AIRCRAFT ELECTRICAL AIRCRAFT ELECTRICAL SYSTEMSSYSTEMS

ObjectivesObjectives

Students will be able to:Students will be able to: Describe the basic components of aircraft Describe the basic components of aircraft

electrical systemelectrical system Explain operation of electrical systemExplain operation of electrical system Interpret aircraft electrical diagramInterpret aircraft electrical diagram Select proper size of wire for installationSelect proper size of wire for installation Describe basic causes of electrical system Describe basic causes of electrical system

malfunctionsmalfunctions

Elements of aircraft electrical Elements of aircraft electrical systemssystems

An aircraft electrical system is mainly An aircraft electrical system is mainly composed of :composed of :

Power sourcesPower sources ComponentsComponents - Control devices- Control devices - Conversion devices- Conversion devices - Protection devices.- Protection devices. Power distribution systemsPower distribution systems Electrical loadsElectrical loads

Electrical Power SourcesElectrical Power Sources

Electricity power sources on an aircraft may Electricity power sources on an aircraft may be classified into two groups :be classified into two groups :

BatteriesBatteries - Lead-acid- Lead-acid - Nickel-cadmium- Nickel-cadmium GeneratorsGenerators - DC generators- DC generators - AC generators- AC generators

A battery is a device that converts A battery is a device that converts chemical energy into electrical energy.chemical energy into electrical energy.

It is a power reservoir that stores energy in It is a power reservoir that stores energy in chemical form.It does not produce energy.chemical form.It does not produce energy.

Its functions are:Its functions are:

- To provide power when no other power - To provide power when no other power source is availablesource is available

- To assist in damping transient loads in - To assist in damping transient loads in the dc systemthe dc system

BatteryBattery

- To provide a short term source during - To provide a short term source during emergency conditionemergency condition

The capacity of battery is measured in The capacity of battery is measured in ampere-hoursampere-hours..

Its normal rate is a little over 24vdc in a Its normal rate is a little over 24vdc in a 28vdc system.28vdc system.

It is automatically recharged when the It is automatically recharged when the engine-driven generator is operational.engine-driven generator is operational.

Two types of batteries are used in aircraftTwo types of batteries are used in aircraft - Lead-acid batteries- Lead-acid batteries - Nickel cadmium batteries- Nickel cadmium batteries

Lead-acid batteryLead-acid battery It is usually found in piston aircraft.It is usually found in piston aircraft. It is made up of cells which have It is made up of cells which have

positive/negative plates of lead and filled positive/negative plates of lead and filled with electrolyte of sulfuric acid and water.with electrolyte of sulfuric acid and water.

Each cell has app. 2.2v, but is simply rated Each cell has app. 2.2v, but is simply rated as 2 v.as 2 v.

It has corrosive effects.It has corrosive effects. Frequent total discharge and remaining Frequent total discharge and remaining

battery in discharged condition for a long battery in discharged condition for a long time will shorten the life of the battery.time will shorten the life of the battery.

Battery CellBattery Cell

Nickel cadmium batteryNickel cadmium battery

They are the most common type of battery They are the most common type of battery used in turboprop and turbojet aircraft.used in turboprop and turbojet aircraft.

They provide electrical discharge at a high They provide electrical discharge at a high rate without voltage drop and accept high rate without voltage drop and accept high charge rates that shorten recharge timecharge rates that shorten recharge time

They may be subject to They may be subject to thermal runawaythermal runaway condition caused by overheating, in which condition caused by overheating, in which the battery destructs itself.the battery destructs itself.

Nickel-cadmium batteryNickel-cadmium battery

CellTerminals

Container

Main connector

Ventpipe

GENERATORSGENERATORS A generator is a machine that converts A generator is a machine that converts

mechanical energy into electrical energy by mechanical energy into electrical energy by the process of electromagnetic induction the process of electromagnetic induction

They are driven by the engine(s), and They are driven by the engine(s), and sometimes by APU.sometimes by APU.

A generator must be rated at adequate A generator must be rated at adequate amperage to drive all the operating amperage to drive all the operating components on its circuit(s). The current components on its circuit(s). The current required to operate each electrical required to operate each electrical component is known as its component is known as its loadload..

All generators produce alternating current All generators produce alternating current naturally.The method which is used to take naturally.The method which is used to take it from the coil will determine if the it from the coil will determine if the generator provides ac or dc to the circuit.generator provides ac or dc to the circuit.

If a If a commutatorcommutator is used for this purpose it is used for this purpose it will be a dc generator. will be a dc generator.

If a If a slip ringslip ring is used it will be an ac is used it will be an ac generator.generator.

DC GENERATORS:DC GENERATORS: CommutatorCommutator in the generator converts the in the generator converts the

ac current to dc current.ac current to dc current. DC generator is designed to supply nearly DC generator is designed to supply nearly

constant voltage. Usually the voltage is constant voltage. Usually the voltage is 28vdc, but there are 270vdc systems 28vdc, but there are 270vdc systems developed recently.developed recently.

They supply current up to 400 A.They supply current up to 400 A. Most aircraft do not have a separate dc Most aircraft do not have a separate dc

generator, but ac produced by ac generator, but ac produced by ac generator is converted to dc to power dc generator is converted to dc to power dc systems.systems.

Basic dc generatorBasic dc generator

Operation of a basic dc generatorOperation of a basic dc generator

Producing steady dcProducing steady dc

AC generator (Alternator)AC generator (Alternator)

On most large aircraft high-load electrical On most large aircraft high-load electrical devices are usually AC powered.devices are usually AC powered.

AC is produced by ac generator which is AC is produced by ac generator which is usually called alternator.usually called alternator.

AC generators use slip rings instead of AC generators use slip rings instead of commutators to provide the current to the commutators to provide the current to the circuits.circuits.

Alternator generates three-phase current, Alternator generates three-phase current, and standard aircraft voltage is 115 vac and standard aircraft voltage is 115 vac with 400 Hz.with 400 Hz.

Basic ac generatorBasic ac generator

Modern electrical power generation Modern electrical power generation typestypes

Mainly two types of electrical power Mainly two types of electrical power generation currently in use on aircraft.generation currently in use on aircraft.

- Constant frequency Integrated drive- Constant frequency Integrated drive

generators (IDG)generators (IDG)

- Variable speed constant frequency- Variable speed constant frequency

(VSCF) generators(VSCF) generators

Constant Speed IDGConstant Speed IDG

It is essential that the alternator output It is essential that the alternator output have a frequency of 400 Hz with a very have a frequency of 400 Hz with a very small tolerance. Thus rotational speed of small tolerance. Thus rotational speed of alternator must remain absolutely alternator must remain absolutely constant.constant.

This is accomplished by using a Constant This is accomplished by using a Constant Speed Drive (CSD) unit between the Speed Drive (CSD) unit between the engine and alternator.engine and alternator.

CSD provides a constant alternator rpm CSD provides a constant alternator rpm within a specified engine rpm range.within a specified engine rpm range.

An alternator and constant speed drive An alternator and constant speed drive unit combination is called integrated drive unit combination is called integrated drive generator (IDG).generator (IDG).

CSD is hydromechanically operated and CSD is hydromechanically operated and needs to be correctly maintained in terms needs to be correctly maintained in terms of oil level and oil cleanliness.of oil level and oil cleanliness.

It is the most commonly used power It is the most commonly used power generation method on today turbine-generation method on today turbine-powered aircraft.powered aircraft.

Integrated Drive GeneratorIntegrated Drive Generator

Constant speed driveConstant speed drive

Constant Speed DriveConstant Speed Drive

VSCFVSCF

In this method, the variable frequency power In this method, the variable frequency power produced by the alternator is converted to produced by the alternator is converted to constant frequency 400 Hz, 115 vac by constant frequency 400 Hz, 115 vac by solid-state devices electronically.solid-state devices electronically.

Thus there is no need for CSD unit.Thus there is no need for CSD unit. It is a new technique and limited in use.It is a new technique and limited in use.

Variable speed constant frequency Variable speed constant frequency generatorgenerator

Power Generation ControlPower Generation Control

Power produced by generators is usually Power produced by generators is usually controlled by generator control unit (GCU).controlled by generator control unit (GCU).

The main functions of this device are:The main functions of this device are: - Act as a voltage regulator- Act as a voltage regulator - Direct current to battery for recharging- Direct current to battery for recharging - Provide circuit and generator protection- Provide circuit and generator protection by disconnecting the generator from theby disconnecting the generator from the system when electrical abnormalitiessystem when electrical abnormalities occur.occur.

Control DevicesControl Devices

These devices are use to initiate and control These devices are use to initiate and control the operation of the circuits.Control devices the operation of the circuits.Control devices includeinclude

SwitchesSwitches RheostatsRheostats RelaysRelays SolenoidsSolenoids

SwitchesSwitches A switch is used to start, to stop, or to A switch is used to start, to stop, or to

change the direction of the current flow in change the direction of the current flow in the circuit.the circuit.

Toggle switches :They are on/off switches Toggle switches :They are on/off switches and extensively used in aircraft electrical and extensively used in aircraft electrical system.system.

Push Switches : They are used primarily Push Switches : They are used primarily for operations of short durationsfor operations of short durations

Rotary switches :When it is necessary to Rotary switches :When it is necessary to select several condition for a circuit, a select several condition for a circuit, a rotary switch may be used.rotary switch may be used.

Rotary SwitchRotary Switch

Toggle switchToggle switch

Rheostats : control the amount of current that Rheostats : control the amount of current that flows throughout the circuit and used as flows throughout the circuit and used as dimmer devices for instrument and cockpit dimmer devices for instrument and cockpit lights.lights.

Relays :Electromagnetic switching devices Relays :Electromagnetic switching devices which are used to remotely control electric which are used to remotely control electric circuits carrying large amount of current.circuits carrying large amount of current.

Solenoids : Remote control devices quite Solenoids : Remote control devices quite similar to relays; but they are designed to similar to relays; but they are designed to move a shaft over a short distance. Thus, move a shaft over a short distance. Thus, solenoids are used as mechanical control solenoids are used as mechanical control devices to operate hydraulic or pneumatic devices to operate hydraulic or pneumatic valves, locking pins, etc.valves, locking pins, etc.

RheostatRheostat

Schematic of a relaySchematic of a relay

RelayRelay

Conversion DevicesConversion Devices

There are many occasions within an There are many occasions within an aircraft electrical system where it is aircraft electrical system where it is required to convert power from one form to required to convert power from one form to another.another.

Typical examples of power conversion are :Typical examples of power conversion are :- Conversion from dc to ac power- Conversion from dc to ac power- Conversion from 115vac to 28 vdc- Conversion from 115vac to 28 vdc- Conversion from one ac voltage level to - Conversion from one ac voltage level to

anotheranother- Battery charging (from 115vac to 28vdc)- Battery charging (from 115vac to 28vdc)

Following devices are used for these Following devices are used for these purposes :purposes :

TransformersTransformers RectifiersRectifiers Transformer-rectifier units (TRUs)Transformer-rectifier units (TRUs) InvertersInverters TransistorsTransistors

Transformers : Used to change ac voltage Transformers : Used to change ac voltage level.level.

Rectifiers : Used to convert ac into high-Rectifiers : Used to convert ac into high-amperage, low-voltage dc.amperage, low-voltage dc.

TRUs: Combination of transformers and TRUs: Combination of transformers and rectifiers and used as main unit to convert rectifiers and used as main unit to convert alternator output to dc in an aircraft alternator output to dc in an aircraft

Inverters : convert 28 vdc power to 26 vac Inverters : convert 28 vdc power to 26 vac power particularly for flight instruments or power particularly for flight instruments or 115 vac.115 vac.

Transistors : Electronic devices that Transistors : Electronic devices that control electron flow and convert ac to dc.control electron flow and convert ac to dc.

TransformerTransformer

Analogy to TRU (ac adapter)Analogy to TRU (ac adapter)

TRUsTRUs

InverterInverter

Analogy to inverter (ups for Analogy to inverter (ups for computer)computer)

Protection DevicesProtection Devices

These devices are used to protect circuits, These devices are used to protect circuits, cables and system components from cables and system components from damage due to failures.damage due to failures.

Fuses Fuses Circuit breakersCircuit breakers DiodesDiodes GCUsGCUs

Fuses : They are designed to protect the Fuses : They are designed to protect the cables against the flow of short-circuit and cables against the flow of short-circuit and excessive current. They break the circuit excessive current. They break the circuit and stop the current flow when the current and stop the current flow when the current exceeds a predetermined value. exceeds a predetermined value.

Circuit breakers :They have same Circuit breakers :They have same function with the fuses.The difference from function with the fuses.The difference from the fuse is that they are resettable, while a the fuse is that they are resettable, while a fuse must be replaced.fuse must be replaced.

Diodes : They are the electronic equivalent Diodes : They are the electronic equivalent to the check valves in hydraulic system. to the check valves in hydraulic system. They allow electricity to flow in one way They allow electricity to flow in one way only. only.

Circuit BreakersCircuit Breakers

Circuit BreakersCircuit Breakers

Primary Causes of Electrical Circuit Primary Causes of Electrical Circuit FailuresFailures

Open circuit :Open circuit :

It is a circuit that is not complete or It is a circuit that is not complete or continuous. This is an uncommanded continuous. This is an uncommanded interruption of electrical power tosome interruption of electrical power tosome components or systems. When an open components or systems. When an open occurs the affected component stop to occurs the affected component stop to operate, but the other components still operate, but the other components still remain in operative condition.remain in operative condition.

Common causes of open circuitCommon causes of open circuit

Short CircuitShort Circuit It occurs when electricity is allowed to It occurs when electricity is allowed to

take a shortcut through or around a take a shortcut through or around a component or system. This is the most component or system. This is the most serious problem. It has two effects :serious problem. It has two effects :

- Affected components have no power- Affected components have no power and fail to operateand fail to operate - Since current will not flow through- Since current will not flow through affected components, the other affected components, the other components will be subjected to highercomponents will be subjected to higher level of current causing them to burnlevel of current causing them to burn out.out.

Common causes of a shortCommon causes of a short

Power DistributionPower Distribution

An electrical distribution system is required An electrical distribution system is required in order to convey the electrical power to in order to convey the electrical power to the equipments and systems that need it.the equipments and systems that need it.

Busbar systems Busbar systems Wire and cablesWire and cables

Busbar SystemBusbar System

In most types of aircraft, output from the In most types of aircraft, output from the generators is sent to one or more generators is sent to one or more conductors before distribution throughout conductors before distribution throughout the system.the system.

These conductors are called busbars and These conductors are called busbars and they act as distribution centers for electric they act as distribution centers for electric power.power.

A busbar system is set up so that each A busbar system is set up so that each power source supplies one or more power source supplies one or more specific buses.specific buses.

Analogy to busAnalogy to bus

A variety of electrical items are hooked up A variety of electrical items are hooked up to each bus for power.to each bus for power.

The buses are interconnected via circuit The buses are interconnected via circuit protection devices which are called bus protection devices which are called bus ties.ties.

Bus tiesBus ties are switches or relays used to are switches or relays used to connect or disconnect buses from one connect or disconnect buses from one another.another.

They serve to isolate failed buses from They serve to isolate failed buses from working ones and/or send electrical power working ones and/or send electrical power to buses that lost their normal power to buses that lost their normal power sources.sources.

Each engine-driven generator, for Each engine-driven generator, for example, normally drives its own example, normally drives its own generator bus. High-current drawn items generator bus. High-current drawn items are connected to these buses.are connected to these buses.

Items like fixed fire extinguishers and Items like fixed fire extinguishers and emergency lights are usually powered off emergency lights are usually powered off battery bus. That way they are powered battery bus. That way they are powered for use even when no generator power is for use even when no generator power is available.available.

Typical 28vdc systemTypical 28vdc system

WIRES AND CABLESWIRES AND CABLES

Wires and cables conduct electrical power Wires and cables conduct electrical power in its various forms and quantities to and in its various forms and quantities to and between equipments.between equipments.

There are various types of wires used in There are various types of wires used in aircraft electrical system. The conductor is aircraft electrical system. The conductor is made of copper or aluminum. The made of copper or aluminum. The insulation material may be nylon, PVC, or insulation material may be nylon, PVC, or fiberglass. fiberglass.

Aircraft wiresAircraft wires

Wire size selectionWire size selection

The wires installed in an aircraft electrical The wires installed in an aircraft electrical system are chosen on the basis of ability to system are chosen on the basis of ability to carry the required currentcarry the required current

- without overheating and - without overheating and

- without producing an excessive voltage- without producing an excessive voltage

drop.drop. Electrical wire charts may be used for this Electrical wire charts may be used for this

purposes. purposes.

Maximum voltage drop in load Maximum voltage drop in load circuitscircuits

Wire chartWire chart

Electrical wire size measurementElectrical wire size measurement

Aircraft wiring diagramsAircraft wiring diagrams

There are different types of electrical There are different types of electrical diagrams available to understand electrical diagrams available to understand electrical systems. These diagrams may be in the systems. These diagrams may be in the following forms :following forms :

- Block diagrams- Block diagrams

- Pictorial diagrams- Pictorial diagrams

- Schematic diagrams- Schematic diagrams Many symbols are used in these Many symbols are used in these

diagrams. diagrams.

Pictorial electric diagramPictorial electric diagram

ELECTRICALDIAGRAM

Aircraft electrical system diagramAircraft electrical system diagram

Electrical loadsElectrical loads Once the aircraft electrical power has Once the aircraft electrical power has

been generated and distributed then it is been generated and distributed then it is available to the aircraft services.available to the aircraft services.

These electrical services are distributed These electrical services are distributed throughout the aircraft and may be broadly throughout the aircraft and may be broadly subdivided into following categories.subdivided into following categories.

MotorsMotors Lighting services Lighting services Heating servicesHeating services AvionicsAvionics

Aircraft Lighting SystemAircraft Lighting System

Lighting system represent an important Lighting system represent an important element of the aircraft electrical services.element of the aircraft electrical services.

External lighting systemsExternal lighting systems Internal lighting systemsInternal lighting systems

External lighting systemExternal lighting system

Provides illumination for such operation as Provides illumination for such operation as landing at night, inspection of icing landing at night, inspection of icing condition, and safety from midair collision.condition, and safety from midair collision.

Most common aircraft exterior lights :Most common aircraft exterior lights : Position lights (navigation lights)Position lights (navigation lights) Landing/taxi lightsLanding/taxi lights Anti-collision lightsAnti-collision lights Inspection lightsInspection lights

Position lights Position lights - Red at left wing tip- Red at left wing tip - Green at right wing tip- Green at right wing tip - White at vertical stabilizer- White at vertical stabilizer Anti-collision lightsAnti-collision lights - Rotating beam lights- Rotating beam lights - Usually at the top of fuselage or tail- Usually at the top of fuselage or tail Landing and taxi lightsLanding and taxi lights - Landing lights at the leading edge of - Landing lights at the leading edge of

wingswings - Taxi lights at nose landing gear - Taxi lights at nose landing gear

Exterior lightsExterior lights

Aircraft exterior lightsAircraft exterior lights

Exterior lightsExterior lights

Exterior lightsExterior lights

Exterior lightsExterior lights

Exterior lightsExterior lights