Aircraft Maintenance Mgt

7/22/2019 Aircraft Maintenance Mgt

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4. AIRCRAFT RELIABILITY (9 HRS)

Aircraft reliability Maintenance schedule & its determination Condition monitoring maintenance Extended range operations (EROPS) & ETOPS Ageing aircraft maintenance production.

Aircraft reliability (schedule reliability)

Topics of interest is schedule reliability

Elements include: Aircraft readiness Down time and repair time Events which can cause schedule interruptions, such as diversions or turn backs

High schedule reliability can be achieved with unreliable aircraft system- as long as the

aircraft can be repaired before each flight, without causing delay.

RELIABILITY : DEFINITIONS

Component reliability. Probability of no failure over a specified period of time underparticular use and environment condition.

System reliability. Determined by the reliability of its components, the way they are puttogether, and the criteria for satisfactory operation. Not all component failures cause system

failure (some component unscheduled removals are later found not to be failures)

Aircraft reliability, commonly referred to as schedule reliability , includes only thosefailures that result in a schedule interruption

Probability of starting & completing a scheduled revenue flight without interruption :r air= [1- (schedule interruptions / total revenue departures)]

.

Reliability & maintainability of digital avionics

MTBF ( mean time between failures) MTBUR (mean time between unscheduled removal) ( MTBUR/MTBF IS called maintenance

efficiency)~50% for electronic components, despite progress in reliability & diagnostics


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TYPES OF FAILURES

Hard . Persists thro several attempts to start the system Soft. Symptoms vanish when power is recycled ( Greatest area of concern for digital

avionics)

Intermittent . Disappears without operator action ( more of a nuisance than danger). Asthese are sources of erratic Maint practices, called Shot Gun Maintenance

Flight Deck Effect.Faithful Maint response to crews perception of a system fault , instead of

discounting if system is found to work well on subsequent ground checks

Fault Isolation & Reporting Method ( FIRM). Combo of:

Fault reporting Manual (FRM)- defines numerical fault codes Fault Isolation Manual (FIM)- defines Maint action reqd

SERIES SYSTEM. i.e. Reliability of a system comprising two items; system fails if either itemfails; failures are statistically independent( no common-cause failures) . R system=

r1xr2 or,unreliability q= q1 +q2

PARALLEL SYSTEM. I.e system fails only if both items fail, again with independence.Q system=Q1XQ2

DESPATCH RELIABILITY. Probability of departing on time. R= 1- [{delays+cancellations} /totaldepartures]

ENROUTE RELIABILITY. Probability of completing a flight plan without a deviation. R= 1- [ {airturn backs+ground turn backs+diversions}/total departures]

GROUND TURN BACK. Occurring before take-off AIR TURN BACK. Occurring after take-off INFLIGHT RELIABILITY . Same as for enroute reliability, but excluding ground turn backs. SCHEDULE RELIABILITY. Probability of starting and completing a scheduled revenue flight

without a schedule interruption. R = 1- {schedule interruptions/total departures}

SCHEDULE INTERRUPTIONS INCLUDE delays, cancellations,turn backs, diversions. Schedule reliability combines component & system reliability , maintainability and

deferability ( i.e ability to defer maintenance of non critical item) and the maintenance

efficiency of the organisation performing repairs.

Types of reliability Other reliability measures

Component Reliability Pilot reports of tech Defects


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System Reliability Engineers Report of Tech defects Dispatch Reliability Ave level of deferred defects carried Delays Ave time to clear deferred defects Ground Turn back Time off service for repairs In-flight Reliability Usage of stdby ac or slack time in the

schedule

Schedule Reliability Maint man hrs per flying hour Schedule Interruptions Maint Cost per flying hourMAINTENANCE SCHEDULE & ITS DETERMINATION

MAINTENANCE STEERING GROUP (MSG ). Modern aircraft maintenance schedules is thedocument produced by the Air Transportation Association (ATA) .

MSG APPROACH SETS LOGICAL RULES BASED ON : Importance of the component or system. Nature of anticipated failures in them Visibility of such faults

Possible corrective actions

Final report is termed maintenance review board (mrb) report.

Older methods. Before msg logic, all aircraft maint was based on preventive replacement orrestoration, commonly referred to as hard time ( fixed lives assigned to parts /tasks).

High wastage of usable components. Most serious disadvantage :it did not actually improve the reliability of aircraft components

& systems.

MSG introduced overhaul or on-condition maint in place of hard time lifting of components MSG-2 included condition-monitored concept for the first time. Reliability centred maintenance ( RCM). Focus on:

Maintenance significant items ( MSIs) and structural significant items (SSI s) Distinction needed between economics & safety

for systems & power plant, MSI defined as that:

Could affect safety, either on ground or in flight


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Undetectable during operations, and/or have significant operational economic impact ,and/or have significant

non-operational economic impact

Top down approach. I.e. Change in emphasis from piece-part to systemapproach.Systems and power plant analysis uses the top down approach,

in which the functional failure of a system is considered in relation to other

systems.

If functional failure of a specific system does affect the operating safety ofthe aircraft, or cause substantial economic penalty, then that system is

maintenance significant& must contain items requiring further

considerations.

BOTTOM UP APPROACH. i.e. Analysis begins with individual componentsand work upwards to system level.

. CONDITION MONITORED MAINTENANCE

Definition. A primary maintenance process under which data on the wholepopulation of specified items in service is analyzed to indicate whether some

allocation of technical resources is required.

Active redundancy. All the redundant items are operating simultaneouslyand share the task; failure of a single item causes the task to be shared

among the remaining items.

Standby redundancy. Only one redundant system is functioning at a time; ifa failure occurs it is necessary to select a standby system.

Three primary maintenance processes

Hard time. A preventive process in which known deterioration of an item,but is limited to an acceptable level by the maintenance actions which are

carried out at periods related to time in service (calendar time, or no ofcycles or no of landings)

On condition. Also a preventive process, but one in which the otem isinspected or tested at specified period to an appropriate standard in order

to determine whether it can continue in service. Inspection or test may

reveal a need for servicing.

Condition monitoring. Not a preventive process. Information on items,gained from operational experience, is collected, analysed and interpreted

on a continuing basis as a means of implementing corrective procedures.


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It is repetitive and continuous, key factor being introduction of aircraftembodying failure tolerant design.

Actuarial analysis. Statistical reliability analysis, performed by examinationof agerelated risk of failure, to determine whether or not a hard time

replacement policy is justifiable.

. Extended range of operations ( EROPS)

Operations beyond the original safety limit of flying time to the nearest airfield with an engine

failure.

This is of particular significance with the two engined aircraft. Original limit on the distance which a twin-engined aircraft might be from a

suitable emergency airfield was 60 minute flying time at normal single-

engine cruise speed.

Operations beyond this distance were extended range, subject to specialprocedures.max single engine speed was 120 min. Increase of 15% (138 min)

possible to obtain after a qualifying period. Further extension up to 180 min

at single engine speed allowable

Design of the aircraft must provide alternative power sources sufficient to permit continuedsafe flight in single-engine operation.

Particular requirements are specified for electrical power, since automatic controls,instruments, navigation and communications depend on this.

Checks for critical items of EROPS related tasks should be signed for by an EROPS qualifiedperson immediately prior to an EROPS flight.

Technical log should ensure adherence to proper minimum equipment procedures.Ageing aircraft

15 yrs/50,000 hrs have now changed to 20 yrs/60,000 hrs sincemanufacture.

As age & fatigue increase, with additional corrosion effects, inspectionreveals more flaws that need repair -- thus maint cost rises until it may be

cheaper to buy a new aircraft than to retain the old one.

Reasons for postponing replacement of ageing ac: aircraft is postponed : Higher costs of new aircraft; Increasing travel demands, needing larger or more numerous

replacements; shortage of ac from manufacturers

Reduced profitability due to price competition


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Ageing aircraft programme& corrosion control All age-related, essential service bulletins are selected , based on factors

such as:

Safety effects Failure risk Inspection difficulty

Corrosion control programme. 3 levels of corrosion:

Level 1. Fact of life, occurs in any good program. Level 2. warning sign. Excessive corrosion that exceeds

allowable limits, taking place between regular inspection

intervals.

Level 3.potential airworthiness concern, requiring promptcorrective action. A sign that the corrosion control program

is not doing its job.

Purpose of an effective Corrosion control programme:

Establishes many inspection areas for each ac. Each area comprises numerous typical corrosion repair zones; repairs and

inspections are made at each zone.

A typical corrosion repair would require removing the damaged surface andpatch repair

Special corrosion inspections are required for aircraft after their 15 th yearof operation.

Every airline has to inspect the equivalent of one aircraft for each area. (itguarantees that every area receives inspection once a year, whether or not

maintenance is planned in that area at less intervals).

5. TECHNOLOGY IN AIRCRAFT MAINTENANCE (8 HRS)

Airlines scheduling (with reference to engg) Product support & spares Maintenance sharing Equipment & tools for aircraft maintenance Aircraft weight control


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Budgetary control. On board maintenance systems Engine monitoring Turbine engine oil maintenance Turbine engine vibration monitoring in aircraft Life usage monitoring Current capabilities of NDT Helicopter maintenanceAirline schedulingwith reference to engineering

Main task of the airline engineer is to produce serviceable aircraftto meetthe flying programme of the airline.

Factors to be taken into account in scheduling aircraft flying andmaintenance:

Demand patterns. Transport is a perishable product.Demand for air transport varies with time.

Seasonal effects. Air tpt is more leisure oriented

Summer. Busiest period of the year. Public holidays. Route effects . Short hauls done in day time. Long hauls require

aircraft to be away from base for days in succession.

Factors to be taken into account in scheduling aircraft flying and maintenance (contd)

Timing. Commercial timing is important for marketing reasons. Business travel has morning peakand evening return peak.

Turn rounds. Aircraft only earn money when they are flyingResources needed for efficient schedule maintenance

Building & ground facilities Trained labor Access & parking Spares supply


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Documentation ( tech manuals) Tools & equipment ContinuityMaintenance response to a problem on the aircraft

GOdefects ( deferred defects, to be repaired at next opportunity) NO GO defects ( depart late/change the aircraft---with an inconvenience to passengers)

Product support and spares

Airlines depend on aircraft manufacturers for the supply of parts and often for the o/h ofspares.

After sales service- status of maint < status of manufacture Airlines in strongest bargain position while placing order for a new fleet Manufacturers recoup income thro high price for spares, expensive tooling needed in

maintenance , unnecessary modifications offered, etc.

Complete aircraft costs tens of millions of $ Cost of engine.. Millions of $

Turbine blades .vanes cost thousands of $

Fasteners cost many times those of equivalent ground based m/c Operators now ask manufacturers to assume more responsibility for reliability and cost of

spares in the early years of use( say, 3 yrs)

Air Transport Association (ATA) set up data base of aircraft parts available online tocustomers( paperless transaction)

Other Provisions for airline-spares inventory problemss:

Selective provisioning of different shortage risks Better forecasting models Selective provisioning for different shortage risks Better forecasting models More use of Real-Time computer systems Just In Time (JIT) policies Manufacturers spares depots


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Processing of Spares. Kinds of Inventory Items: Expendable Rotable (Lifed Component) Contracts Loans New Purchases Compliance of Quality control& Engg business activity essential

Requirements for Inventory Control:

Airworthiness Regulations Company Policies(Inter airline) Maintenance Sharing (agreement

It is one solution to the high cost of facilities and staff needed for modern ac Maint. BestExamples:

ATLAS. Air France,Alitalia,Iiberia, Lufthansa and Sabena. KSSU. KLM,SAS, Swissair & UTA (taken over by Air France)

Consortiaformed originally to support wide body jets in 1970. Allocation of Maint is:

One partner is main provider of Air Frame major check Maint for one ac type;another for engine O/H for one engine model; another for elect compt repair &O/H

etc

Significant savings in hangar & workshops constructionAircraft Weight Control

Operators are required to weigh their ac at regular intervals- (initially


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Max Zero Fuel Wt- Stru limit applied by designer, taking into a/c bending relief offered byfuel load in wing

Max Landing Wt Related to A/F strength & shock absorbing capacity of u/c Max T/Off WtRelated to Structure strength & airfield performance required Max Ramp Wt Including allowance for fuel burnt during taxing from terminal to runway

Crew salaries are more than cost of weight saved Indirect way to control weight is to place a cost on it

Budgetary Control

Essential use of resources and control of costs Communication about plans, motivation to perform and measurement of performance Company divided into Cost Centres for accounting and reporting purposes Budgetary control important part of planning process- manpower, materials & capital eqpt Usually tied to the financial year, with estimate of total revenue, to forecast profitability Budgeting may also include performance statements about quality of serviceon time

delivery, average turn round times or other non monetary events

Single figure targets- late delivery of ac after maint checks; spares demand met


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Documentation systemEngine (Health) Monitoring

Techniques

Oil Analysis Gas Path Analysis Usage Measurement

Attempts to Measure:

Engine operates within acceptable performance limits Trend in performance value indicates need for intervention

Oil Analysis Includes:

> Filter element checks > Micro Visual examination particles > Spectrometric oil analysis

Engine (Health) Monitoring

Techniques

Oil Analysis Gas Path Analysis Usage Measurement

Attempts to Measure:

Engine operates within acceptable performance limits Trend in performance value indicates need for intervention

Oil Analysis Includes:

> Filter element checks , Micro Visual examination particles , Spectrometric oil analysis

Gas Path Analysis- Engine thermodynamic parameters :

Temp, Pressure, Fuel Flow, shaft Speed- reduced to day values for comparison withmanufacturers performance specs

Usage Measurement cycle counting for the more temp critical parts (discs, turbine blades)

Trend Analysis Computer system monitors & analyses engine performance in cruise, in T/O, on

Test bed, Vibration readings, engine control settings & performance simulation. FDR

Engine Parameters Recorded. Engine shaft speeds, Exhaust gas temp, engine pr ratio,vibration levels, fan rotor imbalance, oil temp, fuel flow rate, control settings.


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Aircraft Performance Parameters. Ambient temp, pr, velocity, Mach No, Altitude, Phase offlt, Time, Control lever position

Turbine Engine Oil Monitoring

Monitoring for gas turbine engines can be of:

Oil system Operation.. oil pressure, temperature& quantity, chip detectors. Filter bypassindicators provided as alerts

Oil Debris Monitoring.. Varies from visual to quantitative assessment. Onboard debrismonitoring system. Ground based debris monitoring system. Spectrometric oil Analysis

SOAP analysis

Oil Condition Monitoring. Physical or chemical analysis of oilTurbine Engine Vibration Monitoring

Airborne Vibration Monitoring ( AVM). Fitted to most aircraft, comprising hardware, data&analysis procedures

Engine Faults detectable through AVM. Blade loss or partial loss Blade mounting disturbance Missing rotor parts, such as bolts etc Blade coating delamination Bearing misalignment or wear Oil or debris in rotors Reheat system instability ( Military aircraft)

Vibration sensors are small accelerometers using piezoelectric effects.

Life Usage Monitoring of Engine Parts

Important for aircraft safety & economics Turbine engine parts susceptible to failure by mechanical and thermal stress( fatigue , shock)

and by sustained stress and temp ( creep)

Major engine components exhibit more consistent failure distributions with age, than otheraircraft components

Failure of engine part due to cumulative damage from cyclic or steady stresses fromrotational speed, temp or pressure

Certain engine parts are critical and , hence, life limited if deterioration is age related


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Failure of LifeLimited parts. (Due to fatigue, creep. wear,corrosion , negligence ordefective

manufacture)

Low Cycle Fatigue. Due to repeated high stress applications50,000 cycles Thermal Fatigue. Due to temp diffce causing internal stress Creep. Time dependent effect (e.g., long cruise)Current Capabilities of NDT

For NDT purposes, ac may be grouped as :

Old Aircraft. >15 yrs old. 50 % ac fall in this. Young Aircraft. New inductions < 5 yr old ac. Advanced Aircraft. Using new materials and advanced engine. Will place severe demands on

NDT

Main NDT Techniques.

Visual Aids. Magnetic

Dye Penetrant

Eddy current Ultrasonic RadiographyHelicopter Maintenance

Special problems for aircraft Operation and Maintenance:

Engine failure at certain phases of flight very critical Vibration affects components & structure more in helicopter Low altitude & over water Ops increase exposure to atmospheric turbulence, FOD &

corrosion

One method to deal with the problem is use ofHealth & Usage Monitoring system ( HUMS).

(Requires instruments, FDR and signal processing equipment)

85% serious helicopter incidents & 55% of accidents detectable by HUMS.


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Quantitative Debris Monitoring (QDM) to signal debris count to cockpit instruments or FDRdirectly

Vibration reduction possible by improving main rotor balancing and trackingFuture of Aircraft Maintenance

Predictions for the Airline Industry

Industry as a whole will grow; volumes will grow- so also fuel prices Congestions in the airports; local community resistance to airport development Present under utilized airports will become important Trend of using larger ac at busy airports will continue ( 1000 seater unlikely). No of ac will

; so will demand for maintenance skill & facilities

States will divest themselves of airline ownershipTechnologies Expected to have an Impact in the Future:

Composite materials in structures

Non metallic materials in engines

Automation & software in avionics

Active systems for stability & control

Environmental effects of processes used in manu or maint

Computers , Communications & Information Technology

Predictions for the airline Industry

Reliability and Maintainability of these ac will be higher. Operators have to improve planningskills.

Present Gen of old ac will continue in use after the ageing study program to restore them isimplemented. Final retirement may be determined( up to 30 yrs) by economic or legal

factors.

Supply of spares to these may become difficult Skills & Labor shortage( due to decline in proportion of young people in populations & a

decrease in those taking science subjects at school)

Aircraft will continue to require maintenance in the future Airlines will become much more cost conscious( will it affect safety, cutting cost on trg,

facilities & R&D?-NO)


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Competitive nature of travel markets will raise the emphasis on mktg methods by airlines &the operators will respond quickly to changes in demand or taste.

Engines produce noise, pollution and CO2 . Also contribute to global warming & green houseeffects. Higher standards of pollution levels will come in.

Rising fuel prices will accelerate the retirement of older, less fuel-efficient aircraft andincrease the demand for newer, higher tech ac types.

These ac will demand different maint skills, particularly in avionics, control systems, andmaterials.

Present Gen of old ac will continue in use(upto30 yrs ) till they are retired by eithereconomic or legal factors. Supply of spares for some of these may be difficult. Operators or

maint orgns may have to consider entering spares prodn business.

Shortage of skilled man power will prevail. Importance of wt saving for economic reasons makes maint inevitable.Technologies Expected to have an Impact in the Future:

Composite materials in structures Non metallic materials in engines Automation & software in avionics Active systems for stability & control Environmental effects of processes used in manufacture or maint enance Computers , Communications & Information Technology Computers & Communications.

Since ac maint records are stored on a computer& most Computers will benetworked, airworthiness authorities could check compliance by remote

examination.

Onboard maint systems can transmit data direct to base. Extension of inventory control systems to contractors & manufacturers will provide

complete tracking of parts.

Computer aided planning will allow more examination of alternatives. Maint staff will have to learn new skills in the access & manipulation of data at much

higher volumes& speeds

Safety. Safety Vs Economy. Improvement in air tpt safety by maintenance engineers will haveto come from better understanding of materials , processes & eqpt used.

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Aircraft Maintenance Mgt