188 Aircraft structures and systems There is one small drawback with inertial navigation, and it is the build-up of errors. Any slight inaccuracy in measuring accelerations will be multiplied up by the flight time, and this will give rise to further errors in calculating position. For short journeys with modern systems, the error is small, but extended journeys may see significant errors arise. For the purposes of normal navigation, this is not usually a major issue, but !" cannot be relied upon to position an aircraft accurately enough for automatic landings, for instance. #ver extended flight, the errors may be corrected by using some of the other methods described here, such as $%", where necessary. Fully programmed flight & t is now 'uite possible to fly the aircraft automatically, by programming way points, arrival times, flight levels and so on into the autopilot system. The aircraft system will set the speed, height and bearing as re'uired, making automatic corrections for wind speed and direction, and allowing the flight plan to be modified at any time by the pilot. (y combining some or all of these systems, an aircraft could fly an entire flight without pilot intervention. n the future, it is likely that many civil aircraft will not have pilots at all, but will be pre-programmed to fly a particular route unaided. "ystem failure is largely prevented by having several back-up systems, and as a last resort the aircraft could be controlled from the ground to complete a flight safely. The main problem with this would not be a technical one) it would be passenger resistance to a pilotless aircraft. *owever, once it can be sho+ + that automatic systems are safer than human pilots, this means of flying certainly become available. @CI..?NRiiH3 R q@4 /T0/A "1"T 2" Objectives: to describe the. principles of electrical systems, what power is generated, how it is distributed around an aircraft, what it and the backup systems used in case of an emergency. !T0#34/T#! An aircraft in flight re'uires a significant amount of power, much electrical. /learly, it is not feasible to store enough energ provide large 'uantities of power for the duration of a flight, due to t space and cost involved. (atteries store energy inefficiently, so the must generate the power needed continuously during flight. The power re'uired depends on the electrical demands from the aircraft, of course, allowance must be made for the emergency and failure conditions, so that the aircraft can operate within acceptable safety margins as re'ui the ulations. "ince the primary source 5and ultimately the only source6 of the aircraft+s engines, it is important to appreciate that the outpu these, and hence the electrical output, will vary during flight- as nroaches to land, the engines will be throttled back, hence less powe be lable to run generators. The electrical system must take account of this. n situation where a complete engine failure takes place, the aircraft must way of maintaining at least some function in the electrics, to try to down safely. The loads will depend on the type of aircraft. Typically, a large ai high demands in terms of in-flight entertainment systems 5F 6 and ger-related systems such as food preparation. /onversely, a fighter will ly have a powerful radar system and possibly other weapon-related that will create high demands on the electrical system.
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188 Aircraft structures and systems
There is one small drawback with inertial navigation, and it is the build-up of errors. Any slight inaccuracy in measuring accelerations will be multiplied up by the flight time, and this will give rise to further errors in calculating position. For short journeys with modern systems, the error is small, but
extended journeys may see significant errors arise. For the purposes of normal navigation, this is not usually a major issue, but !" cannot be relied upon to position an aircraft accurately enough for automatic landings, for instance. #ver extended flight, the errors may be corrected by using some of the other methods described here, such as $%", where necessary.
Fully programmed flight
& t is now 'uite possible to fly the aircraft automatically, by programming way points, arrival times, flight levels and so on into the autopilot system. The
aircraft system will set the speed, height and bearing as re'uired, making automatic corrections for wind speed and direction, and allowing the flight plan to be modified at any time by the pilot. (y combining some or all of these systems, an aircraft could fly an entire flight without pilot intervention.
n the future, it is likely that many civil aircraft will not have pilots at all, but will be pre-programmed to fly a particular route unaided. "ystem failure is largely prevented by having several back-up systems, and as a last resort the aircraft could be controlled from the ground to complete a flight safely. The main problem with this would not be a technical one) it would be
passenger resistance to a pilotless aircraft. *owever, once it can be sho++ that automatic systems are safer than human pilots, this means of flying
certainly become available.
@CI..?NRiiH3 R q@4
/T0/A "1"T2"
Objectives: to describe the. principles of electrical systems, how and what
power is generated, how it is distributed around an aircraft, what it is used for,
and the backup systems used in case of an emergency.
!T0#34/T#!
An aircraft in flight re'uires a significant amount of power, much of which is electrical. /learly, it is not feasible to store enough energy in batteries to provide large 'uantities of power for the duration of a flight, due to the weight, space and cost involved. (atteries store energy inefficiently, so the aircraft
must generate the power needed continuously during flight. The power re'uired depends on the electrical demands from the aircraft, of course, but an allowance must be made for the emergency and failure conditions, so that the aircraft can operate within acceptable safety margins as re'uired by the
ulations. "ince the primary source 5and ultimately the only source6 of power the aircraft+s engines, it is important to appreciate that the output capability
these, and hence the electrical output, will vary during flight- as the aircraft
nroaches to land, the engines will be throttled back, hence less power willbe
lable to run generators. The electrical system must take account of this. n situation where a complete engine failure takes place, the aircraft must have way of maintaining at least some function in the electrics, to try to bring the
down safely.
The loads will depend on the type of aircraft. Typically, a large airliner will high demands in terms of in-flight entertainment systems 5F6 and other
ger-related systems such as food preparation. /onversely, a fighter will ly have a powerful radar system and possibly other weapon-related
that will create high demands on the electrical system.
0
The electrical system may be broken down into a number of main areas:
7. %o wer gen erat ion, stor age and con ver sion
= .
course, on the type of aircraft, its si>e,
number of engines and its missions.
/learly a light aircraft such as a
/essna 7?; will need a different setup
from an Airbus A<@9, and different
again from a urofighter Typhoon.
These different types will have other
systems that differ, and again this will
affect the architecture of the electrical
system.
( A "
come in two main types 3/ or
direct current, where the voltage
remains essentially constant with time, and A/ or alternating current, where the voltage varies continuously, usually as a
sinusoid. ach type has advantages and disadvantages,
and the two must exist separately,
although it is possible to convert
between them. 3/ is primarily used at low voltage, and the standard voltage for aircraft is
;@. #ther voltages may be used,
but are usually derived where needed. 3/ may be used to
charge batteries, and these batteries will then return 3/
voltage into the circuit when
re'uired. Bhere a considerable
will demand high currents 5since
power C volts x amps6, and high
currents re'uire thick, heavy
low-voltage circuits, losses due to electrical resistance are higher, and
these losses result in heating of cables. n extreme, overload cases,
electrical fires can result, so the circuits
must be protected from exceeding their
rated current. ow-voltage 3/ is non-
lethal to humans. %rovided the voltages
are the same, two or more 3/ supplies
may be connected 5tied6.
mains supplies for instance, and
77?D;99 on aircraft6. The higher
voltage means that lower currents flow for a given power, hence cables may be
smaller and lighter. "ince alternating current does not show a constant
voltage with time, there are time-related factors to consider - these are
fre'uency, measured in hert> 5*>,
cycles per second6, and phase,
measured as an angle relative to a
reference or datum. f two supplies are
to be connected together, then both the
fre'uency and the phase must be
identical, or large currents will flow
between them. This has important
conse'uences for aircraft, as will be
Electrical systems 787
system is a three-phase system, with
each phase carried in a separate wire
5this is explained later in this chapter6.
oads may use one or all three phases
5rarely two6, and there are different ways of
wiring the phases. 5*ousehold mains is
also three-phase, although each dwelling will generally only receive one phase.
arger consumers such as industrial sites will have a three-phase supply.6
lectricity can only flow around a circuit- a return path is needed 5except in
extreme cases like lightning6. n many
cases this is through the airframe itself
5called +ground+6, although this may not be
sufficient, in which case a bonding lead will
be supplied, or a dedicated ground
connection will be included in the wiring
loom.
As has already been said, it is not feasible
to store enough electrical energy to meet
the demands of the aircraft in flight, so
storage is limited. The vast majority of the
energy used will need to be generated in
flight, and the main method for this is to use
some form of engine-driven generators.
Bith multi engined aircraft, it is normal to fit
one generator to each engine. There will
also often be an auxiliary power unit
5A%46, and this, too, will drive a generator.
ight aircraft will not normally have an A/
system, so a single 3/ generator will be
fitted to each 5or the only6 engine. *owever, the designer has a degree of
choice when it comes to larger
aircraft, depending on the electrical
demands. t is possible to generate only A/ power, and to convert this to supply the 3/
systems. t is likewise possible to generate only 3/ and derive A/ power
by converting i t, although the maximum
power that can reasonably be used is
limited by the current that can be drawn.
The backup systems will often re'uire that
it is possible to convert between the two in
any case. 2ost large airliners and many combat aircraft generate both supplies, but
also have the facility to convert between them.
3 / p o w e r
o n
3irect current is created by using a 3/ generator, which is normally engine driven
from the accessory gearbox. The voltage produced by the generator coils will be
proportional to the speed at which the generator is driven, usually a fixed ratio of
engine speed determined by the gear ratio,
and the load on the system. *owever, to be
usable, the output voltage must be kept
within acceptable limits - typically 79E of
the nominal voltage of ;@. This is
achieved by a regulation system, which
increases the output of the generator
automatically in response to the voltage
falling. The electrical power is generated by
rotating coils of wire inside a
magnetic field, which induces an 2F in the wire. The coils of wire are
78; Aircraft structures and systems
attached to a rotating shaft, and the entire assembly is known as the armature.
n fact, a 3/ generator generates an A/ voltage internally. This is then rectified 5the negative half of the sine wave is inverted6, then smoothed to create a 3/ voltage with an acceptable ripple. 2ost generators contain several coils, each of which will generate a sine wave of different phase from the others 5Figure 7=.76 - when these are combined, the result is a smoother output. The rectification is achieved by a commutator inside the generator - two brushes are in contact with insulated metal contacts that each extend half- way around the generator shaft, so the electrical connection is reversed as the shaft rotates. 3iodes prevent the current flowing backwards between coils in the generator. Bith a simple generator using permanent magnets to provide the field, the induced voltage is proportional to the rotational speed, so the voltage at minimum expected generator speed needs to be sufficient 5in this case, ;@6. For gas turbine engines this is less of an issue than for piston engines, since turbines tend to run at a relatively high speed even at idle.
*owever, this still produces a generator that is larger and heavier than is ideal, and energy is wasted at higher speeds.
(/)
-
Figure 7=.7 0ectified output of internally generated A/. Each set of coils produces a sine wave, which is then rectified to a half-sine by the commutator. With multiple coils, the resulting voltage is shown by the heavy line running along the peaks of each sinusoidal voltage output. The remaining ripple on this line can then be smoothed to produce a clean, steady D voltage, normally !" volts.
Electrical systems 78<
coils pass. The current through the field coils determines the strength of the field, and this is automatically adjusted by the regulator to maintain a constant output voltage. !o method of energy conversion 5which
is what a generator is6 can be
799E efficient- there are always losses. n this case, the losses are due to hysteresis and resistance heating in the windings. n low-power systems, natural convection and radiation may be sufficient to remove this heat, but for large generators, which generate many kilowatts of electrical power, even a small loss results in a lot of heat to be removed. This heat removal is very important to ensure a long, reliable operating life and to avoid fires. The generator will normally have cooling holes around the body, bearings at each end of the shaft to reduce friction and large brushes to allow for large currents without excessive heating. The cooling air may be forced, for instance by using ram-air from a scoop outside the aircraft skin or engine nacelle, and the forward speed of the aircraft creates a small pressure that pushes the air through the generator.
The drive to the generator may be
through a splined shaft, or via a pulley
and belt. arger generators will use a
splined shaft, as it is more robust and
needs little or no maintenance. The
belt-and-pulley arrangement has the
advantage of being simple and light, but
does re'uire routine adjustment to avoid slippage of the belt. This is achieved by
having one of the mountings as a slot, rather than a bolt hole, so one side of the
generator can be swung in or out to
tension the belt. The same system is
used on car alternators. n smaller aircraft, especially those
powered by piston engines, the
the characteristics of being able to
produce significant power over a wide range of rotational speed. n this case a three-phase, fre'uency-wild alternator provides outputs to a rectifier, consisting of a bank of diodes. ffectively, the diodes do the same job as the commutator in a generator. The term +fre'uency wild+ means that the alternator runs at a fixed proportion of the engine speed, which itself varies of course, so that the fre'uency of the output is not fixed. As in the generator, the field supply may be varied to regulate the output voltage. This works because the field strength 5flux6 varies in proportion to the field current. To remove
the generated power, instead of a commutator used in the generator, which alternately contacts each brush as the armature rotates, the alternator armature has slip rings, which are complete, unbroken rings, and each brush runs only on one slip ring.
0egulation may be achieved by a range of methods, but in principle these involve reducing or eliminating the field supply when a threshold voltage, current, or both, is reached. #nce the output falls, the field supply is restored. This happens many times per second - typically ?9 to ;99*>. 2ost modern systems use solid-state regulators rather than the mechanical
7 8 =
reliable than mechanical devices.
which is usually the case, it is
desirable that there is no break in the supply, even for a moment, if
an engine or generator fails in flight. 2any electrical systems are
sensitive to power drop-outs. n
the case of a twin-engine aircraft,
for example, each generator will be
connected to its own 3/ bus
5which will be explained later6. t is
common to have these buses
connected to each other, so that
the generator of one bus can
supply the loads on both if the
other generator output is lost.
*owever, this can cause problems
with the regulators, causing the
generators to +fight+ and each regulator trying to control the
output of both generators. This can de-stabilise the system. The
problem is resolved by connecting
the generators together using
units by ensuring the field currents
are balanced.
g e n e r a t i o n
$enerating A/ power is rather more
complex than 3/ in some ways,
because of the choice of arrangements
that may be used, and the need to
match fre'uency and phase if the
buses are to be interconnected. "ince
the system consists of three phases,
there are several ways in which the
phases may be connected to loads, but that will be explained later.
Bhere the voltage output from a 3/ system may be easily described,
assuming it is fairly steady with time,
the output from an A/ system can be
measured or described in different
ways. "ince the amplitude 5voltage6
varies continually with time, it is
possible to identify the peak voltage
and use that to define the signal.
*owever, the power, measured in
watts, is not e'ual to the peak voltage
multiplied by the peak current. Also, if
the voltage is simply averaged over the
cycle it will be >ero, since the cycle has
a negative half and a positive half. The most common way of specifying the
voltage of an A/ supply is therefore to use the 02", or root-mean-s'uare,
value. The voltage at each point in time
is measured, and s'uared. These
values are then averaged over a whole
cycle and the s'uare root calculated.
This gives what may be considered to
be a usable average value. t turns out
that for a pure sine wave the 02"
value is 9.9 5i.e. ..j;6 times the peak
value.
A/ power, and that is the power
factor. For purely resistive loads, the
current varies directly with voltage, so
multiplying 02" voltage by the 02" current gives the power in
watts. The output of most alternators is large, however, so it
is expressed in kilowatts 5kB6.
Bhere the loads have significant
capacitive or inductive
current and voltage, and this will
change the power delivered and
the power consumed. The
called the power factor 5%F6. For
this
Electrical systems 78?
Figure 14.2 lectrical generator. This is an integrated- drive generator or l#$, which has a constant-speed drive
unit incorporated. The generator %actually an alternator since it generates alternating current& is driven from the accessory gearbo' of a main gas- turbine engine, to which it is attached, and the constant-speed drives converts the variable speed of the engine to a constant speed input to the generator. This means that the fre(uency output of the generator will be constant, and can be matched to the other generator or generators on the aircraft. The three output cables can be clearly seen at top left. This particular l#$ is fitted to a )oeing 767.
reason, the output of an alternator 5and
the power consumption of A/ loads6 is
therefore normally measured in kA
5kilovolt-amps6, where the power factor is
e'ual to the effective power, measured in
kilowatts, divided by the apparent power,
measured in kA. The principle of operation of the
alternator itself, though, is the same as
that already explained for alternator
supplying a 3/ system - a magnetic
field is created within the alternator and the armature, with coils of wire attached,
rotates to induce a voltage. For an A/ system, the output is always in three
phases. n its simplest form, there would be two wires per
1
9
6
e
m
s
Fi x e d- fr e ' u e n c y or fr e ' u e n c y- w il d G
2ost A/ systems used on aircraft use a fixed fre'uency of =99*>, or cycles per second 5each cycle taking ;.? mill iseconds6. This gives several advantages:
7. The impedance1 for loads containing active 5capacitive or inductive6 components, such as motors, is fixed because the fre'uency is fixed. This means that performance of the loads will not change with engine speed, provided the supply voltage can be maintained.
;. %rovided the phases are synchronised between the buses, buses may be linked 5tied6, preventing power drop-outs in the event of partial power loss and allowing load sharing between alternators.
7 mpedance is the e'uivalent of resistance
but for A/ rather than 3/ systems, and it varies
with fre'uency for active loads.
Electrical systems 197
<. "ome loads are based on a fixed reference fre'uency, and in many cases the =99*> supply is sufficiently precise. Bith variable- fre'uency supplies, this reference fre'uency must be created separately.
*owever, this system does have some drawbacks. n order to maintain a fixed fre'uency and phase synchronisation over a range of engine speeds, the alternators must be driven at an exactly constant speed. This is normally performed using either a constant- speed drive unit 5/"346, and in some cases this is built into the alternator, which is then known as an 3$ or integrated-drive generator 5Figure 7=.;6. n either case, the constant-speed drive must provide a continuously variable transmission ratio, and allow very precise adjustment of the speed and phase to maintain synchronisation. To do this, /"34s employ a hydraulic coupling -
a hydraulic pump and a hydraulic motor in series. The speed of the pump is at the proportion of the engine speed determined by the drive ratios in the engine+s accessory gearbox, and the delivery is fixed by the design of the pump. The motor is a swash-plate motor, and so its displacement per revolution can be varied by changing the angle of the swash-plate. 5"wash-plate motors and pumps are described in /hapter Fifteen.6 "ince it must use the fluid output from the motor at exactly the same rate as it is delivered, reducing the displacement will cause the speed to increase, giving control over the ratio of the hydraulic transmission. A governor then controls the transmission so that its speed remains constant over the entire speed range of the aircraft+s engines.
/"34s are complex and unreliable components. They are also heavy,
1%hase;
1%hase<
...
Figure 14.3 "tar and delta wiring. Three-phase loads %and generator coils& may be connected in two ways. The configuration shown on the left is called star wiring, where each phase of the load is wired from one supply phase to a common return %often called ground&, seen here as the point where
... ...
be higher if the impedance is the same. 7owever no return connection is re(uired.
Figure 14.4 Three-phase electrical supply. The three-phase supply consists of three separate voltage supplies, each phased at F;9J to the others. The potential of each line is at /01 2 peak with respect to aircraft ground %earth&, and !44 2 peak with respect to the other phases. 8o a three-phase supply contains three live lines and one ground. 8ince the three lines are out of phase, each line acts as a return to the others at some part of the cycle, or 9ground9 may be used, depending on how the e(uipment is wired.
7 8 @
e
m
s
discharged as heat. #bviously, a way of eliminating them would be very attractive. (ecause the alternator would no longer run at a constant speed, such a system would have variable fre'uency, and phase synchronisation between alternators would not be possible. The loads within the electrical system would therefore need to be able to cope with a fre'uency-wild system. "ince it is no longer possible to tie A/ buses together
when their individual alternators are running, an alternator failure would result in a momentary drop-out of power on the bus it supplies, until a contactor can be operated to tie another bus and restore power. This issue can be critical, and can affect systems design and choice of components.
4ntil recently, fre'uency-wild systems were rare. *owever, with
increasing pressures to reduce weight and
energy losses and increase reliability, the
benefits offered are more attractive.
$ r o u n d
p o w e r
Figure 14.! $round-power cart. A ground-power cart, seen here in the foreground alongside an aircraft tug, supplies A and*or D power to the aircraft through a ground-power receptacle. "t allows some of the electrical systems on the aircraft to be powered without the need to run the aircraft9s engines or au'iliary power unit, reducing fuel use and noise. #n most large aircraft, the hydraulics may also be run using the electrical hydraulic pumps provided as part of the backup
systems. %hoto: / (arcenilla
Electrical systems 788
and embarking and disembarking passengers. 0unning the main engines wastes fuel, as they must be run at very little load just to provide electrical power, and using the batteries will mean they are not fully charged for the next flight, removing the safety margin they provide. t is common to use an auxiliary power unit 5A%46 if power is needed for short periods. A%4s are described in /hapter "ixteen. For extended periods, or when power is re'uired intermittently, ground power is preferred. This consists of a cable that connects onto a receptacle on the outside of the aircraft, supplying A/, 3/ or both. The other end of the cable is connected to a ground-power cart, consisting of a large diesel engine and the appropriate electrical generators, together with controls. "uch a cart is often known in the 4K as a +*ouchin+, after a manufacturer of these items. "ome airports provide ground power through cabled supplies at the stands.
% o w e r
s t o r a g e
result of losing power on all engines - multiple generator failures are otherwise virtually
unheard of. This energy is stored in batteries, which
under normal operation are
constantly charged from one of the main electrical supplies. "ome batteries are of the lead-acid type, as used in most cars, although nickel-cadmium batteries are also used, which are capable of a higher energy density and can supply higher currents for short periods. (atteries would typically be located in the forward avionics bay in an airliner. (attery voltage is commonly ;@, and re'uire an A/-powered charger or other means of supplying a higher voltage to drive the charging current. ;= batteries are also found, which gives ade'uate supply voltage in an emergency
and means the batteries can be charged directly from a ;@ 3/ bus without re'uiring any voltage conversion. *owever, this system gives less control over the battery state and rate of charging. Typically, a separate battery is used for starting the auxiliary power unit.
n emergency cases, the first operation is to remove every single non essential drain on the electrical system. This is called load shedding and happens automatically, since only the systems that are absolutely essential, such as basic instruments and flight controls, are connected to the battery bus. n the event of failure, this bus is automatically disconnected from the other buses by the bus-tie contactor, and is then powered solely by the batteries. The bus-tie contactor allows loads on the battery bus to be driven by the main 3/ system under normal operation, but allows rapid
2
contactor is normally designed not to
allow current to flow from the battery
bus to the main 3/ bus5es6, preventing
inadvertent discharge of the batteries.
/ommonly, the battery bus is split
into two - the +hot battery bus+,
connected directly to the batteries,
and a separate battery bus, connected
through a bus tie 5see %ower
3istribution6. Bhere it is essential to
supply some A/ power in an
emergency, 3/ from the batteries can
be converted to A/, as described
below.
3/ power are generated by engine
mounted generators. *owever, in
smaller aircraft, and under certain conditions in large aircraft, it is more
convenient, or necessary, to convert
Figure 7=.L Transformer-rectifier. :ormally located in the avionics bay in an airliner or combat aircraft, the transformer-rectifier takes three- phase A and converts it to D. The thinner multi core cable entering the unit at upper-centre is the input, and the output is through the two separate cables below it. The output cables are much thicker- because the voltage has been transformed down, the output current will be higher for a given power.
Electrical systems ;97
between the two types. This will allow both supplies to be maintained in emergencies, and simplifies the power systems on aircraft of a si>e that does not warrant complex electrical systems. n both cases, there are two distinct operations re'uired - voltage conversion and conversion between A/ and 3/.
/onversion from 77?D;99 A/ to
;@ 3/ is carried out by transformer rectifiers 5T04s6, and is probably the
easier to understand. The high-voltage
A/ supply passes through a
transformer, reducing its voltage to around ;@. This signal, still at =99*> A/, must now be converted to 3/. This is
performed by a rectifier, which mirrors 5rectifies6 the negative half of the A/ wave. /ombining the rectified outputs of the three phases gives a signal that is already reasonably steady, as the rectified outputs overlap 5Figure 7=.;6. A smoothing circuit then reduces the ripple on the rectified signal, so that it more-closely resembles a pure 3/ signal.
/onversion from 3/ to A/ is slightly more difficult. The first units used a 3/
motor to drive an alternator, all contained within a single unit known as
a rotary inverter. Although they are still
used occasionally, they generally are of
low efficiency and are unreliable.
2odern systems use a device called a static inverter, and as the name
suggests these have no moving parts.
The 3/ supply is first converted to A/
using electronics 5a wave generator6.
This is then amplified and regulated to
produce the re'uired 77?D;99 output
at the desired power output. ach phase must be created separately, although
the three are usually combined into a single physical unit. The three phases
must, of course, be synchronised so that the correct phase relationship is
maintained between them. %ower conversion capacity from 3/
to A/ is 'uite limited due to the
currents needed from 3/ - it is not possible to generate all the A/ power
needed for a large aircraft from any 3/ source, as the current draw from the 3/
supplies would be enormous. For this
reason, conversion is usually limited to
specific re'uirements, or to supply the
minimum e'uipment needed in an
emergency.
(
the engines, and also in some
circumstances at the auxiliary power unit, a distribution system is needed to
take the power to where it is to be used. The first point at which the
supplies and distribution system come together is the buses 5sometimes
called busbars6 -a series of large
copper bars or strips capable of
carrying all the current re'uired for the
whole aircraft. These may be located
on or near the flight deck, or in an
avionics bay,
2"2 Aircraft
structures and
Electrical systems 2"3
typically beneath the flight deck, although other locations are possible. #ne copper bar is needed for 3/, and a further one for each A/ phase, although the combination of three phases is normally referred to as the +A/ bus+.
,+ (.)
where multiplegenerators are used. #n a typical airliner there might be two *+ t g c.
x M5 N 3/ buses, a separate battery bus,an emergency 3/ bus or hotbattery bus,
"%0-) .
( 9 .)
5. 6
/OP,#+l
l
and two or more A/ buses - perhaps one per engine for multi- engine
aircraft. #f course, if a fre'uency-wild
A/ system is used, then a separate
A/ bus must be provided for each
alternator, and they cannot be connected unless an alternator has
failed. For load sharing, it is normal to provide a method of linking the buses
together - known as bus-tie contactors, or simply bus ties. n normal flight
0 ,=77+5 00.900+.0 33 23
+9+ // / "##
.,#-
5all systems working6, these may be connected or they may be disconnected and only tied in the
event of supply failures. "ome 3/
bus ties are configured to allow current
in one direction only, and are known as
reverse-current contactors. n some
'
% . #
)
"
be prioritised: there is a +2ain+ 3/ bus and an +ssential+ one, with a
reverse-current contactor installed
may flow from the 2ain bus to the
ssential one, but not in the reverse
direction. The principle is that the
more-important e'uipment is run from
the ssential bus, so that if the 2ain
3/ generator fails the loads on the
2ain system will effectively be shed
automatically, reducing the load on
the ssential generator.
#
o &
The conn ectio ns betw een buse s, and man y other circu its, are princ ipall y
contr olled by relay s - elect
r i c a l
simultaneously, which is necessary
for three-phase supplies of course, and may even be wired to open
some contacts and close others in a single operation 5for instance
changeover contacts, where a
(/)
distribution
Figure 14.7 Aircraft electrical system. The electrical system in most aircraft is (uite comple', with parallel redundant systems for safety. All essential systems con be supplied from more than one of these systems, and the systems themselves can be cross-fed if re(uired. 3n this way no single failure, and only a limited number of unlikely multiple failures, can cause a complete loss of electrical power. llustration courtesy 0aytheon /orporate Pets nc.
From the buses, power is distributed
around the aircraft. This generally
takes the form of a wiring +ring+ -
both ends of a complete loop are connected to the bus, with suitable
circuit protection. This allows the current to flow either way around the
circuit, reducing peak currents and reducing
,9
9
2"4 Aircraft structures and systems
Figure 14.8 /ircuit breakers and breaker panel. This is the panel above the crew9s heads on a )oeing 767, showing ius= some of the many circuit breakers needed. 8ome breakers here have been 9pulled9 to disconnect them, and in this case have been fitted with flags to indicate this %inset&, as the aircraft is undergoing maintenance. A small collar fits under the breaker head to ensure if is not inadvertently closed, to protect maintenance personnel working on the aircraft.
the chance of mechanical
circuit. The wiring takes the
form of round, stranded wires,
similar to mains cable, bundled
into larger groups called looms.
/onnectors at suitable points
connected where re'uired.
airliner.
wire is determined by the cross-section of its
conductors. Thus a high voltage system can deliver a given amount of
power with a smaller current, hence re'uiring thinner wires, than a
low-voltage one. *owever, low voltages are safer. A ;@ system
will cause little or no sensation of electric
shock, and is non-lethal, to someone coming into contact with it, whereas voltages above about 7##A are considered
lethal. The actual probability of death
depends on the current passing through the
person+s body rather than
Electrical systems 2"!
voltage, so the lethal voltage varies widely, depending on the current+s path through the body, the person involved, whether it is A/ or 3/, and several other factors. *igh- voltage 3/ causes muscle clenching 5spasm6, sometimes preventing release of the person+s grip on the source of electricity, whereas A/ does not 5it causes alternate spasm and release as the current reverses6.
t+s important that the wire is correctly rated for the current it will carry, and that it is proof against moisture and other degradation. 1iany aircraft, especially military aircraft, may stay in service for half a century or more. The insulation is typically PTFE 5polytetrafluoroethane6, although new polymers are being developed all the time. t needs to provide good mechanical strength and abrasion resistance to prevent chafing, good electrical resistance and be resistant to chemical, water and biological attack 5fungi, insects, rodents6 and also to ultraviolet light. The conductors within the wires are invariably of stranded copper, providing good electrical conductivity and allowing the wire to bend and flex without breaking.
oads are often placed on distribution circuits in logical groups. n the event of problems, the crew must know that if they choose to isolate 5switch off6 a circuit, the systems that will be lost are logically grouped. For this reason, there are 'uite a lot of separate distribution circuits, allowing the crew to be very selective about what they disable. ach circuit is protected, as described below.
/ i r c u i t
o t e c t i o n
f too much current is allowed to pass through a cable or loom, the energy losses due to the electrical resistance of the cable will cause the cable to overheat, and if not stopped this could cause a fire. A fire on an aircraft in flight is generally considered to be catastrophic 5leading to loss of life6. (y design, all circuits must be rated for the maximum load they can carry, and protection is installed to prevent this maximum being exceeded. n domestic appliances, this is normally achieved using a fuse - a thin piece of wire that will melt above a specific current, breaking the circuit. Fuses are also used in aircraft, but another device known as a circuit breaker is faster and more flexible in use. 5!ote that circuit breakers are not the same as contact breakers, and carry out a different function.6 /ircuit breakers can be manually disconnected 5+pulled+6 to disconnect circuits, either in flight or during ground servicing 5Figure 7=.@6.
/ircuit breakers are grouped together in one or more electrical distribution panels, and have a small button on the top. Bhen connected, the button is pushed in, and if they trip, or are pulled, the button sticks out. A white or red marker band under the button makes it easy to see if a breaker is out. They are configured so that the circuit cannot be enabled by holding in the button, preventing anyone manually overriding circuit protection. "ome circuit breakers have a separate trip button.
2
" 6
A
i
# A 3 "
The types of electrical loads on an aircraft are very varied. There are thus different ways of categorising them, but for the purposes of this book we can consider large loads and small loads, and A/ and 3/ loads.
T
y
p
i
Actuation 5motors, linear actuators6 Anti-ice, deicing and de-misting
ntake and probe heating
#ther loads
Although it is not a hard-and-fast rule, it is generally found that large loads will be
powered from the A/ system, since it is capable of delivering high power with
moderate currents, making the wiring
lighter. t is also generally the case that
the majority of e'uipment on the flight
deck is powered from the ;@ 3/
airliners use the same system, which is to start the A%4 from its own batteries, then use either electrical power or bleed air from the A%4 to start the main engines - one at a time to limit the power re'uired. Bhere electrical starters are used, they are normally 3/ machines. The windings and internal components are fre'uently arranged so that after starting is completed the device will be turned by the engine and become a 3/ generator. "uch devices are known as starter-generators. The benefit of these is a significant weight saving compared to separate components.
"ome aircraft, notably the Airbus A<@9, use electrical actuators for
primary flight controls, as well as hydraulic ones. This gives a significant reduction in the number of systems re'uired, because it is possible to
provide the necessary redundancy shared between the electrical and hydraulic systems. An electric motor or linear actuator would not provide the right combination of response speed and power to provide an effective actuator, so the ones used for flight controls are electro-hydraulic actuators - the electrical supply is used to drive a hydraulic pump, and the actuation is performed hydraulically. This makes the actuator self-contained 5apart from the electrical connection6, reducing the weight since hydraulic supply pipes are not re'uired. Anti-ice, deicing and de-misting
devices are generally similar in function
- they use electrical heating to prevent
the build-up of ice on critical surfaces,
such as engine intakes, propeller
blades and leading edges. Anti-ice may
use chemical methods as an alternative,
which will create very little load on the
electrical system. ighting may be a major consumer of electrical power. nternal lighting on
passenger aircraft is mandatory, for safety reasons, and may be powered by
Electrical systems 2"7
A/ or 3/. There is also a re'uirement for emergency lighting, both to illuminate the cabin and for floor-level exit lighting. This emergency lighting must be capable of being supplied from the batteries in case of complete engine failure, or post-landing. /ockpit lighting is very important, of course. (oth ambient lighting and instrument lighting are provided, and are adjustable in intensity. Ambient lighting is kept at a low level during flight to avoid reducing the crew+s night vision. "mall reading lights on goose-neck stems are provided to allow the reading of charts and checklists.
xternal lighting serves a number of purposes on an aircraft. "trobe lights are fitted to the fuselage - intensely bright lights that flash to draw attention to the aircraft in flight, making it visible to other air traffic. !avigation lights are located at the wingtips and at the tail, showing the orientation of the aircraft. This is important to other air users, as they need to know the direction other aircraft are travelling in. A red light is fitted at the left 5port6 wingtip and a green light at the right 5starboard6. Bhite lights are fitted at the tail. Flashing red anti-collision lights may also be fitted to the upper and
lower fuselage.
#ther lighting may be fitted to help the crew see the state of the aircraft at night. Bing lights can be fitted, normally at the leading-edge root, so the entire leading edge can be seen from the flight deck at night. This allows the crew to see visually whether any ice has formed. 2any airliners are fitted with a logo light, on each side of the fin, ensuring that the operator+s branding can be clearly seen.
lectrical control systems are used in a wide variety of applications, from anti- skid brakes to windscreen wipers, flaps and undercarriages.
A /
l o a d s
An important point must be made here regarding the wiring of A/ loads. As
there are three phases, the loads may be connected in either of two different ways, known as star wiring and delta wiring. As has already been seen,
since there is a phase difference between the three phases, there is a voltage difference between any two phases at a given point in time. The voltage between phases is therefore ;99 peak for an aircraft system. 0elative to ground 5the airframe6 there is a voltage of only 77? peak on each phase. For a given power, the current drawn tends to be higher in the delta configuration.
2 "
8
A
i
r
c
r
a
f
t
"
As the electrical system is vital to the safe and effective operation of an
aircraft, so it is important to ensure that any failures will still allow the aircraft to
continue safe flight. "ystems are designed on the assumption that at some point in their life, failures will
occur. These failures can be predicted, and the results analysed. $enerally, no single failure must be allowed to create
a significant ha>ard, and predictable multiple failures must also be allowed for. This is the main reason why the
systems on board an aircraft
The provision of bus ties and power conversion has already
been covered in this chapter. A/ and 3/ generators are generally extremely reliable, and the major concern in the electrical system is the engine-out case. This is especially important with twin-engine aircraft, since a failure of one engine leaves little spare electrical capacity. n particular, &tops operation 5xtended Twin #%eration"6, which is flight over large expanses of water in twin-engine aircraft, is now common. An engine failure may occur when the aircraft is a long way from dry land, so it is important that the loss of a propulsion unit is not compounded by a major loss of electrical power, and the A%4 is regarded as a safety-critical item. A%4s are described in detail in /hapter "ixteen, but the provision of
electrical power from the A%4 is relevant here. The A%4 will drive at least one electrical generator 5and some large aircraft have two A%4s6, giving an additional power source to that provided by the main engines. n the event of a loss of a generator or an engine in flight, starting the A%4 is standard procedure. n the unlikely, but not unknown, event of a second engine failure, the A%4 will continue to provide power, and therefore maintain flight controls. t also alleviates some of the additional load on the remaining engine5s6 in taking over some of the load needed to power the electrical
s y s t e m .
0 a m - a i r
t u r b i n e s
n the event of total engine and A%4 failure in flight, normally from running out of fuel, many aircraft have a ram- air turbine 50AT6 that is deployed automatically from a stowage bay, usually under the fuselage on an airliner. t acts like a windmill, and drives a generator 5usually 3/6 and sometimes a hydraulic pump. "ince the A%4 will take some time to start, this provides vital time to keep control of the aircraft until the A%4 reaches full power. f the problem is lack of fuel, the A%4 will not start at all, of course. Although the power available from the 0AT is very limited compared to an A%4 or engine, it is capable of supplying much more power than the batteries are, giving the crew vital time in which to attempt to overcome the emergency. The batteries are, of course, the final resort.
Electrical systems 2"9
rather than loss of engines, the electrical system may be driven from the hydraulic system. lectrical pumps provide a backup for the hydraulic system, but these may be run in reverse, using the pump as a hydraulic motor and the electrical motor as a 3/ generator. This provides a further element of redundancy in both systems.
2#0-/T0/ A!3 A-
/T0/ A0/0AFT
Bith increasing demands on designers and operators to improve the environmental performance of their aircraft, and to reduce fuel costs, there is a move towards reducing the reliance on air drawn from the engines to supply the cabin pressurisation system, using electrical operation instead. This process is being extended towards the goal of powering all the onboard systems electrically, replacing hydraulics and pneumatics. This end-point is known as the all'electric aircraft, with intermediate stages being referred to as the more'
electric aircraft.
The drawback of taking compressor bleed air from the engines is that, as both the amount of air drawn and the speed of the engines, hence the engine air flow, vary independently, it is not possible to make full allowance for this +lost+ air
when designing the engines, and this makes the engines less efficient during some phases of operation. 0eplacing engine air bleed with larger electrical generators allows engine design to be better optimised for all flight phases, saving fuel and reducing emissions. The savings can be considerable. "ince the air used for cabin air systems leaves the engine at high temperature, and needs to be cooled before it can be used 5see /hapter "ixteen6, the waste heat is another loss of efficiency that can be improved.
*owever, this new approach is a radical change to the way in which aircraft systems are designed and is not without its problems. The amount of electrical power to supply all of the needs of the aircraft re'uires extremely large generators, and these are bulky and heavy. $enerators normally run more slowly than the engine shafts that drive them, re'uiring a gearbox to reduce the speed, and this reduces efficiency and adds weight to the engine. deally, the generator could run at engine shaft speed, eliminating the gearbox. Bork to make these changes to both engine and generator designs continues, as does the re-design of cabin-air systems to use electric pumps instead of bleed air. At the time of writing, a number of
There is one small drawback with inertial navigation, and it is the build-up of errors. Any slight inaccuracy in measuring accelerations will be multiplied up by the flight time, and this will give rise to further errors in calculating position. For short journeys with modern systems, the error is small, but
extended journeys may see significant errors arise. For the purposes of normal navigation, this is not usually a major issue, but !" cannot be relied upon to position an aircraft accurately enough for automatic landings, for instance. #ver extended flight, the errors may be corrected by using some of the other methods described here, such as $%", where necessary.
Fully programmed flight
& t is now 'uite possible to fly the aircraft automatically, by programming way points, arrival times, flight levels and so on into the autopilot system. The
aircraft system will set the speed, height and bearing as re'uired, making automatic corrections for wind speed and direction, and allowing the flight plan to be modified at any time by the pilot. (y combining some or all of these systems, an aircraft could fly an entire flight without pilot intervention.
n the future, it is likely that many civil aircraft will not have pilots at all, but will be pre-programmed to fly a particular route unaided. "ystem failure is largely prevented by having several back-up systems, and as a last resort the aircraft could be controlled from the ground to complete a flight safely. The main problem with this would not be a technical one) it would be
passenger resistance to a pilotless aircraft. *owever, once it can be sho++ that automatic systems are safer than human pilots, this means of flying
certainly become available.
@CI..?NRiiH3 R q@4
/T0/A "1"T2"
Objectives: to describe the. principles of electrical systems, how and what
power is generated, how it is distributed around an aircraft, what it is used for,
and the backup systems used in case of an emergency.
!T0#34/T#!
An aircraft in flight re'uires a significant amount of power, much of which is electrical. /learly, it is not feasible to store enough energy in batteries to provide large 'uantities of power for the duration of a flight, due to the weight, space and cost involved. (atteries store energy inefficiently, so the aircraft
must generate the power needed continuously during flight. The power re'uired depends on the electrical demands from the aircraft, of course, but an allowance must be made for the emergency and failure conditions, so that the aircraft can operate within acceptable safety margins as re'uired by the
ulations. "ince the primary source 5and ultimately the only source6 of power the aircraft+s engines, it is important to appreciate that the output capability
these, and hence the electrical output, will vary during flight- as the aircraft
nroaches to land, the engines will be throttled back, hence less power willbe
lable to run generators. The electrical system must take account of this. n situation where a complete engine failure takes place, the aircraft must have way of maintaining at least some function in the electrics, to try to bring the
down safely.
The loads will depend on the type of aircraft. Typically, a large airliner will high demands in terms of in-flight entertainment systems 5F6 and other
ger-related systems such as food preparation. /onversely, a fighter will ly have a powerful radar system and possibly other weapon-related
that will create high demands on the electrical system.
0
The electrical system may be broken down into a number of main areas:
7. %o wer gen erat ion, stor age and con ver sion
= .
course, on the type of aircraft, its si>e,
number of engines and its missions.
/learly a light aircraft such as a
/essna 7?; will need a different setup
from an Airbus A<@9, and different
again from a urofighter Typhoon.
These different types will have other
systems that differ, and again this will
affect the architecture of the electrical
system.
( A "
come in two main types 3/ or
direct current, where the voltage
remains essentially constant with time, and A/ or alternating current, where the voltage varies continuously, usually as a
sinusoid. ach type has advantages and disadvantages,
and the two must exist separately,
although it is possible to convert
between them. 3/ is primarily used at low voltage, and the standard voltage for aircraft is
;@. #ther voltages may be used,
but are usually derived where needed. 3/ may be used to
charge batteries, and these batteries will then return 3/
voltage into the circuit when
re'uired. Bhere a considerable
will demand high currents 5since
power C volts x amps6, and high
currents re'uire thick, heavy
low-voltage circuits, losses due to electrical resistance are higher, and
these losses result in heating of cables. n extreme, overload cases,
electrical fires can result, so the circuits
must be protected from exceeding their
rated current. ow-voltage 3/ is non-
lethal to humans. %rovided the voltages
are the same, two or more 3/ supplies
may be connected 5tied6.
mains supplies for instance, and
77?D;99 on aircraft6. The higher
voltage means that lower currents flow for a given power, hence cables may be
smaller and lighter. "ince alternating current does not show a constant
voltage with time, there are time-related factors to consider - these are
fre'uency, measured in hert> 5*>,
cycles per second6, and phase,
measured as an angle relative to a
reference or datum. f two supplies are
to be connected together, then both the
fre'uency and the phase must be
identical, or large currents will flow
between them. This has important
conse'uences for aircraft, as will be
Electrical systems 787
system is a three-phase system, with
each phase carried in a separate wire
5this is explained later in this chapter6.
oads may use one or all three phases
5rarely two6, and there are different ways of
wiring the phases. 5*ousehold mains is
also three-phase, although each dwelling will generally only receive one phase.
arger consumers such as industrial sites will have a three-phase supply.6
lectricity can only flow around a circuit- a return path is needed 5except in
extreme cases like lightning6. n many
cases this is through the airframe itself
5called +ground+6, although this may not be
sufficient, in which case a bonding lead will
be supplied, or a dedicated ground
connection will be included in the wiring
loom.
As has already been said, it is not feasible
to store enough electrical energy to meet
the demands of the aircraft in flight, so
storage is limited. The vast majority of the
energy used will need to be generated in
flight, and the main method for this is to use
some form of engine-driven generators.
Bith multi engined aircraft, it is normal to fit
one generator to each engine. There will
also often be an auxiliary power unit
5A%46, and this, too, will drive a generator.
ight aircraft will not normally have an A/
system, so a single 3/ generator will be
fitted to each 5or the only6 engine. *owever, the designer has a degree of
choice when it comes to larger
aircraft, depending on the electrical
demands. t is possible to generate only A/ power, and to convert this to supply the 3/
systems. t is likewise possible to generate only 3/ and derive A/ power
by converting i t, although the maximum
power that can reasonably be used is
limited by the current that can be drawn.
The backup systems will often re'uire that
it is possible to convert between the two in
any case. 2ost large airliners and many combat aircraft generate both supplies, but
also have the facility to convert between them.
3 / p o w e r
o n
3irect current is created by using a 3/ generator, which is normally engine driven
from the accessory gearbox. The voltage produced by the generator coils will be
proportional to the speed at which the generator is driven, usually a fixed ratio of
engine speed determined by the gear ratio,
and the load on the system. *owever, to be
usable, the output voltage must be kept
within acceptable limits - typically 79E of
the nominal voltage of ;@. This is
achieved by a regulation system, which
increases the output of the generator
automatically in response to the voltage
falling. The electrical power is generated by
rotating coils of wire inside a
magnetic field, which induces an 2F in the wire. The coils of wire are
78; Aircraft structures and systems
attached to a rotating shaft, and the entire assembly is known as the armature.
n fact, a 3/ generator generates an A/ voltage internally. This is then rectified 5the negative half of the sine wave is inverted6, then smoothed to create a 3/ voltage with an acceptable ripple. 2ost generators contain several coils, each of which will generate a sine wave of different phase from the others 5Figure 7=.76 - when these are combined, the result is a smoother output. The rectification is achieved by a commutator inside the generator - two brushes are in contact with insulated metal contacts that each extend half- way around the generator shaft, so the electrical connection is reversed as the shaft rotates. 3iodes prevent the current flowing backwards between coils in the generator. Bith a simple generator using permanent magnets to provide the field, the induced voltage is proportional to the rotational speed, so the voltage at minimum expected generator speed needs to be sufficient 5in this case, ;@6. For gas turbine engines this is less of an issue than for piston engines, since turbines tend to run at a relatively high speed even at idle.
*owever, this still produces a generator that is larger and heavier than is ideal, and energy is wasted at higher speeds.
(/)
-
Figure 7=.7 0ectified output of internally generated A/. Each set of coils produces a sine wave, which is then rectified to a half-sine by the commutator. With multiple coils, the resulting voltage is shown by the heavy line running along the peaks of each sinusoidal voltage output. The remaining ripple on this line can then be smoothed to produce a clean, steady D voltage, normally !" volts.
Electrical systems 78<
coils pass. The current through the field coils determines the strength of the field, and this is automatically adjusted by the regulator to maintain a constant output voltage. !o method of energy conversion 5which
is what a generator is6 can be
799E efficient- there are always losses. n this case, the losses are due to hysteresis and resistance heating in the windings. n low-power systems, natural convection and radiation may be sufficient to remove this heat, but for large generators, which generate many kilowatts of electrical power, even a small loss results in a lot of heat to be removed. This heat removal is very important to ensure a long, reliable operating life and to avoid fires. The generator will normally have cooling holes around the body, bearings at each end of the shaft to reduce friction and large brushes to allow for large currents without excessive heating. The cooling air may be forced, for instance by using ram-air from a scoop outside the aircraft skin or engine nacelle, and the forward speed of the aircraft creates a small pressure that pushes the air through the generator.
The drive to the generator may be
through a splined shaft, or via a pulley
and belt. arger generators will use a
splined shaft, as it is more robust and
needs little or no maintenance. The
belt-and-pulley arrangement has the
advantage of being simple and light, but
does re'uire routine adjustment to avoid slippage of the belt. This is achieved by
having one of the mountings as a slot, rather than a bolt hole, so one side of the
generator can be swung in or out to
tension the belt. The same system is
used on car alternators. n smaller aircraft, especially those
powered by piston engines, the
the characteristics of being able to
produce significant power over a wide range of rotational speed. n this case a three-phase, fre'uency-wild alternator provides outputs to a rectifier, consisting of a bank of diodes. ffectively, the diodes do the same job as the commutator in a generator. The term +fre'uency wild+ means that the alternator runs at a fixed proportion of the engine speed, which itself varies of course, so that the fre'uency of the output is not fixed. As in the generator, the field supply may be varied to regulate the output voltage. This works because the field strength 5flux6 varies in proportion to the field current. To remove
the generated power, instead of a commutator used in the generator, which alternately contacts each brush as the armature rotates, the alternator armature has slip rings, which are complete, unbroken rings, and each brush runs only on one slip ring.
0egulation may be achieved by a range of methods, but in principle these involve reducing or eliminating the field supply when a threshold voltage, current, or both, is reached. #nce the output falls, the field supply is restored. This happens many times per second - typically ?9 to ;99*>. 2ost modern systems use solid-state regulators rather than the mechanical
7 8 =
reliable than mechanical devices.
which is usually the case, it is
desirable that there is no break in the supply, even for a moment, if
an engine or generator fails in flight. 2any electrical systems are
sensitive to power drop-outs. n
the case of a twin-engine aircraft,
for example, each generator will be
connected to its own 3/ bus
5which will be explained later6. t is
common to have these buses
connected to each other, so that
the generator of one bus can
supply the loads on both if the
other generator output is lost.
*owever, this can cause problems
with the regulators, causing the
generators to +fight+ and each regulator trying to control the
output of both generators. This can de-stabilise the system. The
problem is resolved by connecting
the generators together using
units by ensuring the field currents
are balanced.
g e n e r a t i o n
$enerating A/ power is rather more
complex than 3/ in some ways,
because of the choice of arrangements
that may be used, and the need to
match fre'uency and phase if the
buses are to be interconnected. "ince
the system consists of three phases,
there are several ways in which the
phases may be connected to loads, but that will be explained later.
Bhere the voltage output from a 3/ system may be easily described,
assuming it is fairly steady with time,
the output from an A/ system can be
measured or described in different
ways. "ince the amplitude 5voltage6
varies continually with time, it is
possible to identify the peak voltage
and use that to define the signal.
*owever, the power, measured in
watts, is not e'ual to the peak voltage
multiplied by the peak current. Also, if
the voltage is simply averaged over the
cycle it will be >ero, since the cycle has
a negative half and a positive half. The most common way of specifying the
voltage of an A/ supply is therefore to use the 02", or root-mean-s'uare,
value. The voltage at each point in time
is measured, and s'uared. These
values are then averaged over a whole
cycle and the s'uare root calculated.
This gives what may be considered to
be a usable average value. t turns out
that for a pure sine wave the 02"
value is 9.9 5i.e. ..j;6 times the peak
value.
A/ power, and that is the power
factor. For purely resistive loads, the
current varies directly with voltage, so
multiplying 02" voltage by the 02" current gives the power in
watts. The output of most alternators is large, however, so it
is expressed in kilowatts 5kB6.
Bhere the loads have significant
capacitive or inductive
current and voltage, and this will
change the power delivered and
the power consumed. The
called the power factor 5%F6. For
this
Electrical systems 78?
Figure 14.2 lectrical generator. This is an integrated- drive generator or l#$, which has a constant-speed drive
unit incorporated. The generator %actually an alternator since it generates alternating current& is driven from the accessory gearbo' of a main gas- turbine engine, to which it is attached, and the constant-speed drives converts the variable speed of the engine to a constant speed input to the generator. This means that the fre(uency output of the generator will be constant, and can be matched to the other generator or generators on the aircraft. The three output cables can be clearly seen at top left. This particular l#$ is fitted to a )oeing 767.
reason, the output of an alternator 5and
the power consumption of A/ loads6 is
therefore normally measured in kA
5kilovolt-amps6, where the power factor is
e'ual to the effective power, measured in
kilowatts, divided by the apparent power,
measured in kA. The principle of operation of the
alternator itself, though, is the same as
that already explained for alternator
supplying a 3/ system - a magnetic
field is created within the alternator and the armature, with coils of wire attached,
rotates to induce a voltage. For an A/ system, the output is always in three
phases. n its simplest form, there would be two wires per
1
9
6
e
m
s
Fi x e d- fr e ' u e n c y or fr e ' u e n c y- w il d G
2ost A/ systems used on aircraft use a fixed fre'uency of =99*>, or cycles per second 5each cycle taking ;.? mill iseconds6. This gives several advantages:
7. The impedance1 for loads containing active 5capacitive or inductive6 components, such as motors, is fixed because the fre'uency is fixed. This means that performance of the loads will not change with engine speed, provided the supply voltage can be maintained.
;. %rovided the phases are synchronised between the buses, buses may be linked 5tied6, preventing power drop-outs in the event of partial power loss and allowing load sharing between alternators.
7 mpedance is the e'uivalent of resistance
but for A/ rather than 3/ systems, and it varies
with fre'uency for active loads.
Electrical systems 197
<. "ome loads are based on a fixed reference fre'uency, and in many cases the =99*> supply is sufficiently precise. Bith variable- fre'uency supplies, this reference fre'uency must be created separately.
*owever, this system does have some drawbacks. n order to maintain a fixed fre'uency and phase synchronisation over a range of engine speeds, the alternators must be driven at an exactly constant speed. This is normally performed using either a constant- speed drive unit 5/"346, and in some cases this is built into the alternator, which is then known as an 3$ or integrated-drive generator 5Figure 7=.;6. n either case, the constant-speed drive must provide a continuously variable transmission ratio, and allow very precise adjustment of the speed and phase to maintain synchronisation. To do this, /"34s employ a hydraulic coupling -
a hydraulic pump and a hydraulic motor in series. The speed of the pump is at the proportion of the engine speed determined by the drive ratios in the engine+s accessory gearbox, and the delivery is fixed by the design of the pump. The motor is a swash-plate motor, and so its displacement per revolution can be varied by changing the angle of the swash-plate. 5"wash-plate motors and pumps are described in /hapter Fifteen.6 "ince it must use the fluid output from the motor at exactly the same rate as it is delivered, reducing the displacement will cause the speed to increase, giving control over the ratio of the hydraulic transmission. A governor then controls the transmission so that its speed remains constant over the entire speed range of the aircraft+s engines.
/"34s are complex and unreliable components. They are also heavy,
1%hase;
1%hase<
...
Figure 14.3 "tar and delta wiring. Three-phase loads %and generator coils& may be connected in two ways. The configuration shown on the left is called star wiring, where each phase of the load is wired from one supply phase to a common return %often called ground&, seen here as the point where
... ...
be higher if the impedance is the same. 7owever no return connection is re(uired.
Figure 14.4 Three-phase electrical supply. The three-phase supply consists of three separate voltage supplies, each phased at F;9J to the others. The potential of each line is at /01 2 peak with respect to aircraft ground %earth&, and !44 2 peak with respect to the other phases. 8o a three-phase supply contains three live lines and one ground. 8ince the three lines are out of phase, each line acts as a return to the others at some part of the cycle, or 9ground9 may be used, depending on how the e(uipment is wired.
7 8 @
e
m
s
discharged as heat. #bviously, a way of eliminating them would be very attractive. (ecause the alternator would no longer run at a constant speed, such a system would have variable fre'uency, and phase synchronisation between alternators would not be possible. The loads within the electrical system would therefore need to be able to cope with a fre'uency-wild system. "ince it is no longer possible to tie A/ buses together
when their individual alternators are running, an alternator failure would result in a momentary drop-out of power on the bus it supplies, until a contactor can be operated to tie another bus and restore power. This issue can be critical, and can affect systems design and choice of components.
4ntil recently, fre'uency-wild systems were rare. *owever, with
increasing pressures to reduce weight and
energy losses and increase reliability, the
benefits offered are more attractive.
$ r o u n d
p o w e r
Figure 14.! $round-power cart. A ground-power cart, seen here in the foreground alongside an aircraft tug, supplies A and*or D power to the aircraft through a ground-power receptacle. "t allows some of the electrical systems on the aircraft to be powered without the need to run the aircraft9s engines or au'iliary power unit, reducing fuel use and noise. #n most large aircraft, the hydraulics may also be run using the electrical hydraulic pumps provided as part of the backup
systems. %hoto: / (arcenilla
Electrical systems 788
and embarking and disembarking passengers. 0unning the main engines wastes fuel, as they must be run at very little load just to provide electrical power, and using the batteries will mean they are not fully charged for the next flight, removing the safety margin they provide. t is common to use an auxiliary power unit 5A%46 if power is needed for short periods. A%4s are described in /hapter "ixteen. For extended periods, or when power is re'uired intermittently, ground power is preferred. This consists of a cable that connects onto a receptacle on the outside of the aircraft, supplying A/, 3/ or both. The other end of the cable is connected to a ground-power cart, consisting of a large diesel engine and the appropriate electrical generators, together with controls. "uch a cart is often known in the 4K as a +*ouchin+, after a manufacturer of these items. "ome airports provide ground power through cabled supplies at the stands.
% o w e r
s t o r a g e
result of losing power on all engines - multiple generator failures are otherwise virtually
unheard of. This energy is stored in batteries, which
under normal operation are
constantly charged from one of the main electrical supplies. "ome batteries are of the lead-acid type, as used in most cars, although nickel-cadmium batteries are also used, which are capable of a higher energy density and can supply higher currents for short periods. (atteries would typically be located in the forward avionics bay in an airliner. (attery voltage is commonly ;@, and re'uire an A/-powered charger or other means of supplying a higher voltage to drive the charging current. ;= batteries are also found, which gives ade'uate supply voltage in an emergency
and means the batteries can be charged directly from a ;@ 3/ bus without re'uiring any voltage conversion. *owever, this system gives less control over the battery state and rate of charging. Typically, a separate battery is used for starting the auxiliary power unit.
n emergency cases, the first operation is to remove every single non essential drain on the electrical system. This is called load shedding and happens automatically, since only the systems that are absolutely essential, such as basic instruments and flight controls, are connected to the battery bus. n the event of failure, this bus is automatically disconnected from the other buses by the bus-tie contactor, and is then powered solely by the batteries. The bus-tie contactor allows loads on the battery bus to be driven by the main 3/ system under normal operation, but allows rapid
2
contactor is normally designed not to
allow current to flow from the battery
bus to the main 3/ bus5es6, preventing
inadvertent discharge of the batteries.
/ommonly, the battery bus is split
into two - the +hot battery bus+,
connected directly to the batteries,
and a separate battery bus, connected
through a bus tie 5see %ower
3istribution6. Bhere it is essential to
supply some A/ power in an
emergency, 3/ from the batteries can
be converted to A/, as described
below.
3/ power are generated by engine
mounted generators. *owever, in
smaller aircraft, and under certain conditions in large aircraft, it is more
convenient, or necessary, to convert
Figure 7=.L Transformer-rectifier. :ormally located in the avionics bay in an airliner or combat aircraft, the transformer-rectifier takes three- phase A and converts it to D. The thinner multi core cable entering the unit at upper-centre is the input, and the output is through the two separate cables below it. The output cables are much thicker- because the voltage has been transformed down, the output current will be higher for a given power.
Electrical systems ;97
between the two types. This will allow both supplies to be maintained in emergencies, and simplifies the power systems on aircraft of a si>e that does not warrant complex electrical systems. n both cases, there are two distinct operations re'uired - voltage conversion and conversion between A/ and 3/.
/onversion from 77?D;99 A/ to
;@ 3/ is carried out by transformer rectifiers 5T04s6, and is probably the
easier to understand. The high-voltage
A/ supply passes through a
transformer, reducing its voltage to around ;@. This signal, still at =99*> A/, must now be converted to 3/. This is
performed by a rectifier, which mirrors 5rectifies6 the negative half of the A/ wave. /ombining the rectified outputs of the three phases gives a signal that is already reasonably steady, as the rectified outputs overlap 5Figure 7=.;6. A smoothing circuit then reduces the ripple on the rectified signal, so that it more-closely resembles a pure 3/ signal.
/onversion from 3/ to A/ is slightly more difficult. The first units used a 3/
motor to drive an alternator, all contained within a single unit known as
a rotary inverter. Although they are still
used occasionally, they generally are of
low efficiency and are unreliable.
2odern systems use a device called a static inverter, and as the name
suggests these have no moving parts.
The 3/ supply is first converted to A/
using electronics 5a wave generator6.
This is then amplified and regulated to
produce the re'uired 77?D;99 output
at the desired power output. ach phase must be created separately, although
the three are usually combined into a single physical unit. The three phases
must, of course, be synchronised so that the correct phase relationship is
maintained between them. %ower conversion capacity from 3/
to A/ is 'uite limited due to the
currents needed from 3/ - it is not possible to generate all the A/ power
needed for a large aircraft from any 3/ source, as the current draw from the 3/
supplies would be enormous. For this
reason, conversion is usually limited to
specific re'uirements, or to supply the
minimum e'uipment needed in an
emergency.
(
the engines, and also in some
circumstances at the auxiliary power unit, a distribution system is needed to
take the power to where it is to be used. The first point at which the
supplies and distribution system come together is the buses 5sometimes
called busbars6 -a series of large
copper bars or strips capable of
carrying all the current re'uired for the
whole aircraft. These may be located
on or near the flight deck, or in an
avionics bay,
2"2 Aircraft
structures and
Electrical systems 2"3
typically beneath the flight deck, although other locations are possible. #ne copper bar is needed for 3/, and a further one for each A/ phase, although the combination of three phases is normally referred to as the +A/ bus+.
,+ (.)
where multiplegenerators are used. #n a typical airliner there might be two *+ t g c.
x M5 N 3/ buses, a separate battery bus,an emergency 3/ bus or hotbattery bus,
"%0-) .
( 9 .)
5. 6
/OP,#+l
l
and two or more A/ buses - perhaps one per engine for multi- engine
aircraft. #f course, if a fre'uency-wild
A/ system is used, then a separate
A/ bus must be provided for each
alternator, and they cannot be connected unless an alternator has
failed. For load sharing, it is normal to provide a method of linking the buses
together - known as bus-tie contactors, or simply bus ties. n normal flight
0 ,=77+5 00.900+.0 33 23
+9+ // / "##
.,#-
5all systems working6, these may be connected or they may be disconnected and only tied in the
event of supply failures. "ome 3/
bus ties are configured to allow current
in one direction only, and are known as
reverse-current contactors. n some
'
% . #
)
"
be prioritised: there is a +2ain+ 3/ bus and an +ssential+ one, with a
reverse-current contactor installed
may flow from the 2ain bus to the
ssential one, but not in the reverse
direction. The principle is that the
more-important e'uipment is run from
the ssential bus, so that if the 2ain
3/ generator fails the loads on the
2ain system will effectively be shed
automatically, reducing the load on
the ssential generator.
#
o &
The conn ectio ns betw een buse s, and man y other circu its, are princ ipall y
contr olled by relay s - elect
r i c a l
simultaneously, which is necessary
for three-phase supplies of course, and may even be wired to open
some contacts and close others in a single operation 5for instance
changeover contacts, where a
(/)
distribution
Figure 14.7 Aircraft electrical system. The electrical system in most aircraft is (uite comple', with parallel redundant systems for safety. All essential systems con be supplied from more than one of these systems, and the systems themselves can be cross-fed if re(uired. 3n this way no single failure, and only a limited number of unlikely multiple failures, can cause a complete loss of electrical power. llustration courtesy 0aytheon /orporate Pets nc.
From the buses, power is distributed
around the aircraft. This generally
takes the form of a wiring +ring+ -
both ends of a complete loop are connected to the bus, with suitable
circuit protection. This allows the current to flow either way around the
circuit, reducing peak currents and reducing
,9
9
2"4 Aircraft structures and systems
Figure 14.8 /ircuit breakers and breaker panel. This is the panel above the crew9s heads on a )oeing 767, showing ius= some of the many circuit breakers needed. 8ome breakers here have been 9pulled9 to disconnect them, and in this case have been fitted with flags to indicate this %inset&, as the aircraft is undergoing maintenance. A small collar fits under the breaker head to ensure if is not inadvertently closed, to protect maintenance personnel working on the aircraft.
the chance of mechanical
circuit. The wiring takes the
form of round, stranded wires,
similar to mains cable, bundled
into larger groups called looms.
/onnectors at suitable points
connected where re'uired.
airliner.
wire is determined by the cross-section of its
conductors. Thus a high voltage system can deliver a given amount of
power with a smaller current, hence re'uiring thinner wires, than a
low-voltage one. *owever, low voltages are safer. A ;@ system
will cause little or no sensation of electric
shock, and is non-lethal, to someone coming into contact with it, whereas voltages above about 7##A are considered
lethal. The actual probability of death
depends on the current passing through the
person+s body rather than
Electrical systems 2"!
voltage, so the lethal voltage varies widely, depending on the current+s path through the body, the person involved, whether it is A/ or 3/, and several other factors. *igh- voltage 3/ causes muscle clenching 5spasm6, sometimes preventing release of the person+s grip on the source of electricity, whereas A/ does not 5it causes alternate spasm and release as the current reverses6.
t+s important that the wire is correctly rated for the current it will carry, and that it is proof against moisture and other degradation. 1iany aircraft, especially military aircraft, may stay in service for half a century or more. The insulation is typically PTFE 5polytetrafluoroethane6, although new polymers are being developed all the time. t needs to provide good mechanical strength and abrasion resistance to prevent chafing, good electrical resistance and be resistant to chemical, water and biological attack 5fungi, insects, rodents6 and also to ultraviolet light. The conductors within the wires are invariably of stranded copper, providing good electrical conductivity and allowing the wire to bend and flex without breaking.
oads are often placed on distribution circuits in logical groups. n the event of problems, the crew must know that if they choose to isolate 5switch off6 a circuit, the systems that will be lost are logically grouped. For this reason, there are 'uite a lot of separate distribution circuits, allowing the crew to be very selective about what they disable. ach circuit is protected, as described below.
/ i r c u i t
o t e c t i o n
f too much current is allowed to pass through a cable or loom, the energy losses due to the electrical resistance of the cable will cause the cable to overheat, and if not stopped this could cause a fire. A fire on an aircraft in flight is generally considered to be catastrophic 5leading to loss of life6. (y design, all circuits must be rated for the maximum load they can carry, and protection is installed to prevent this maximum being exceeded. n domestic appliances, this is normally achieved using a fuse - a thin piece of wire that will melt above a specific current, breaking the circuit. Fuses are also used in aircraft, but another device known as a circuit breaker is faster and more flexible in use. 5!ote that circuit breakers are not the same as contact breakers, and carry out a different function.6 /ircuit breakers can be manually disconnected 5+pulled+6 to disconnect circuits, either in flight or during ground servicing 5Figure 7=.@6.
/ircuit breakers are grouped together in one or more electrical distribution panels, and have a small button on the top. Bhen connected, the button is pushed in, and if they trip, or are pulled, the button sticks out. A white or red marker band under the button makes it easy to see if a breaker is out. They are configured so that the circuit cannot be enabled by holding in the button, preventing anyone manually overriding circuit protection. "ome circuit breakers have a separate trip button.
2
" 6
A
i
# A 3 "
The types of electrical loads on an aircraft are very varied. There are thus different ways of categorising them, but for the purposes of this book we can consider large loads and small loads, and A/ and 3/ loads.
T
y
p
i
Actuation 5motors, linear actuators6 Anti-ice, deicing and de-misting
ntake and probe heating
#ther loads
Although it is not a hard-and-fast rule, it is generally found that large loads will be
powered from the A/ system, since it is capable of delivering high power with
moderate currents, making the wiring
lighter. t is also generally the case that
the majority of e'uipment on the flight
deck is powered from the ;@ 3/
airliners use the same system, which is to start the A%4 from its own batteries, then use either electrical power or bleed air from the A%4 to start the main engines - one at a time to limit the power re'uired. Bhere electrical starters are used, they are normally 3/ machines. The windings and internal components are fre'uently arranged so that after starting is completed the device will be turned by the engine and become a 3/ generator. "uch devices are known as starter-generators. The benefit of these is a significant weight saving compared to separate components.
"ome aircraft, notably the Airbus A<@9, use electrical actuators for
primary flight controls, as well as hydraulic ones. This gives a significant reduction in the number of systems re'uired, because it is possible to
provide the necessary redundancy shared between the electrical and hydraulic systems. An electric motor or linear actuator would not provide the right combination of response speed and power to provide an effective actuator, so the ones used for flight controls are electro-hydraulic actuators - the electrical supply is used to drive a hydraulic pump, and the actuation is performed hydraulically. This makes the actuator self-contained 5apart from the electrical connection6, reducing the weight since hydraulic supply pipes are not re'uired. Anti-ice, deicing and de-misting
devices are generally similar in function
- they use electrical heating to prevent
the build-up of ice on critical surfaces,
such as engine intakes, propeller
blades and leading edges. Anti-ice may
use chemical methods as an alternative,
which will create very little load on the
electrical system. ighting may be a major consumer of electrical power. nternal lighting on
passenger aircraft is mandatory, for safety reasons, and may be powered by
Electrical systems 2"7
A/ or 3/. There is also a re'uirement for emergency lighting, both to illuminate the cabin and for floor-level exit lighting. This emergency lighting must be capable of being supplied from the batteries in case of complete engine failure, or post-landing. /ockpit lighting is very important, of course. (oth ambient lighting and instrument lighting are provided, and are adjustable in intensity. Ambient lighting is kept at a low level during flight to avoid reducing the crew+s night vision. "mall reading lights on goose-neck stems are provided to allow the reading of charts and checklists.
xternal lighting serves a number of purposes on an aircraft. "trobe lights are fitted to the fuselage - intensely bright lights that flash to draw attention to the aircraft in flight, making it visible to other air traffic. !avigation lights are located at the wingtips and at the tail, showing the orientation of the aircraft. This is important to other air users, as they need to know the direction other aircraft are travelling in. A red light is fitted at the left 5port6 wingtip and a green light at the right 5starboard6. Bhite lights are fitted at the tail. Flashing red anti-collision lights may also be fitted to the upper and
lower fuselage.
#ther lighting may be fitted to help the crew see the state of the aircraft at night. Bing lights can be fitted, normally at the leading-edge root, so the entire leading edge can be seen from the flight deck at night. This allows the crew to see visually whether any ice has formed. 2any airliners are fitted with a logo light, on each side of the fin, ensuring that the operator+s branding can be clearly seen.
lectrical control systems are used in a wide variety of applications, from anti- skid brakes to windscreen wipers, flaps and undercarriages.
A /
l o a d s
An important point must be made here regarding the wiring of A/ loads. As
there are three phases, the loads may be connected in either of two different ways, known as star wiring and delta wiring. As has already been seen,
since there is a phase difference between the three phases, there is a voltage difference between any two phases at a given point in time. The voltage between phases is therefore ;99 peak for an aircraft system. 0elative to ground 5the airframe6 there is a voltage of only 77? peak on each phase. For a given power, the current drawn tends to be higher in the delta configuration.
2 "
8
A
i
r
c
r
a
f
t
"
As the electrical system is vital to the safe and effective operation of an
aircraft, so it is important to ensure that any failures will still allow the aircraft to
continue safe flight. "ystems are designed on the assumption that at some point in their life, failures will
occur. These failures can be predicted, and the results analysed. $enerally, no single failure must be allowed to create
a significant ha>ard, and predictable multiple failures must also be allowed for. This is the main reason why the
systems on board an aircraft
The provision of bus ties and power conversion has already
been covered in this chapter. A/ and 3/ generators are generally extremely reliable, and the major concern in the electrical system is the engine-out case. This is especially important with twin-engine aircraft, since a failure of one engine leaves little spare electrical capacity. n particular, &tops operation 5xtended Twin #%eration"6, which is flight over large expanses of water in twin-engine aircraft, is now common. An engine failure may occur when the aircraft is a long way from dry land, so it is important that the loss of a propulsion unit is not compounded by a major loss of electrical power, and the A%4 is regarded as a safety-critical item. A%4s are described in detail in /hapter "ixteen, but the provision of
electrical power from the A%4 is relevant here. The A%4 will drive at least one electrical generator 5and some large aircraft have two A%4s6, giving an additional power source to that provided by the main engines. n the event of a loss of a generator or an engine in flight, starting the A%4 is standard procedure. n the unlikely, but not unknown, event of a second engine failure, the A%4 will continue to provide power, and therefore maintain flight controls. t also alleviates some of the additional load on the remaining engine5s6 in taking over some of the load needed to power the electrical
s y s t e m .
0 a m - a i r
t u r b i n e s
n the event of total engine and A%4 failure in flight, normally from running out of fuel, many aircraft have a ram- air turbine 50AT6 that is deployed automatically from a stowage bay, usually under the fuselage on an airliner. t acts like a windmill, and drives a generator 5usually 3/6 and sometimes a hydraulic pump. "ince the A%4 will take some time to start, this provides vital time to keep control of the aircraft until the A%4 reaches full power. f the problem is lack of fuel, the A%4 will not start at all, of course. Although the power available from the 0AT is very limited compared to an A%4 or engine, it is capable of supplying much more power than the batteries are, giving the crew vital time in which to attempt to overcome the emergency. The batteries are, of course, the final resort.
Electrical systems 2"9
rather than loss of engines, the electrical system may be driven from the hydraulic system. lectrical pumps provide a backup for the hydraulic system, but these may be run in reverse, using the pump as a hydraulic motor and the electrical motor as a 3/ generator. This provides a further element of redundancy in both systems.
2#0-/T0/ A!3 A-
/T0/ A0/0AFT
Bith increasing demands on designers and operators to improve the environmental performance of their aircraft, and to reduce fuel costs, there is a move towards reducing the reliance on air drawn from the engines to supply the cabin pressurisation system, using electrical operation instead. This process is being extended towards the goal of powering all the onboard systems electrically, replacing hydraulics and pneumatics. This end-point is known as the all'electric aircraft, with intermediate stages being referred to as the more'
electric aircraft.
The drawback of taking compressor bleed air from the engines is that, as both the amount of air drawn and the speed of the engines, hence the engine air flow, vary independently, it is not possible to make full allowance for this +lost+ air
when designing the engines, and this makes the engines less efficient during some phases of operation. 0eplacing engine air bleed with larger electrical generators allows engine design to be better optimised for all flight phases, saving fuel and reducing emissions. The savings can be considerable. "ince the air used for cabin air systems leaves the engine at high temperature, and needs to be cooled before it can be used 5see /hapter "ixteen6, the waste heat is another loss of efficiency that can be improved.
*owever, this new approach is a radical change to the way in which aircraft systems are designed and is not without its problems. The amount of electrical power to supply all of the needs of the aircraft re'uires extremely large generators, and these are bulky and heavy. $enerators normally run more slowly than the engine shafts that drive them, re'uiring a gearbox to reduce the speed, and this reduces efficiency and adds weight to the engine. deally, the generator could run at engine shaft speed, eliminating the gearbox. Bork to make these changes to both engine and generator designs continues, as does the re-design of cabin-air systems to use electric pumps instead of bleed air. At the time of writing, a number of
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