AIRCRAFT ELECTRICAL
SYSTEMS
Objectives
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Students will be able to:
Describe the basic components of aircraft
electrical system
Explain operation of electrical system
Interpret aircraft electrical diagram
Select proper size of wire for installation
Describe basic causes of electrical system
malfunctions
Elements of aircraft electrical
systems
 An aircraft electrical system is mainly
composed of :
 Power sources
 Components
- Control devices
- Conversion devices
- Protection devices.
 Power distribution systems
 Electrical loads
Electrical Power Sources
Electricity power sources on an aircraft may
be classified into two groups :
 Batteries
- Lead-acid
- Nickel-cadmium
 Generators
- DC generators
- AC generators
Battery
 A battery is a device that converts
chemical energy into electrical energy.
 It is a power reservoir that stores energy in
chemical form.It does not produce energy.
 Its functions are:
- To provide power when no other power
source is available
- To assist in damping transient loads in
the dc system
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- To provide a short term source during
emergency condition
The capacity of battery is measured in
ampere-hours.
Its normal rate is a little over 24vdc in a
28vdc system.
It is automatically recharged when the
engine-driven generator is operational.
Two types of batteries are used in aircraft
- Lead-acid batteries
- Nickel cadmium batteries
Lead-acid battery
 It is usually found in piston aircraft.
 It is made up of cells which have
positive/negative plates of lead and filled
with electrolyte of sulfuric acid and water.
 Each cell has app. 2.2v, but is simply rated
as 2 v.
 It has corrosive effects.
 Frequent total discharge and remaining
battery in discharged condition for a long
time will shorten the life of the battery.
Battery Cell
Nickel cadmium battery
 They are the most common type of battery
used in turboprop and turbojet aircraft.
 They provide electrical discharge at a high
rate without voltage drop and accept high
charge rates that shorten recharge time
 They may be subject to thermal runaway
condition caused by overheating, in which
the battery destructs itself.
Nickel-cadmium battery
Cell
Terminals
Vent
pipe
Container
Main
connector
GENERATORS
 A generator is a machine that converts
mechanical energy into electrical energy by
the process of electromagnetic induction
 They are driven by the engine(s), and
sometimes by APU.
 A generator must be rated at adequate
amperage to drive all the operating
components on its circuit(s). The current
required to operate each electrical
component is known as its load.
 All generators produce alternating current
naturally.The method which is used to take
it from the coil will determine if the
generator provides ac or dc to the circuit.
 If a commutator is used for this purpose it
will be a dc generator.
 If a slip ring is used it will be an ac
generator.
 DC GENERATORS:
 Commutator in the generator converts the
ac current to dc current.
 DC generator is designed to supply nearly
constant voltage. Usually the voltage is
28vdc, but there are 270vdc systems
developed recently.
 They supply current up to 400 A.
 Most aircraft do not have a separate dc
generator, but ac produced by ac
generator is converted to dc to power dc
systems.
Basic dc generator
Operation of a basic dc generator
Producing steady dc
AC generator (Alternator)
 On most large aircraft high-load electrical
devices are usually AC powered.
 AC is produced by ac generator which is
usually called alternator.
 AC generators use slip rings instead of
commutators to provide the current to the
circuits.
 Alternator generates three-phase current,
and standard aircraft voltage is 115 vac
with 400 Hz.
Basic ac generator
Modern electrical power generation
types
Mainly two types of electrical power
generation currently in use on aircraft.
- Constant frequency Integrated drive
generators (IDG)
- Variable speed constant frequency
(VSCF) generators
Constant Speed IDG
 It is essential that the alternator output
have a frequency of 400 Hz with a very
small tolerance. Thus rotational speed of
alternator
must
remain
absolutely
constant.
 This is accomplished by using a Constant
Speed Drive (CSD) unit between the
engine and alternator.
 CSD provides a constant alternator rpm
within a specified engine rpm range.
 An alternator and constant speed drive
unit combination is called integrated drive
generator (IDG).
 CSD is hydromechanically operated and
needs to be correctly maintained in terms
of oil level and oil cleanliness.
 It is the most commonly used power
generation method on today turbinepowered aircraft.
Integrated Drive Generator
Constant speed drive
Constant Speed Drive
VSCF
 In this method, the variable frequency power
produced by the alternator is converted to
constant frequency 400 Hz, 115 vac by
solid-state devices electronically.
 Thus there is no need for CSD unit.
 It is a new technique and limited in use.
Variable speed constant frequency
generator
Power Generation Control
 Power produced by generators is usually
controlled by generator control unit (GCU).
 The main functions of this device are:
- Act as a voltage regulator
- Direct current to battery for recharging
- Provide circuit and generator protection
by disconnecting the generator from the
system when electrical abnormalities
occur.
Control Devices
 These devices are use to initiate and control
the operation of the circuits.Control devices
include
 Switches
 Rheostats
 Relays
 Solenoids
Switches
 A switch is used to start, to stop, or to
change the direction of the current flow in
the circuit.
 Toggle switches :They are on/off switches
and extensively used in aircraft electrical
system.
 Push Switches : They are used primarily
for operations of short durations
 Rotary switches :When it is necessary to
select several condition for a circuit, a
rotary switch may be used.
Rotary Switch
Toggle switch
Rheostats : control the amount of current that
flows throughout the circuit and used as
dimmer devices for instrument and cockpit
lights.
Relays :Electromagnetic switching devices
which are used to remotely control electric
circuits carrying large amount of current.
Solenoids : Remote control devices quite
similar to relays; but they are designed to
move a shaft over a short distance. Thus,
solenoids are used as mechanical control
devices to operate hydraulic or pneumatic
valves, locking pins, etc.
Rheostat
Schematic of a relay
Relay
Conversion Devices
There are many occasions within an
aircraft electrical system where it is
required to convert power from one form to
another.
Typical examples of power conversion are :
- Conversion from dc to ac power
- Conversion from 115vac to 28 vdc
- Conversion from one ac voltage level to
another
- Battery charging (from 115vac to 28vdc)
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Following devices are used for these
purposes :
Transformers
Rectifiers
Transformer-rectifier units (TRUs)
Inverters
Transistors
 Transformers : Used to change ac voltage
level.
 Rectifiers : Used to convert ac into highamperage, low-voltage dc.
 TRUs: Combination of transformers and
rectifiers and used as main unit to convert
alternator output to dc in an aircraft
 Inverters : convert 28 vdc power to 26 vac
power particularly for flight instruments or
115 vac.
 Transistors : Electronic devices that
control electron flow and convert ac to dc.
Transformer
Analogy to TRU (ac adapter)
TRUs
Inverter
Analogy to inverter (ups for
computer)
Protection Devices
 These devices are used to protect circuits,
cables and system components from
damage due to failures.
 Fuses
 Circuit breakers
 Diodes
 GCUs
 Fuses : They are designed to protect the
cables against the flow of short-circuit and
excessive current. They break the circuit
and stop the current flow when the current
exceeds a predetermined value.
 Circuit breakers :They have
same
function with the fuses.The difference from
the fuse is that they are resettable, while a
fuse must be replaced.
 Diodes : They are the electronic equivalent
to the check valves in hydraulic system.
They allow electricity to flow in one way
only.
Circuit Breakers
Circuit Breakers
Primary Causes of Electrical Circuit
Failures
 Open circuit :
It is a circuit that is not complete or
continuous. This is an uncommanded
interruption of electrical power tosome
components or systems. When an open
occurs the affected component stop to
operate, but the other components still
remain in operative condition.
Common causes of open circuit
 Short Circuit
It occurs when electricity is allowed to
take a shortcut through or around a
component or system. This is the most
serious problem. It has two effects :
- Affected components have no power
and fail to operate
- Since current will not flow through
affected components, the other
components will be subjected to higher
level of current causing them to burn
out.
Common causes of a short
Power Distribution
An electrical distribution system is required
in order to convey the electrical power to
the equipments and systems that need it.
 Busbar systems
 Wire and cables
Busbar System
 In most types of aircraft, output from the
generators is sent to one or more
conductors before distribution throughout
the system.
 These conductors are called busbars and
they act as distribution centers for electric
power.
 A busbar system is set up so that each
power source supplies one or more
specific buses.
Analogy to bus
 A variety of electrical items are hooked up
to each bus for power.
 The buses are interconnected via circuit
protection devices which are called bus
ties.
 Bus ties are switches or relays used to
connect or disconnect buses from one
another.
 They serve to isolate failed buses from
working ones and/or send electrical power
to buses that lost their normal power
sources.
 Each
engine-driven
generator,
for
example, normally drives its own
generator bus. High-current drawn items
are connected to these buses.
 Items like fixed fire extinguishers and
emergency lights are usually powered off
battery bus. That way they are powered
for use even when no generator power is
available.
Typical 28vdc system
WIRES AND CABLES
 Wires and cables conduct electrical power
in its various forms and quantities to and
between equipments.
 There are various types of wires used in
aircraft electrical system. The conductor is
made of copper or aluminum. The insulation
material may be nylon, PVC, or fiberglass.
Aircraft wires
Wire size selection
 The wires installed in an aircraft electrical
system are chosen on the basis of ability to
carry the required current
- without overheating and
- without producing an excessive voltage
drop.
 Electrical wire charts may be used for this
purposes.
Maximum voltage drop in load
circuits
Wire chart
Electrical wire size measurement
Aircraft wiring diagrams
 There are different types of electrical
diagrams available to understand electrical
systems. These diagrams may be in the
following forms :
- Block diagrams
- Pictorial diagrams
- Schematic diagrams
 Many symbols are used in these
diagrams.
Pictorial electric diagram
ELECTRICAL
DIAGRAM
Aircraft electrical system diagram
Electrical loads
 Once the aircraft electrical power has
been generated and distributed then it is
available to the aircraft services.
 These electrical services are distributed
throughout the aircraft and may be broadly
subdivided into following categories.
 Motors
 Lighting services
 Heating services
 Avionics
Aircraft Lighting System
Lighting system represent an important
element of the aircraft electrical services.
 External lighting systems
 Internal lighting systems
External lighting system
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Provides illumination for such operation as
landing at night, inspection of icing
condition, and safety from midair collision.
Most common aircraft exterior lights :
Position lights (navigation lights)
Landing/taxi lights
Anti-collision lights
Inspection lights
 Position lights
- Red at left wing tip
- Green at right wing tip
- White at vertical stabilizer
 Anti-collision lights
- Rotating beam lights
- Usually at the top of fuselage or tail
 Landing and taxi lights
- Landing lights at the leading edge of
wings
- Taxi lights at nose landing gear
Exterior lights
Aircraft exterior lights
Exterior lights
Exterior lights
Exterior lights
Exterior lights