast

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forward or rear of the jacking points or to check the fuel load of the aircraft, to bring the

centre of gravity within safe limits as specified in the Maintenance Manual.

Each jacking point may have a load limit which, if exceeded, could result in

structural damage. To avoid exceeding this limit it may be necessary to install hydraulic

or electric load cells. Any special requirements should be listed in the Maintenance

Manual.

Micro-switches, attached to the undercarriage legs, and operated by the

extension of the shock absorbers (weight-on switches), are used to operate various

electrical circuits, This operation may not be desirable, so circuits should be isolated, by

tripping circuit breakers or removing fuses as necessary.

Aircraft should always be as structurally complete as possible before jacking, It

is essential that any stressed panels which have been removed are re-installed. Failure to

do this may result in distortion or failure of the structure.

8.2.2 Aircraft Jacks Aircraft jacks are intended for raising and lowering loads and should not be used

for supporting the loads for long periods, Where a load must remain raised for a long

period, it should be supported on blocks or trestles after it has been jacked to the

required height. The most common types of aircraft jacks are the pillar, trolley, bipod,

tripod and the quadrupod hydraulic jacks. There are several sizes of jacks, with

capacities ranging from 4000 kg and greater.

The Pillar hydraulic jack consists of a cylinder assembly, a fluid container and a

hydraulic pump which, when operated, forces fluid from the container into the cylinder

and raises the ram. A release valve is provided which, when opened, causes the fluid in

the cylinder to return to the container and the ram to descend.

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Figure 8-10 Components of a jack

Because of possible hydraulic failure, some jacks are provided with a

mechanical locking collar which, when wound down, will prevent the jack from

lowering. An air/filler valve, which vents the return side to atmosphere, may also be

provided. This should always be open when the jack is operated.

Bipod, Tripod and Quadrupod jacks are used, to raise an aircraft for various

servicing operations. Their methods of operation and hydraulic mechanisms are similar

to the pillar jack. They consist of a hydraulic unit, supported by the relevant number of

legs (two, three or four).

Figure 8-11 Typical jack positions

Because of the problems involved in raising an aircraft and to avoid injury to

personnel or damage to the aircraft, care should be taken to use the correct type of jack

as stated in the Maintenance Manual. Each jack should be used with the correct adapter

head.

The tripod jack comprises a hydraulic unit with three equally spaced legs. The

jack is designed for a vertical lift only and not for a lift involving lateral movement of

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the jack (such as when raising one side of the aircraft for a wheel change). The resulting

side thrust may cause anyone of the following:

Serious damage to the ram, due to the bending load

Distortion of the jack legs

Damage to the aircraft, due to the .jack head slipping out of the jacking pad

Shearing of the jacking pad fastener

Dragging sideways of the serviceable tyre.

To change a single wheel, a pillar jack may be used, while two tripod jacks may

be used to raise the complete aircraft (or a bipod jack may be used). The bipod

arrangement overcomes the limitations of the tripod jack for an 'arc' lift. On this type of

jack, two fixed legs provide the support and a third, trailing leg, follows the lift and

steadies the load during the lift. The maximum angle of arc should not be more than 6º.

The quadruped jack is used more commonly as it possesses the advantages of

both types of jack. Two legs are fixed and two are adjustable. This jack may be used as

a bipod jack, by removing the adjustable legs, or as an adjustable, stable jack with one

extra leg added. All four legs may be locked solid, by slight adjustment of both

adjustable legs.

Transportation wheels are often permanently attached to some jacks while they

may be provided as detachable units on other jacks. The wheels facilitate easy

movement of the jacks that would otherwise need to be dragged around the hangar.

Jacks, alternatively, can be dismantled for easier transportation.

8.2.3 Jack Maintenance and General notes Aircraft jacks should always be positioned correctly and the load raised and

lowered gradually.

All jacks should be stored in the fully retracted position, kept clean and free

from corrosion. Moving parts should be lubricated regularly and the jack should be

exercised if it is not used frequently.

Jack replenishment is usually through the air valve, up to the level of the bottom

of the air valve. Low oil level is indicated by inability to lift to maximum height, whilst

over-filling is indicated by leakage of oil when the jack is fully extended.

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The following jacking precautions and procedures must be kept in mind:

Precautions

As a safety precaution, small aircraft should normally be jacked inside a hangar.

Larger aircraft may be jacked outside, provided they are positioned nose into wind; the jacking surface is level and strong enough to support the weight, and that any special instructions, stated in the Maintenance Manual, are observed.

A maximum wind speed, stated for jacking outside, can also be found within the Maintenance Manual. The aircraft to be jacked should be chocked fore and aft and the brakes positioned to OFF (brakes released). If the brakes are inadvertently left in the ON position (brakes applied) stress could be introduced to the landing gear or to the aircraft structure, due to weight redistribution as the aircraft is raised.

Procedures

While the following procedures will, generally, ensure safe and satisfactory

jacking of most aircraft, precedence must always be given to the procedures and

precautions specified in the relevant Maintenance Manual.

One person should co-ordinate the operation and one person should control each

jacking point. On larger aircraft a levelling station will also need to be

monitored and all members of the team may need to be in radio or telephone

communication with the co-ordinator.

Checks should be made on the aircraft weight, its fuel state, and centre of

gravity, to ensure they are within the specified limits as detailed in the

Maintenance Manual. The aircraft should be headed into wind (if it is in the

open), the main wheels chocked fore and aft, the brakes released and the

undercarriage ground locks installed.

It is vital that the earth cable be connect to the earth point on the aircraft and it

must be ensured that there is adequate clearance above every part of the aircraft

and that there is clearance for lifting cranes or other equipment, which may be

required.

Jacking pads should be attached to the jacking points and adapters provided for

the jacks as required. Load cells may also be included if needed.

The jacks should be positioned at each jacking point and checks made, to

confirm that the jacks are adjusted correctly (i.e. release valve closed, jack body

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vertical, weight evenly distributed about the legs when the adapters are located

centrally in the jacking pads, and the weight of the aircraft is just being taken by

the jacks).

Before jacking commences, the chocks must be removed and then the aircraft

should be raised slowly and as evenly as possible. Whilst jacking is in progress,

the locking collars should be continually wound down, keeping them close to the

body of the jack. When the aircraft is raised to the correct height, the locking

collar should be fully tightened down.

When jacking is complete, then supports may be placed under the wings and

fuselage as indicated in the Maintenance manual.

Note: As previously stated, a pillar (bottle) jack and an adapter are often used for

raising a single undercarriage for changing a single wheel. Alternatively a

trolley jack or stirrup jack may be used. The remaining wheels should be

checked to prevent aircraft movement, and it may be specified that a tail support

be located when raising a nose undercarriage. The jack should be raised only

enough to lift the unserviceable wheel clear of the ground.

8.2.4 Trestles These are provided to support to aircraft structures (main planes, fuselages etc.)

and may also be used to support the complete aircraft. Various types are available

including plain wooden trestles that are purpose-built and not adjustable. Trestles

should only be used at designated strong parts of the structure. It will normally be

shown in the Maintenance Manual where they should be positioned. Lines are often

painted on the aircraft to show where the trestle beam is positioned

The 'Universal' trestle is made up from lengths of angle iron, bolts and nuts, and

has two jacking heads. By using different lengths of angle iron, trestles of various sizes

can be produced. The wooden beam across the jacking heads may be replaced by a

wooden former, which is cut to the curvature of the component it supports.

Padding is normally attached to the former, to prevent damage to the aircraft

finish. The two jacking heads, which are hand-operated screw jacks, enable the beam to

be adjusted to suit the angle of the component.

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Although the trestles have 'jacking heads', they should only be used for

supporting a load, and not for attempting to raise parts of the aircraft. Damage may be

caused to the aircraft if attempts are made to do any more than support the structure.

The Tail' trestle is not suitable for heavy loads and must only be used for

supporting a load vertically. Adjustment in height is made by a screw thread. In the

same manner as a universal trestle, the beam can be made in the same shape as the

contour of the aircraft.

8.2.5 Lowering Aircraft From The Jacks Before lowering the aircraft to the ground, all equipment, trestles, work stands

etc. should be moved clear of the aircraft, to prevent collision or contact with the

aircraft structure. The wheels should be rotated by hand, to ensure the brakes are off.

The jacks should be lowered together, by opening their respective release valves, and

the locking collars (if used) unscrewed (but kept close to the jack body), whilst the jacks

are lowered, The jacks should be fully lowered after the aircraft is resting on its wheels

and the release valves then closed.

On no account should the top of the jacks be handled until the jack is clear of the

aircraft. It is common for the aircraft shock absorbers to stick and to suddenly collapse,

resulting in damage to equipment or serious injury to parts that might be between the

aircraft and jack.

After the aircraft is lowered and the jacks removed, the jacking pads and

adapters should be removed and the chocks placed in position. Any fuses or circuit

breakers should be re-set in their correct position.

8.2.6 Slinging Slings may be required for lifting various parts of an aircraft during

maintenance, repair, dismantling and assembly. Sometimes a complete aircraft may

need to be lifted for transportation or to clear a runway quickly.

The use of the correct equipment for lifting aircraft parts will minimise the risk

of damage to the aircraft and personnel. A list of special equipment is usually in the

front of the Maintenance Manual. This list will usually include special slings to be used

on the aircraft and any other special equipment or tools required.

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Slings may be of the three-point type, as used for lifting-main planes, while

other types, used for lifting engines, fuselages or other large items may be provided

with spreader bars or struts.

Before removing a main plane, the opposite main plane must be supported with

trestles. To attach a sling, some aircraft have special slinging points with threaded holes

in the airframe, which are used to accommodate the eye or fork-end bolts of the sling.

These holes are normally sealed, with removable plugs, when not in use. As an

alternative to screw-in devices, some slings are used in conjunction with strong straps

that pass under the component to be lifted.

8.2.7 Lifting Tackle The following is a list of safety precautions that must be used when using lifting tackle:

Do not exceed the safe working load of the lifting devices

Do not leave a suspended load unattended at any time

Do not walk or work under a suspended load

Do not tow the hoist at greater than walking pace

Do not tow the hoist, other than by hand, when a load is suspended from the

lifting hook

Do not allow the load to swing, especially when it is being hand-towed

Do not using a hoist or crane on soft ground

Do not use a crane or hoist if the lifting tackle shows signs of damage.

Wire rope, chain or fibre rope may be used for lifting purposes. Before use, the

tackle should be inspected to ensure that it is serviceable, is of the correct type and,

when used, that the Safe Working Load (SWL) is not exceeded. The SWL should be

stated on an identification plate, attached to the lifting sling, and should never be

removed from the sling.

Wire Rope is used with cranes, hoists, gantries and various slings. Before use,

the wire rope, splices and attachments should be inspected for damage such as wear,

corrosion and broken wires.

In use, care should be taken that the rope does not kink under load. Before

multiple leg wire rope slings are used, they should be laid out on the floor to ensure

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shackles are correctly attached and the fittings are not twisted. Knotting of ropes, to

shorten them, is prohibited.

Wire rope slings may be treated against corrosion by immersion in oil and the

surplus oil wiped off, but this treatment must not be applied to slings used for oxygen

cylinders. They must always be free from oil or grease.

Chains are used with cranes and various types of sling. Before use, all chains

must be inspected for damage such as cracks, distortion, excessive wear and 'socketing'.

Socketing is the name given to the grooves, produced in the ends of links, when

the links wear against each other. Any reduction in diameter will render the chain

unserviceable.

Fibre rope slings may be used for lifting lighter components, and are made from

natural fibres such as sisal, hemp or nylon fibres. They must be inspected for frayed

strands, pulled splices, excessive wear and deterioration.

When not in use, fibre rope slings should be hung on pegs, in a sheltered

position, and free from dampness. Immediately before use, the rope should be opened

up, by slightly untwisting the strands, to ensure they are not damaged or mildewed

internally. A damaged or mildewed fibre rope sling should not be used, and it must be

destroyed, by cutting into small, unusable sections, before final disposal.

In addition to before-use checks on the rope, all loaded components such as

pulley blocks, shackles, pins, spreader bars and hooks are to be inspected for excessive

wear, cracks and flaws. Moving parts must be lubricated periodically.

8.2.8 Hoisting At times an aircraft must be hoisted, rather than jacked. When this is done,

follow the manufacturer's recommendations in detail. Use a hoist of sufficient capacity

and, where necessary, place spreader bars between the cables to prevent side loads on

the attachment points.

8.2.9 Mooring (Picketing) An aircrafts lightweight construction coupled with its airfoil-shaped wings and

tail surfaces or rotors makes it highly susceptible to damage from wind.

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The best protective measure you can take to help ensure an aircraft's safety is to

put it in a hanger. However, at times a hanger may not be available. If this is the case,

an aircraft should be securely tied down and its controls firmly locked in place. For

example, most aircraft are equipped with internal control locks that hold the control

surfaces in a streamlined position. However, since these locks secure the cockpit

control, there is still a possibility that if severe forces were exerted on an aircrafts

control surfaces. damage to the control actuating system could result. To prevent this,

control surface battens are often used to hold a control surface in a streamline position.

Figure 8-12 Aircraft Picketing points

These battens are clamped against a fixed surface and should be lined with one-

inch foam rubber. Furthermore, battens should be painted red and have a long red

streamer attached so they are easy to see. This helps prevent a pilot from inadvertently

leaving them on the controls prior to a flight.

If a tail wheel aircraft is tied down facing into the wind, its elevator should be

locked in the full up position so the wind forces the tail down. On the other hand, if a

tailwheel - type aircraft is tied down facing away from the wind, the elevator should be

locked in the full down position.

If a severe wind is expected, spoiler boards can be secured to the top surface of a

wing to destroy lift. These spoilers are often made of 2 x 2 boards on which a one-inch

strip of foam rubber is attached. holes are drilled through the boards so they can be

secured with nylon rope. The nylon rope is tied around the wing to hold the spoiler

parallel with the wing span approximately one-quarter of the wings width back from the

leading edge.

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Figure 8-13 Control battens are used to securely lock an aircrafts control surfaces in place. They should have red streamers attached to help prevent their inadvertently being left on the aircraft when it is prepared for flight.

Figure 8-14 If an aeroplane must be left outside in a windstorm, spoilers can be lashed to the top of the wing to pre-vent generation of lift.

Special care must be taken when securing a set of spoilers so that the ropes are

not pulled too tight and the wings leading or trailing edge damaged. Furthermore, scraps

of carpet or foam rubber should be placed under the rope where it contacts the wing to

prevent damage to the aircrafts finish.

In addition to securing the control surfaces and attaching spoilers to the wing, all

doors and windows should be secured so they cannot be blown open. Furthermore, all

engine openings should be covered to keep blowing dirt from entering the engine

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compartment and the engine itself. Pitot heads should also be covered to exclude water

and dirt.

Figure 8-15 Tiedown chain attachment

When it comes to tying down an aircraft, most airports have a tiedown area with

anchors permanently embedded into a hard-surfaced ramp. However, at some airports,

aircraft are tied down to cables that run the length of the flight line. With either method,

an aircraft should be secured so that it is headed as nearly into the wind as is practical

with as much separation between it and adjacent aircraft as possible.

When parked, an aircrafts nose wheel should be locked in a straight ahead

position so the aircraft cannot move from side to side, or weathervane in the wind. In

addition, tie downs should be secured to each wing and to the tail. Although aircraft can

be tied down with either rope or chain, ropes normally provide the strongest attachment.

Nylon is the strongest material for rope, although Dacron and yellow polypropylene

also provide sufficient strength. Manila rope should be avoided, if possible. since it has

a tendency to shrink when it gets wet, as well as mildew and rot from exposure to

weather.

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Figure 8-16 When securing an aircraft on a crowded ramp, observe the minimum clearances shown in this diagram.

When inserting rope through aircraft tiedown rings the rope should be pulled

snug and secured with a bowline knot. The rope should not be pulled tight enough to put

a strain on the wing, but must keep the aircraft from rocking back and forth excessively.

Proper tension allows about one inch of movement. However, if manila rope is used, a

little extra slack must he allowed in the event the rope shrinks.

Figure 8-17 The bowline is the most generally used knot for tying an aircraft with ropes.

Chains are used at many airports. Although they have a much longer life than

rope and are easier to use, but they are not as strong as the proper size rope. If chains are

used, they must be secured to an aircraft by passing the chain through the tiedown ring,

then sticking one link through a link in the standing chain and fastening it in place with

a snap. Do not allow the snap to take any of the strain, since it is not made for this

purpose.

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In addition to securing the wings, aeroplanes with nose wheels should be tied

down with one rope through the nose gear tied own ring and two ropes through the tail

tied own ring. The ropes from the tail should pull away at a 45 degree angle to each side

of the tail. Furthermore, make sure that the wheels are blocked with properly fitting

chocks in front of and behind the wheels.

Figure 8-18 When securing an aircraft with a chain, do not depend upon the snap to carry

any strain. The snap should be used only to secure a free link through a standing link.

8.2.10 Seaplanes Seaplanes can be secured by towing them into shallow water or onto a beach or

by securing them to a dock or tree. If left in the water. some of the float compartments

should be flooded to add weight and assist the tiedown ropes in holding the aircraft.

8.2.11 Skiplanes Ski-equipped aircraft are often caught out in storms with no protection from

high winds. When this happens, loose snow can be packed around the skis, then doused

with water so they freeze in. The tiedown ropes can also be frozen into ice to secure.

8.2.12 Typical Small Aircraft Mooring Procedures When mooring small aircraft in the open, the aircraft, if possible, should be

parked head into the wind. The control surfaces should be secured with the internal

control lock and the brakes applied.

Care must, however, be exercised in extremely cold weather and parking brakes

must not be set if there is a danger that accumulated moisture may freeze the brakes.

Another danger, in cold weather, exists when the brakes are overheated, because, if they

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are set in this condition, serious distortion and cracking of the brake (and wheel)

components can occur as they cool down.

Ropes, cables, or chains should be attached to the wing mooring (tie-down)

points, and their opposite ends secured to ground anchors. A tie down rope (no chains or

cables) should be fastened to the exposed portion of the engine mount and the opposite

end of the rope also secured to a ground anchor.

The middle of a rope should be attached to the tail tie-down ring and each end of

the rope pulled, at a 45° angle, and secured to a ground tie-down point either side of the

tail. A control lock should be applied to the pilot's control column. If a control lock is

not available, then the control may be tied back with a front seat belt.

These aircraft are usually equipped with a spring-loaded steering system that

affords protection against normal wind gusts. However, if extremely high winds are

anticipated, additional external locks may be installed.

8.2.13 Typical Large Aircraft Mooring Procedures These may only require picketing in very strong wind conditions. The maximum

wind-speed will normally be stated in the Maintenance Manual (including gusting

winds). The aircraft should be headed into wind and the parking brakes applied.

Cables or chains should be attached from the aircraft picketing points to

prepared anchorages. In some instances the picketing cables are special components and

include a tension meter that is used to apply a pre-load to the cable.

If an aircraft is to be parked for a longer period, then additional precautions must

be taken. Landing gear down-locks must be installed (if so equipped) and all openings

such as static vents and engine intakes should be covered or blanked off to prevent the

ingress of dirt, birds, insects and all forms of precipitation.

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Figure 8-19 Typical aircraft blanks

8.2.14 Chocking of Aircraft When aircraft are parked, it is normal to place a chock ahead and behind at least

one wheel set. The parking brakes are usually left in the 'off' position once chocks are in

position, to allow the heat, generated by the brakes, to dissipate evenly.

Figure 8-20 Typical aircraft chocks

At high wind speeds, it is normal to chock all the wheels and apply the brakes (if

they have cooled). Some aircraft chocks can be chained together, to give a more secure

hold. During ground runs (and especially those involving high power), it is common

sense to place chocks at the front of all main wheel sets, to reinforce the parking brake.

8.3 Aircraft Storage

If an aircraft is de-activated for an extended time it will need to be protected

against corrosion, deterioration and environmental conditions during its period of

storage.

The following notes are based on the storage procedures applicable to BAe 146

aircraft that have been de-activated for periods in excess of 30 days and up to a

maximum of 2 years. It is not intended for the information given here to be complete,

but merely to give the student examples of some of the activities performed. Specific

details of an aircraft's storage procedures can be found in the Chapter of the relevant

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Maintenance Manual. A list of equipment and materials is normally given. This will,

typically, include:

Hydraulic fluid and lubricating oils and greases

Specialised water-displacing fluids and corrosion-preventative compounds

Aircraft covers and blanks

Plastic sheeting and adhesive tape.

Prior to the storage period certain tasks are completed. These may include

replacing the tyres with 'dummy' tyres (those not suitable for flight), or the raising of the

pressures of the normal ones. The various tanks are either filled (water), drained (toilet),

or part-filled (fuel). If the aircraft has propellers, they must be feathered, to prevent

them rotating in the wind. (they may also be restrained by straps).

Generally there would be an initial procedure, this being repeated at specified

intervals, as shown in Tables 8-2 (a) and 8-2 (b). If no repeat interval is given, then the

item is only done initially.

Once the aircraft has been prepared, there are routine, weekly checks to keep it

in good condition.

Table 8-2 (a) Typical Aircraft Storage Tasks

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Table 8-2 (b) Typical Aircraft Storage Tasks

To allow the circulation of air around the inside of the aircraft, all the doors and

curtains are fixed open, whilst all the external doors and panels are shut. The battery

will be removed from the aircraft and kept in the battery bay.

More active checks might be done on the two-weekly checks. These checks will

probably involve re-installing the battery, running the engines for a period and

functionally testing a number of the aircraft's systems that require the engines operating.

The flight controls might require cycling throughout their ranges and, if dummy tyres

are not fitted, the aircraft must be moved slightly to prevent 'flat spots' forming on the

tyres.

In addition, when power plants are stored separately, their fuel and oil systems

must be inhibited and all their external mechanisms protected with grease or other

suitable preservative. They must be stored in a clean, warm, dry atmosphere with

inspections at intervals to check for deterioration. Some engines are stored in an airtight

bag, which has moisture-absorbent crystals (a desiccant) inside.

After the storage period all of the covers, blanks and preservative compounds

will need to be removed. All of the systems will need to be restored to their original

condition prior to aircraft use. A further set of procedures will be followed, similar to

those previously discussed.

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When the aircraft is to be returned to service, it is simply a case of initially

removing all covers, blanks and tie-downs. Once access to the inside of the aircraft is

obtained and the battery reinstalled, all of the systems must be checked and tested.

All the tanks must be replenished to their correct levels and all pressure vessels

will require their gases charging to their normal operating pressures. If the cabin

furnishings, such as seats, carpets and galleys have been removed, they are to be

inspected and, when serviceable, reinstalled in the cabin.

As already stated, the foregoing summaries are only examples of the form that a

basic aircraft storage procedure might take. If the aircraft is smaller or larger and more

complex it will require a different form of inspection and routine checking.

The correct storage procedures will be found in the relevant aircraft's Maintenance Manual.

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REFERENSI

1. Maintenance Practices, Europe Aviation Safety Autority (EASA) Part 66 Module 7

2. Nick Bonacci, ‘ Aircraft Sheet Metal”, Digital Aviation Reference Library, 2009

3. Dale Crane, Airframe, Aviation Maintenance Technician Series, ASA, Systems, Volume 2,ISBN 1-56027-340-2

4. Dale Crane, Aircraft Powerplants , Aviation Maintenance Technician Series, ASA,

Systems, Volume 2,ISBN 1-56020-240-2

5. Jeppesen, ‘ A & P Technician Powerplant Textbook”, Jeppesen Sanderson, Inc., 2004.

6. JAR Conversion B1, Module 5 Digital Technique Electronic Instrument Systems,

KLM-UK Engineering, 2007

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