ELEC4504 Avionics Systems 175CHAPTER 15. Electrical SystemsAvionics and other aircraft systems require electrical power to operate. Typically the aircraft engines have electri-

cal generators incorporated in them to provide this power.

15.1. Basic equations:

Power = Voltage x Current

Voltage drop in a wire = Current x wire resistance

Power dissipated in a wire = (Current)2 x wire resistance OR (Voltage)2/wire resistance

Generator Load

current

Voltage (gen) Voltage (load)

Figure 84: General Power Circuit

In the diagram above the voltage at the load is less than the voltage at the generator due to the voltage drop in

the wire. Thus the power received by the load is less than the power produced by the generator by an amount

equal to the power dissipated in the wire.

15.2. Types of Aircraft power

DC (direct current)

DC power is standardized at 28 volts. It is relatively easy to generate but, because of the low voltage large

currents are required to transfer significant amounts of power. E.g. A one kilowatt load would draw about

35 amperes which is more than double the capacity of a normal house circuit.

To minimize the power loss through the wire, the wire must be of large diameter which increases the

weight.

One advantage of 28VDC is that there is less danger of electrical shock or arcing

AC (alternating current)

Aircraft alternating current operates at 400 Hz rather than the 60 Hz of the domestic system. The reason

for the higher frequency is that smaller (and lighter) transformers are needed at this frequency.

The voltages used are 115 V (for power transmission) and 26 V (for synchro excitation)

Alternating current has the advantage that the voltage can be altered easily with transformers.

It has the disadvantage that there is more danger of electrical shock and that it generates electromagnetic

fields which can interfere with the operation of some equipment.

176 ELEC4504 Avionics Systems Comparison of power requirements of MIL-STD 704 and DO160

Voltage MIL-STD-704 DO160

ac voltage (volts)

Normal 108.0 - 118.0 104.0-122.0

Over/Undervoltage 100.0-125.0 97.0-134.0

Emergency 108.0-118.0 (steady state) NA

ac frequency (Hz)

Normal 393-407 380-420

Abnormal 375-425 (Same as Normal)

Emergency 360-440 (steady state) 350-440

dc voltage (volts)

Normal 20.0-29.0 (28.0 nominal) 22.0-29.5 (27.5 nominal)

Emergency 18.0-29.0 (steady state) 18.0

Starting 12.0-29.0 (steady state) 10.0 for 15 seconds

15.3. Power Generation

The main source of power are the generators on the aircraft engines. These convert mechanical power to electrical

power by using the principle that by changing the amount of magnetic flux passing through a loop of wire will

cause a current to flow in that loop and a voltage to appear at the terminals

15.3.1 DC generation

If the output were taken directly from the terminals of the loop, the resulting voltage would be AC at a frequen-

cy determined by the rate of rotation of the loop. If, however a commutator is added, then the voltage will al-

ways be of the same sign since the commutator changes the sign just as the voltage reverses direction. Thus the

resulting voltage and current have a non-zero (DC) average. Although such a simple generator would provide

useful power through the use of a filter to get rid of the alternating component, it is more efficient to use several

loops and several commutators to provide a smooth current flow.

The magnetic field may be provided by a permanent magnet (which makes the generator heavy) or it may be

provided by the output voltage itself. Note that prior to starting there will be a residual magnetic field in the

electromagnet so that it will be self-starting.

ELEC4504 Avionics Systems 17715.3.2 AC generation

AC generation is simpler than DC generation in that commutation is not required. However, since the frequen-

cy of the output voltage is determined by the rate of rotation of the shaft, the power generated is not usually at

400Hz. This is usually called wild AC.

One way around this problem is to devise a mechanical constant speed drive between the engine and the gen-

erator. These are usually hydro-mechanical devices which are usually complex, expensive and prone to failure.

They are, however used in most large aircraft

The latest approach is to live with the variable frequency and to design the electrical equipment to tolerate the

varying frequency. This development is in conjunction with the trend to more electrical systems including con-

trol surface actuators which combine of electrical and hydraulic devices in

electrohydrostatic actuators (EHAs)

Another approach is to convert the wild AC to DC in a transformer/rectifier. The transformer changes the volt-

age down to 28 Volt and the rectifier converts the AC to DC.

LOAD

AC

GENERATOR

+

-

12

34

FILT

ER

Figure 85:

Full Wave Rectifier

In the diagram the rectifying diodes conduct current only in the direction of the arrow. Thus when the polarity

of the generator output is as shown, the current flow will be through diode 1, through the load from left to right

and back to the generator through diode 3. When the polarity is reversed the current will flow through diode 4,

through the load from left to right and back to the generator through diode 2. Thus the current flow through

the load is in the same direction for both polaritites. The resulting current flow still has a high AC component

(called ripple) and this is removed by the use of a filter .

The resulting output can then be used to service the DC devices in the aircraft.

For the AC equipment, a device called an inverter is used. Older inverters were simply a DC motor mounted

on the same shaft as an AC generator with a governor to maintain the correct speed to generate 400 Hz. More

modern inverters use a solid state design in which the DC drives a 400 Hz oscillator.

178 ELEC4504 Avionics Systems 15.3.3 Auxiliary Power Units (APU)

To provide power to operate aircraft systems on the ground most larger aircraft are equipped with an APU.

This is simply a small engine (usually turbine) which drives a generator. It is usually installed in the tail. This

unit can generate enough power to run the air conditioning system, the lighting and most of the avionics (espe-

cially the INUs if installed)

Note: some from of reverse current protection is required so that the engines do not drive the APU.

Some APUs can be operated in the air in an emergency

15.3.4 Ground power Units

Most large airports and maintenance facilities have ground power units which are capable of providing both

AC and DC power. The aircraft has a standard design socket into which the ground power unit can be plugged

15.3.5 Batteries

Batteries are used primarily to provide power to start the APU (if any) or to start the engines if an APU or GPU

is not available. They can be used in an emergency but the capacity would probably not be sufficient to run very

much for very long.

Almost all aircraft batteries are of the Nickel Cadmium (NiCd). These have the advantage of not emitting flam-

mable gases when being charged. (Lead Acid batteries emit hydrogen during the charging cycle). Another ad-

vantage of the NiCd battery is that it maintains its output voltage throughout most of its discharge cycle though

this characteristic makes it difficult to determine the state of charge.

One problem with NiCd batteries is that of thermal runaway. During recharge (for example after engine start)

it is possible for the battery to heat up. The batterys internal electrical resistance decreases with increased tem-

perature and thus the charging current increases which leads to further heating etc. etc.. If left unchecked, the

battery may explode or set fire to its surroundings. For this reason battery temperature sensors are installed and

the pilot can monitor the temperature during the period after a start.

15.4. Wiring

15.4.1 General

In order to transmit power from the sources to the user equipment it is necessary to provide wiring. Because

of the critical nature of the wiring in aircraft, MIL specification wire is usually used (MIL-W-22759)

ELEC4504 Avionics Systems 17915.4.2 Wire types and sizes

Wire usually consists of a centre conductor covered by and one or more layers of insulation and/or protection.

Since the temperature where the wire has to be installed may vary considerably, several types of conductor are

available:

a) - ordinary copper is suitable for temperatures up to about 105 C

b) - silver coated copper is used for temperatures up to 200 C

c) - nickle coated copper is used for temperatures up to 260 C

15.4.2.1 Insulation

As well as insulating the wire from contact with other wires and the airframe, the covering also must be

able to withstand abrasion (from vibration), hydraulic and other fluids. It also must not support

combustion.

It also must not produce toxic fumes if it is heated to high temperatures.

Typical coverings are:

a) - Polyvinylchoride (PVC) which is good up to 105 C

b) - Silicone rubber is good to 200 C and is highly flexible

c) - TFE Fluorocarbon is used for high temperature applications and is resistant to most fluids

15.4.2.2 Sizes

Wire sizes are designated by the American Wire Gauge (AWG) system in which smaller wires are assigned

larger numbers

e.g. AWG 22 has a diameter of 0.025 in.

AWG 14 has a diameter of 0.064 in.

15.4.2.3 Selection of wire.(free air, bundling)

The selection of wire for a particular application starts with the electrical requirements:

a) what current is going to be carried?

b) what is the length of the run?

c) what is the allowable voltage drop?

for 28 V system - 1 Volt

for 115 V system - 4 volts

From this information the wire gauge can be determined.

Note that different gauges are required for single wires and wires in bundles. This is due to the power

dissipation vs temperature requirement.

Once the wire gauge has been determined, the type of conductor and insulation can be chosen for the

expected temperatures in the area of installation.

180 ELEC4504 Avionics Systems 15.4.2.4 Identification of wire

All wire over 3 inches in length installed in an aircraft must bear an identification number to aid in

installation and maintenance.

Normally the convention followed is as in the following example.:

2NAV152B22

where the 2NAV indicates navigation system number 2, 152 indicates the numerical sequence of the wire,

B indicates it is the second segment in the run and 22 indicates the wire gauge.

On a wiring drawing it would be indicated as shown in

VOR 2 2NAV152A22 2NAV152B223 3 3 4

FLIGHT MANAGEMENT

SYSTEM

Figure 86: Example of Wiring Drawing

Twisted shielded wire is indicated as follows. The shield must be continuous electrically through connectors

VOR 2

2NAV152A22 2NAV152B226 6 6 17 7 7 2

8 8 8 3

Figure 87: Wiring Drawing for Twisted, Shielded Pair

15.4.2.5 Circuit Protection

Malfunctions in equipment can cause them to draw much more current than the wiring was originally

designed for. To prevent wires overheating and possible causing a fire, circuit protection devices are

installed in all circuits. There is usually one circuit protector for each unit. In order to protect the wiring

protection devices are installed as close to the source of power as possible.

ELEC4504 Avionics Systems 18115.4.2.5.1 Fuses

Fuses are essentially a strip of metal or wire which has a relatively low melting point such that it

will melt or fuse when the current exceeds a specified value. There are two main type called

slow blow and fast blow. Slow blow fuses are designed for use in circuits where there may be

momentary surges in current that do not pose a threat to the wiring. In these cases having the

fuse blow for each surge would be an unnecessary nuisance. Fast blow fuses are used where

surges are not expected or where they would cause damage.

The main problem with fuses is that once they have blown they have to be replaced (once the

cause of the high current has been removed). This means that spares must be available.

Fuses are mainly used in line replaceable units (LRUs) such as ILS receivers, communications

transceivers etc. i.e. black boxes. This is because malfunctions in these units can not be recti-

fied in flight and thus replacing a fuse would be useless.

15.4.2.5.2 Circuit Breakers

Circuit breakers isolate the circuit by means of a mechanical trip device which is activated by

the heating of a bimetallic element. They have the advantage that they can be reset once the

fault has been remedied for instance the failed item could be turned off and the circuit breaker

reset in flight.

Circuit breaker panels are usually located in the cockpit so that the pilots have direct control

over the system. In some cases a circuit breaker can be pulled or activated manually to isolate

an LRU which does not have a direct switch control.

15.4.2.5.3 Connectors

Connectors are a necessary evil in aircraft wiring. They are used to carry electrical power and

signals through pressure bulkheads and to removable parts of the aircraft and to the LRUs them-

selves.

Most aircraft connectors are of the Mil Spec variety and come in a bewildering array of types,

sizes and configurations. They are typically very expensive and have long lead times.

As shown in the diagram connectors can have a variety of numbers and arrangement of pins and

sockets. Also they come with bulkhead, panel and cable fittings. The inserts can be pins or sock-

ets and the outer shell can have exterior threads or have a ring with mating interior threads.

Some connector types have a bayonet or push and twist type of locking mechanism instead of

screw threads. these are usually keyed to ensure that the cables are mated to the correct connec-

tors.

182 ELEC4504 Avionics Systems

Figure 88: Types of Aircraft Connectors

15.4.2.5.4 Reliability

In spite of the many years of development and the use of miltary specifications, connectors still

remain a weak link in the avionics systems.

In a recent study the USAF estimated that 40% of all avionics faults were caused by connectors

15.4.2.6 Busses and Interconnects

As with the example of the control system of the A320, electrical systems are organized to provide

redundant services in the case of a failure of parts of the system.

The power feeds to equipment are organized as busses and are designated as follows:

ELEC4504 Avionics Systems 183Emergency: absolutely necessary in the case of a crash or wheels-up landing etc.

Essential: Required to ensure safe flight in the event of an in-flight emergency

Non-essential: those items which can be shut down in an in-flight emergency

Figure 89: Typical Aircraft Power Distribution

Note: in the above diagram vital means emergency.

184 ELEC4504 Avionics Systems

CHAPTER 15. Electrical SystemsAvionics and other aircraft systems require electrical power to operate. Typically the aircraft engines have electrical generators incorporated in them to provide this power.

15.1. Basic equations:Power = Voltage x CurrentVoltage drop in a wire = Current x wire resistancePower dissipated in a wire = (Current)2 x wire resistance OR (Voltage)2/wire resistanceFigure 84:General Power CircuitIn the diagram above the voltage at the load is less than the voltage at the generator due to the voltage drop in the wire. Thus...

15.2. Types of Aircraft powerDC (direct current)AC (alternating current)Comparison of power requirements of MIL-STD 704 and DO160

15.3. Power GenerationThe main source of power are the generators on the aircraft engines. These convert mechanical power to electrical power by using...

15.3.1 DC generationIf the output were taken directly from the terminals of the loop, the resulting voltage would be AC at a frequency determined by...The magnetic field may be provided by a permanent magnet (which makes the generator heavy) or it may be provided by the output v...

15.3.2 AC generationAC generation is simpler than DC generation in that commutation is not required. However, since the frequency of the output volt...One way around this problem is to devise a mechanical constant speed drive between the engine and the generator. These are usual...The latest approach is to live with the variable frequency and to design the electrical equipment to tolerate the varying freque...electrohydrostatic actuators (EHAs)Another approach is to convert the wild AC to DC in a transformer/rectifier. The transformer changes the voltage down to 28 Volt and the rectifier converts the AC to DC.Figure 85:

Full Wave RectifierIn the diagram the rectifying diodes conduct current only in the direction of the arrow. Thus when the polarity of the generator...The resulting output can then be used to service the DC devices in the aircraft.For the AC equipment, a device called an inverter is used. Older inverters were simply a DC motor mounted on the same shaft as a...15.3.3 Auxiliary Power Units (APU)To provide power to operate aircraft systems on the ground most larger aircraft are equipped with an APU. This is simply a small...Note: some from of reverse current protection is required so that the engines do not drive the APU.Some APUs can be operated in the air in an emergency

15.3.4 Ground power UnitsMost large airports and maintenance facilities have ground power units which are capable of providing both AC and DC power. The aircraft has a standard design socket into which the ground power unit can be plugged

15.3.5 BatteriesBatteries are used primarily to provide power to start the APU (if any) or to start the engines if an APU or GPU is not available. They can be used in an emergency but the capacity would probably not be sufficient to run very much for very long.Almost all aircraft batteries are of the Nickel Cadmium (NiCd). These have the advantage of not emitting flammable gases when be...One problem with NiCd batteries is that of thermal runaway. During recharge (for example after engine start) it is possible for ...

15.4. Wiring15.4.1 GeneralIn order to transmit power from the sources to the user equipment it is necessary to provide wiring. Because of the critical nature of the wiring in aircraft, MIL specification wire is usually used (MIL-W-22759)

15.4.2 Wire types and sizesWire usually consists of a centre conductor covered by and one or more layers of insulation and/or protection. Since the temperature where the wire has to be installed may vary considerably, several types of conductor are available:a) - ordinary copper is suitable for temperatures up to about 105 Cb) - silver coated copper is used for temperatures up to 200 Cc) - nickle coated copper is used for temperatures up to 260 C

15.4.2.1 Insulationa) - Polyvinylchoride (PVC) which is good up to 105 Cb) - Silicone rubber is good to 200 C and is highly flexiblec) - TFE Fluorocarbon is used for high temperature applications and is resistant to most fluids

15.4.2.2 Sizes15.4.2.3 Selection of wire.(free air, bundling)a) what current is going to be carried?b) what is the length of the run?c) what is the allowable voltage drop?

15.4.2.4 Identification of wireFigure 86:Example of Wiring DrawingTwisted shielded wire is indicated as follows. The shield must be continuous electrically through connectors

Figure 87:Wiring Drawing for Twisted, Shielded Pair

15.4.2.5 Circuit Protection15.4.2.5.1 Fuses15.4.2.5.2 Circuit Breakers15.4.2.5.3 ConnectorsFigure 88:Types of Aircraft Connectors

15.4.2.5.4 Reliability

15.4.2.6 Busses and InterconnectsFigure 89:Typical Aircraft Power DistributionNote: in the above diagram vital means emergency.