THE AERONAUTICAL JOURNAL MARCH 1999 159

Aircraft flight control systems

D. McLean
Department of Aeronautics & Astronautics
University of Southampton, UK

ABSTRACT
This paper presents a short account of the flight control systems used few such aircraft are equipped either with automatic flight control
in commercial transport, military combat and general aviation aircraft. systems (AFCS) or with more than rudimentary avionics. Although
The effects of aircraft safety, reliability and weather delays on few in relation to the total, there are a number of general aviation aircraft,
satisfactory aircraft operations are shown to be significant reasons particularly twin-engined types, which are able to operate in instrument
for the extensive use of flight control systems. The principles of meteorological conditions (IMC) and, consequently, are well-
flight control, the sensors and actuators required and the various equipped with avionic suites and AFCS. Military combat aircraft
modes which can be selected are treated, together with a short operate over more extensive flight envelopes and are usually
account of the primary flying controls and the use of manual reversion equipped with specialised avionics and weapons systems. They have
in emergency situations. The paper concludes with a consideration AFCS which provide a number of additional functions not usually
of the fly-by-wire (FBW) and fly-fby-light concepts, and covers found on civilian airliners or general aviation aircraft. This paper
relaxed static stability, carefree manoeuvring and the use of canards aims to give an account of flight control systems which relates to all
before discussing some FBW flight control systems which are used three classes of aircraft. Specialist functions which are to be found
in passenger aircraft. on particular classes of aircraft will also be briefly discussed.

1.0 INTRODUCTION 2.0 SAFETY AND RELIABILITY

In modern aviation, flight control systems are used in commercial The flight of any aircraft usually comprises eight distinct phases, as
transport, general aviation and military combat aircraft. Of these shown in Fig. 1. For a typical 800 nautical mile sector flight by a
classes of aircraft, general aviation is much the largest, but relatively commercial aircraft the longest phase is cruise which occupies about

26.1% 40.5%

Figure 1. Flight phases shown as fractions of flight time and world-wide hull losses per flight phase.

Review Paper No. 004.

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 160 T H E AERONAUTICAL JOURNAL MARCH 1999

64% of the total flight time. The shortest phases are: set by the pilot when it has been reached without further intervention
Take-off by the pilot. Moreover, a particular route can be specified and the
Initial climb route hold mode can be engaged. This mode needs information
Final approach from an inertial reference system. If such a system is not fitted to the
Landing. aircraft then, as an alternative, VOR (VHF omni-range) hold can be
Taken together, these phases occupy about 8% of the total flight selected which uses VOR information from the navigation
time. But from inspection of Fig. 1 it can be seen that almost two- receivers. A sequence of such modes can be selected continuously
thirds (67%) of all the hull losses which have occurred world-wide until the landing phase. If the aircraft is fitted with an automatic
in the period 1959-1990 have occurred during these phases of flight. landing system then, when it is engaged, the aircraft can touch down
It is in these four flight phases that any AFCS being used has to be automatically. What cannot yet be done with commercial airliners is
most reliable since the risk of total loss of the aircraft is greatest, for to automatically taxi the aircraft back from the runway to the terminal
the aircraft is flying at relatively low speed and height. The importance buildings or to takeoff under automatic control. Both these
of avoiding aircraft hull losses is self-evident. For example, from manoeuvres require continuous pilot action.
lata statistics, there were 22 such hull losses in 1994, eight of those Many commercial aircraft are fitted with a flight management
accidents occurring in adverse weather. 666 passengers and 56 crew system (FMS). In 1985, for example, a flight management and
lost their lives. In many of those accidents, the aircraft were guidance computer (FMGC) was introduced on the A320. Both the
using flight control systems with only limited authority: the pilot was FMS and FMGC take into account more than the selection of the
flying the aircraft manually. It is important that every method of various AFCS modes. Because it has been provided with a large
assisting pilots to fly with safety, including enhancement of the memory and extensive computing capacity an FMS can directly
authority of the AFCS, should be used to improve the safety of flying.
select an optimum flight path from takeoff. Usually the optimality
The reliability required is usually expressed as one failure in 107 criterion is minimum fuel burn. However, constraints such as noise
flying hours or, if it is assumed that an aircraft operates for 3,000 flying avoidance after takeoff, or passing over a beacon at a prescribed
hours per year, one failure every three thousand years. The figure of height to meet air traffic control 'requirements', can be taken into
10"7 is based upon the probability of occurrence of natural death account in the search for the optimum climb trajectory. Azimuthal
within the next hour of a person in good health. Assuming a life navigation can also be regulated by the FMS: a pilot merely inputs
expectancy of 75 years that probability is about 10~6 (actually the required routes and waypoints. From the results of experiments
1-52 x 10-*); by taking a figure of 10-7 it means that the flight can be carried out with a Boeing 727 and Lockheed L-1011 it has been
completed even if one pilot dies in flightt.To provide an AFCS with found that automatic flight management can reduce fuel consumption
a reliability of 10"7 requires the use of redundancy, the use of several by 3%.
channels of identical equipment carrying out the same functions
simultaneously, and being monitored continuously for any disparity
between channels. Such a solution makes the system expensive and
heavier than if only a single channel had been possible. 4.0 PRINCIPLES OF FLIGHT CONTROL
Although safety is of paramount importance in aviation, in
In addition to the engines and control surfaces which can be used to
commercial aviation regularity of service is also an important
change the present state of motion of an aircraft, every aircraft contains
consideration. The need to operate safely in adverse weather is
some motion sensors which provide measures of the changes which
essential for the economic health of airlines. It has been estimated
have occurred in the measured motion variables as the aircraft
that each year in the USA some 24,000 commercial flights are
diverted or delayed because of bad weather. With average delays being responds to the pilot's commands, or as it encounters a disturbance.
about six hours, and with the average number of passengers involved The signals from such sensors can be used as inputs to the aircraft
in each flight being about 200, the total passenger-hours involved in instruments to provide the pilots with an appropriate cockpit display,
these delays is about 30 x 106 with an attendant annual cost to or they can be used as feedback signals for an AFCS. The general
airlines of about $l-5bn. The cause of 60% of the delays has been structure of an AFCS<2> can be represented as a block diagram such
attributed to the weather, with the cause of another 35% being attributed as that shown in Fig. 2. The purpose of the flight controller is to
to airport capacity limitations. (However, such capacity-limit delays compare the commanded motion signals with the signals from the
are partly caused by the knock-on effects of earlier weather delays). sensors measuring the aircraft motion and, if there is any difference
In the European Community it is estimated that there are about between them, to generate according to the designed control law the
330,000 hours of delay each year. Even though three quarters of the command signals which are applied to the actuators. These produce
delays were less than about 20 minutes, it is obvious that, for the torques and forces which deflect the control surfaces by the
commercial airliners at least, AFCS and avionic systems which amounts required to result in the appropriate control moment or
assist aircraft to operate in all weather conditions are essential for force being applied to the aircraft.
operational efficiency. In flight-control systems the following aspects need to be
distinguished:
a) The development of forces and moments for the purpose of
establishing an equilibrium state of motion for an aircraft (trim)
3.0 FLIGHT MODES and for restoring a disturbed aircraft to that equilibrium state,
A modern passenger aircraft flies a sequence of automatic sub-phases while regulating within specified limits the aircraft's deviation
(referred to as modes) which are selected continuously by the pilot. from the trim state, are regarded as constituting flight control.
For example, when the aircraft has taken off, an attitude hold mode b) Regulating the aircraft's deviation from the trim state, its
can be selected, and if the aircraft is fitted with an auto-throttle, an response, is frequently referred to as stabilisation.
airspeed hold mode can also be chosen. Most airlines impose an upper c) Guidance means determining the path to be followed and the
limit on pitch attitude on takeoff, such as +16°, to ensure passenger speed to be maintained by the aircraft.
comfort. The pilot can also select the height (or flight level) at which Flight control systems are interfaces between the guidance systems
he wishes the aircraft to fly after the climb is completed; by selecting and the aircraft being guided<3> The flight control system receives
height hold mode the aircraft will continue to fly at the height level correction commands from the guidance system and it provides as
outputs appropriate deflections of the necessary control surfaces to
cause a required change in the motion of the aircraft. If an aircraft is
t For a more prosaic account of the origin of the 10"7figuresee Pelegrin and to execute the guidance commands properly in relation to the
HollisterUaQS)!1). ground, it has to be provided with information about how the aircraft
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 MCLEAN AIRCRAFT FLIGHT CONTROL SYSTEMS 161

Manual Reversion

MANUAL
Primary Flight Controller Control Mechanical Aircraft
Flying Surface AIRCRAFT DYNAMICS Motion
Linkage
Control AUTO (Control Law) Actuators
+ Command
Actuator
Control
Surface
Deflections

Guidance Guidance Command

System
Feedback signals
.+ Sensors
11+

Figure 2. Block diagram of an AFCS.

< Sensor
Noise
\
/

is oriented so that right turn, left turn, up, down, roll right, roll left,
for example, are related to the airborne geometrical reference. For
5.0 PRIMARY FLYING CONTROLS
nearly eighty years it has been common practice to provide aircraft Any flight control system is an arrangement of elements which enables
with reference coordinates for flight control by using gyroscopes. controlling forces and moments to be applied to the aircraft in some
For example, an attitude indicator (pitch and roll) effectively provides required fashion. These elements belong to three groups:
a horizontal reference plane with an accuracy of a few degrees. pilot input elements
Similarly the turn indicator, which shows the aircraft's turning left or system output elements
right, is a gyroscopic device. These indicators have a very limited intervening linkages and connections.
accuracy of a few degrees. Moreover, they can have drift rates of The pilot input elements are referred to as the primary flying
about 10° per hour. Present day navigation requires that for each controls; they are elements moved by the pilot to cause the control
hour of operation after an inertial fix the accumulated error in distance surfaces to move. The principal primary flying controls are the pitch,
must not be greater man 1 -5km. A distance of about 95kmt corresponds roll and yaw controls whose use affects motion about the transverse,
to an angle between local gravitational directions of one degree (1°). the longitudinal and the normal axes respectively, although each may
Thus, the sensors used in modern AFCS have drift rates about two cause simultaneous motion about the other axes. Thrust is controlled
orders better than the figure quoted above, with a corresponding via the throttle levers: in combat aircraft engine nozzle area can also
improvement in accuracy. To achieve this performance ring-laser be used to control thrust. In Fig. 3 is shown the cockpit layout of a
gyroscopes and force-balance accelerometers are now used. twin-engined general aviation aircraft. The yoke is the primary flying

t The distance from Eastbourne (England) to Boulogne (France).

Dual exhaust
Dual r.p.m. gauge Magnetic compass
gas temperature
Dual manifold pressure gauge 'System fault <EGT) gauge
annunciation / L / R oil/cylinder
Altimeter
Air speed indicator head temperature
Attitude indicator gauges
Horizontal
situation Clock
indicator
(HSI)
-Fuel pressure
Gyro pressure
gauge
Turn and bank
indicator
Vertical speed -L/R fuel quantity
indicator
Alternators' 'Avionics cluster
ammeter
Radio magnetic /
indicator (RMI) "•Mixture
VHF omnirange Propcllors blade pitch controls
(VOR) indicator
Instrument landing system (ILS) meter
Pitch trim wheel

Figure 3. Cockpit layout.

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 162 THE AERONAUTICAL JOURNAL MARCH 1999

control used for pitch and roll control. When the yoke is pulled 6.0 FLY-BY-WIRE AND FLY-BY-LIGHT
towards or pushed from the pilot, the elevator on the aircraft is
moved accordingly. When the yoke is rotated to the left or the right SYSTEMS
the ailerons are moved. Yaw control is effected by means of pedalst The term 'fly-by-wire' (FBW) means that the pilot commands aircraft
which a pilot pushes with his feet. These pedals cause the rudder to motion rather than the pilot commanding control surface deflection.
move. In this type of small aircraft the links between these primary The difference between FBW and systems with electrical signalling
flying controls and the control surfaces are by means of cables and is that the latter involve merely a simple replacement of the mechanical
pulleys. Such an arrangement means that the aerodynamic forces acting link between the primary flight controls and the control surface actuators
on the control surfaces have to be countered directly by the pilot. To by an electrical link, whereas in the former aircraft motion sensors
maintain a control surface at any fixed position for a prolonged period are essential elements of the flight control system in addition to the
means that the pilot must continuously maintain the required flight controller which processes the feedback signals from the sensors
counter-force, which is difficult and fatiguing. To provide pilots with to provide the command signals to the control surface actuators. Fly-
physical relief in this task, aircraft are fitted with trim wheels which by-light (FBL) systems are identical to FBW systems except that the
a pilot adjusts until the command initially set on the primary flying medium by which signals are transmitted is a fibre optic cable. One
control is set on the control surface. The pilot is then relieved of the of the technical reasons for proposing the substitution of fibre optics
need to sustain the counter-force. There are trim wheels for pitch, for wires was the supposition that the AFCS would be thereby
roll and yaw. The yaw wheel is sometimes referred to as nose trim. protected from the injurious effects of high intensity radiation. A few
In fast military combat aircraft and large transport aircraft the years ago a study found that the view the FBW systems were
aerodynamic forces acting on the control surfaces are so great that it inherently vulnerable to microwave radiation was unfounded'4). In
has become impossible for a human pilot to sustain the force required that study digital FBW flight control systems were found to be
to deflect the control surface. As an illustration of the forces involved, susceptible to microwave radiation only at very high power levels,
and to show how the use of actuators is required in modern aircraft, the intensity level of the microwave field which first caused damage
an example of a representative short haul jet-engined passenger aircraft being about 35,OOOV/m. Fibre optic systems were found to be
is considered. Its parameters are given in Table 1. susceptible to interference, however, basically because the opto-
electronics had been incorrectly shielded: the interconnections with
Cruising speed 420kt the electronic components were found to be the weakest link. With the
Height 6,850m use of carefully-prepared connections, however, FBL systems are
Wing surface area 770m2 now generally considered to be superior in performance to FBW
Surface area of elevator 10-0m2 systems. In the evaluation described by Nordwall all the system up-
Moment arm of elevator 160m sets were digital although the signal levels at the analogue circuits
Hinge moment coefficient 0-2 involved were three times higher than similarly configured digital cir-
Lift coefficient 0-385 cuits. The earliest FBW flight control systems, in the F-l 11 and Tor-
Weight at cruising 40,000kg nado for example, had safeguards against total failure of the electri-
No of passengers 104 cal inputs by having manual reversion. Now such manual reversion
Maximum elevator deflection 23° is usually dispensed with, although Boeing has retained it in its 777
series. However, fibre-optic transmission offers much superior
Table 1 information capacity handling rates, and for that reason alone it will
become the preferred signalling medium in future avionics and AFCS.
From these parameters it can be shown that the maximum elevator
moment which can be produced is 170,OOONm which requires a
force of 10,625N. The lift generated by the aircraft in cruising flight 7.0 RELAXED STATIC STABILITY
is 392,600N. Since no human could produce the continual force If a FBW system is used to provide an aircraft with artificial stability
required to generate the maximum elevator moment, it is necessary then a more rearward e.g. location can be used which results in a
to use an actuator, usually electro-hydraulic. The command signals reduction in the tail load needed to trim the aircraft. The consequences
to these actuators are voltages supplied either by the AFCS of such a reduction in tail load is an increase in the lift at the
controller or directly from a suitable transducer on the primary flying trimmed condition, and an attendant reduction in the drag. These
control itself. By using an actuator to provide the control force the changes allow the designer to use smaller tail surfaces (which are
pilot need only provide the tiny force to move the transducer. However, usually sized by their maximum lift capability). The improved
it now becomes necessary to provide the pilot with some artificial aerodynamic efficiency can be used to enhance the aircraft's
feel i.e. a force, representative of the forces created by the aircraft performance, particularly in sustained turn rates for close aerial combat
manoeuvre, has to be produced on the primary flying control by an or, by virtue of the improvement in LID ratio, to produce a lighter
artificial feel system. Such 'feel' is a necessary cue to a pilot and is and less expensive aircraft. This approach is particularly beneficial
essential for flying the aircraft successfully. for tail-less aircraft. Such aircraft at high lift suffer from a high trim
In the event of an electrical or hydraulic failure the actuator could drag, and even loss of lift at the trimmed flight condition. Inevitably
cease to function and the control surface would not move: the aircraft this adversely affects an aircraft's combat performance.
would then be out of control. To avoid this situation most civilian
and military aircraft retain a direct, parallel, mechanical connection
from the primary flying control to the corresponding control surface. 8.0 THE USE OF CANARDS
(See the dotted line in Fig. 2). This mechanical connection is
intended for use only in an emergency. With such a link available, Air superiority fighters such as Eurofighter, Gripen, Lavi, and Rafale
an AFCS is said to have the capability of manual reversion. In such have all incorporated an unstable canard into their designs to
emergencies, since the forces which can be produced by a pilot are accomplish the required mission performance objectives.
very limited, only small deflections of the control surfaces could be If an aircraft uses a stable canard there is a loss of performance
achieved. because the location (by definition) of the canard ahead of the e.g.
means that a large proportion of the lift is generated by the foreplane,
which creates high drag because of the canard's high span loading.
t Not shown in Fig. 3. In a tail-less aircraft which uses a stability augmentation system
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 MCLEAN AIRCRAFT FLIGHT CONTROL SYSTEMS 163

(SAS), for example, the trim penalty is smaller, but the aft e.g. location is considered, the effect of structural bending on the aircraft's flying
is restricted because the effectiveness of downward-deflected flaperons qualities can be illustrated. Suppose the XB-70 is cruising (flight
reduces with deflection: beyond 20° there is very little control phase category A) at M = 2-5 at a height of 19,000m. Then the true
effectiveness at high incidence. Thus, the CG must be located either air speed is l,438kt and the air density is 0-104kgnr3. The weight of
forward of the ideal location (where the control deflection needed to the aircraft is 1,710,460N and the slope of the lift curve, CL , is
trim the aircraft reduces the maximum L/D ratio) or at that precise approximately 1-4. The surface area of the wing is 678m2. From
location, in which case there is a severe restriction on the usable lift these data the acceleration sensitivity, nz , can be easily determined
(angle of attack) because the aircraft is unable to recover from a to be about 15-5. The flying qualities specification in DEF STAN
higher angle of attack. Using an unstable canard avoids that situation. 00-970 limits the maximum value of a> s2p//iz for level I and flight
phase category A to 3-6. Hence, the upper limit of the natural frequency
for level I flying qualities is 7-5rad/s. The actual natural frequency of
the XB-70 is 2 0 rad/s. If there exists a longitudinal bending mode
9.0 CAREFREE MANOEUVRING with a natural frequency of, say, 10-0rad/s, then the interaction of
With a FBW system it is possible to limit the pilot's capacity to apply this bending mode with the short period mode could result in a new
severe control inputs when the structural or performance limits of the effective natural frequency of about 8-0rad/s which would mean that
aircraft are approached. Thus, the aircraft can be prevented from the flying qualities possessed by the XB-70 at the stated flight condition
entering a spin or being structurally overstressed. If 'carefree would then be only level II.
manoeuvring' is provided, the pilot can devote his full attention to Next are the stability augmentation systems (SAS) which augment
combat, and the loss rate in service can be reduced. Many aircraft the dynamic stability of the aircraft in pitch, roll, and yaw. Pitch rate
losses in peacetime have been attributed to spinning accidents during SAS and yaw dampers provide the required flying qualities and
training. Many losses of USAF F-4s occurred in Vietnam as a result improve passenger comfort in turbulent flight. They also assist in
of stall/spin induced by over-rapid evasive turns to avoid missile improving the manual control of the aircraft and can alleviate the
threats. What FBW can achieve is to provide a safety barrier for the effects on the aircraft's response to shifts in the e.g. The roll rate
pilot: without it there is often just a precipice. damper is particularly important in combat aircraft, as it provides
However, there is a limit to what FBW can achieve. The performance improved lateral response. It is also essential in commercial transport
is limited by the time lags which accumulate in the system from discrete aircraft to ensure the effectiveness of the lateral path control systems.
signal processing, signal conditioning and the necessary inclusion of These SAS modes are essential for AFCS controlled flight: they
aliasing filters and structural and noise suppression filters. These lags constitute the inner loops. Usually these modes are not selectable,
eventually make it impossible to maintain sufficient margins of system but in general aviation aircraft equipped with AFCS it is common to
stability to accommodate the unpredictable differences between allow selection of the yaw damper. This arises because the AFCS is
actual and design values of aerodynamic and inertial factors, or even implemented in series with the primary flying controls and the motion
when there is used a wide variety of stores and loads. of the rudder pedals caused by the action of the yaw damper can be
In conventional aircraft, the dynamic behaviour is governed by irritating to a pilot.
such features as the tail size, location of the e.g., the dihedral of the The attitude control modes which are always selectable, and
wing, the size of the fin, the control balance and the mass. In FBW most often receive their command signals from guidance or weapons
aircraft the behaviour is governed by the flight control system and systems, are pitch, bank angle and heading. These constitute the main
the aerodynamic characteristics of the basic aircraft may have only a feedback control systems in the AFCS. The speed control systems
small effect on the handling qualities of the controlled aircraft. are next. In commercial airliners the speed is held by controlling the
thrust of the engines1. In some combat aircraft speed is controlled by
increasing drag by means of a speed brake. When a Mach hold
mode is incorporated, the system controls the aircraft's Mach num-
10.0 FLIGHT CONTROL FUNCTIONS ber by means of the stabilator or elevator. Note that in certain AFCS
Modern AFCS often have modes which are not selectable by the pilot: modes attitude and speed control systems must interact with one
as soon as the master electrical switch is ON these functions are another; for example, on a glide slope coupled approach the aircraft
operating and continue to operate for the remainder of the flight. One descends along a fixed slope with the engines almost fully throttled
satisfactory method of discussing AFCS modes is to consider them back. Such approaches can save time and fuel and, because of the
in reducing order of bandwidth. Functions involving control of the encroachment of urban development on airports (or vice-versa), must
aircraft's flexibility, such as gust load alleviation, structural load be quiet. Thus, throttle control of the engine speed and longitudinal
alleviation, and ride control are usually only provided on military attitude control of the flight path have to be carried out simultaneously.
aircraft, but are now being fitted to some commercial airliners^ and Such operation is the basis of an integrated flight control system.
commuter aircraft. They constitute elements of what is termed active Finally, in AFCS, there are the modes associated with flight path
control technology (ACT). Such functions are not easy to design, control: G/S hold (glide slope hold), LOC hold (localiser coupled
requiring many carefully located sensors and very fast acting actuators. approach), VOR hold (VOR coupled flight), height hold. The
The frequencies of the lowest bending modes of the fuselage or the effectiveness of these modes is assessed by their accuracy in
wing are generally much higher than the frequencies associated with maintaining the set path even in the presence of atmospheric
the rigid body motion. However, future structural development suggests disturbances. In addition to these modes, the AFCS must provide
that these frequencies will get closer, bringing with it the severe satisfactory performance in relation to certain transient phases when
problem of tight coupling between the rigid-body and flexible motion changing or establishing modes e.g. height change, localiser, glide
of the aircraft. Such coupling will severely affect the aircraft's flying slope and VOR capture, automatic landing (flare) and go-around
qualities. modes.
Changes of required values of the natural frequency of the short- In specialist aircraft, such as an SST, the AFCS might also have
period mode for different levels of flying qualities for the three phases an automatic regulation of e.g. location (achieved by controlled
(A, B and C) of flight are generally laid down in airworthiness fuel transfer). In helicopters automatic station-keeping is often provided
documents, such as Part 6 of Volume 1 of DEF STAN 00-970. For
example, if the experimental American military aircraft, the XB-70,
t If the speed is too great on approach, say, on a B747 for example, then it
t The Boeing B 777 and Airbus A320 are examples of such aircraft. can be reduced sharply by using speed brakes. The point is that the speed
brakes are not used continuously.
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 164 T H E AERONAUTICAL JOURNAL MARCH 1999

for helicopters engaged in search and rescue (SAR) operations. and Douglas Aircraft Corp)*6' The system has no mechanical or elec-
Stall/spin prevention systems and sideslip suppression systems are trical backup. The throttle quadrant on the aircraft, a twin turbofan
functions commonly provided on fast, agile combat aircraft. Many Beechjet, was modified to accommodate three rotary variable
aircraft now have their engines controlled by a full authority digital differential transformers to sense the throttle position. (These differential
engine control (fadec) system. Such engine systems are essential el- transformers are to be replaced with fibre optic sensors when they
ements of the aircraft's speed control system. have been developed). LEDs are used to transform electronic to optical
signals which are transmitted along three independent fibre optic
paths to the engine. The diode receivers transform the optical signals
to electronic signals which are then used as inputs to a separate fault-
11.0 PASSENGER AIRCRAFT WITH FBW tolerant computer. These computers compare data and if one has an
FLIGHT CONTROL SYSTEMS engine setting which is out of tolerance it is rejected. If the three settings
are close, a median value is selected and used to activate a torque
In general, FBW passenger aircraft have conventional handling in
motor to drive the shaft on the hydromechanical fuel valve. Independent
roll and yaw: the traditional spiral, Dutch roll and rolling subsidence
modes still exist, although roll control tends to be fast in response to sensors provide feedback on the position of the shaft of the fuel
pilot commands, with heavy (deadbeat) damping, to provide precise valve, thus closing the loop without the need for a central computer.
control of bank angle. On the Boeing 777, for example, to prevent As a result, data transmission rates are minimized, although the
pilots exceeding bank angle boundaries, the roll force on the column fibre-optic network provides data transmission rates in the range of
increases as the bank angle nears 35°. FBW enables more complex 1 -3 megabits per second. One advantage of such a system is that it
inter-axis coupling than the traditional rudder crossfeed for roll/yaw removes the problem of any relaxation in the tension of the mechanical
coordination which results in negligible sideslip even in extreme ma- cable linking the throttle quadrant and the engine. Maintaining the
noeuvres. In the Boeing 777, for example, the 'yaw gust damper' adjustment of that cable tension during the service life of the aircraft
(which is independent and separate from the standard yaw damper is a problem. Sometimes, after 100-200hr of aircraft operation,
on the aircraft) is fitted to improve passenger ride quality. It senses Beech has had to adjust cable tension, or add a cable tension adjustment
any lateral gust and immediately applies rudder to alleviate loads on mechanism to take up the slack. The fly-by-light system is expected
the vertical fin — a feature of benefit to flight attendants, in particular, to eliminate the need for both the adjustments and the related
who are often manoeuvring beverage or food carts in aircraft aisles mechanisms.
during turbulence. The Boeing 777 has a FBW system which allows Another advantage of such a system is the possibility of saving a
the longitudinal static margin to be relaxed — a 6% positive static weight of 100kg which equates to one passenger with baggage —
margin is maintained — which has allowed the use of a smaller horizontal this is most significant for an aircraft with a current capacity of 8-9
tail with a corresponding increase in the aircraft's efficiency. Some passengers. The system was developed by Beech in nine months using
wing weight was saved by incorporating a gust load alleviation system off-the-shelf components and successful flight trials have taken
and stall protection is provided by increasing control column forces place.
gradually with increases in angle of attack. Pilots cannot trim out
these forces as the aircraft near stall speed or the angle of attack limit.
A pilot can stall the aircraft, but only by applying considerable control
force. At the upper end of the flight envelope, overspeed protection 12.0 FUTURE FLIGHT CONTROL SYSTEMS
is also provided by increasing the column control force. It is a commonplace observation that the future can never take place
These envelope protection features are lost, and the handling qualities until the present is past. Or, to put it more simply, 'you can only predict
of the aircraft are slightly degraded, if the pilot selects on a cockpit things after they have happened'*7*. That being so, it is difficult to
switch a conversion from digital to analogue-only control. There is say with assurance what will be the full extent of the application and
also continuous manual reversion provided in basic pitch and roll development of flight control systems in the future. In essence, control
control through cables to the horizontal stabiliser and the spoiler is a technology which works by increasing complexity: if any flight
panels. task can be accomplished without the use of an AFCS, then it should
In the Airbus A320 sidestick controllers are used. The pitch control be. This choice does not injure the reliability of the aircraft and
law on that aircraft type is basically a flight path rate command/flight reduces the cost. Whenever it is difficult to fully accomplish a
path angle hold system and there is extensive provision of flight mission, for whatever reason, then automatic control is required.
envelope protection. In particular, the low speed protection system is Such control uses two kinds of information: process, usually
especially effective. Initially the lateral control for the A320 was not expressed in terms of dynamic models, and data, usually in the form
so favourably received by pilots but has been improved subsequently*5*. of direct measurements*8'. The complexity of any control enables
As in the Boeing 777 the bank angle is limited to 35°; there is also interaction to take place which, in turn, determines the perceived
low speed stall protection at high angle of attack. The FBW system complexity. If the interaction is to become more interactive the
works through an integrated flight management and guidance system complexity of the control system has to increase. However, it be-
(FMGS) which comprises the AFCS, the speed control system, the comes impossible to handle arbitrarily large amounts of
FMS and an electronic flight instrument system (EFIS) and an complexity1.
electronic centralised aircraft monitor (ECAM) display (the A320 To reduce uncertainty by increasing complexity, as is routinely
has six ECAMs). done in AFCS, is inherently difficult for it is possible that such
In these Airbus FBW aircraft turn coordination is provided by an action will result in either of two kinds of catastrophe:
aileron-rudder interconnect. There is pitch coordination in turns. A (/) a data catastrophe, in which the amount of data needed to resolve
speed control system maintains either VREF or the speed which the uncertainty becomes unbounded;
obtained at engagement. And there is a wing leveller (for bank angles (ii) a process catastrophe, in which the process action needed to
less than 3°). There is no mechanical back-up. Because of this, the avoid the uncertainty becomes unbounded.
Airbus system must be able to detect and correct any failure in the Such catastrophic conditions in modern transport and combat aircraft
signal generator (analogue or digital), in the processors, or in the AFCS are easy to identify. Process catastrophes correspond to crashes
control surface actuators within a few milliseconds. Equipment has in which the aircraft or its systems fail. Data catastrophes cause
to be triplicated, or in some cases, quadruplicated with automatic
'majority voters' and there is some provision for system reconfiguration.
. One new fly-by-light system is the engine control system developed +
These statements correspond to MacFarlane's three laws of interaction,
by Beech Aircraft (in association with its parent cQmpany Raytheon which are analogous to the laws of thermodynamics.
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https://doi.org/10.1017/S0001924000064976
 MCLEAN AIRCRAFT FLIGHT CONTROL SYSTEMS 165

crashes which occur because of mode confusion, or what has been lead in the future to the entire removal from aircraft of the control
attributed coarsely in the past to 'pilot error'. As FBW technology column and rudder pedals, with their associated cables, pushrods and
has been more widely adopted, more AFCS modes have been bellcranks.
achieved. The resultant increase in automation has frequently Nowadays, combat aircraft must have reduced radar cross-sectional
increased the difficulties for pilots. In combat aircraft, for example, areas. There are many techniques which can contribute to such a
the primacy of piloted aircraft in theatres of war is evident in the vigorous reduction, but the avoidance of sharp edges, such as those produced
development of the next generation of combat aircraft, such as the as a result of deploying an aircraft's control surfaces, is a leading
Typhoon, the F-22, and the Jast. These aircraft have been designed contributory factor. Hence future AFCS must achieve the required
to depend upon the innate capacity of an human pilot to make swift stability augmentation and dynamic response with the least possible
decisions and also upon that pilot's capacity to adapt to unfamiliar use of flap-type surfaces. It appears to be very probable, therefore,
flight situations. The possible application to AFCS of neural network that future combat aircraft will be tailless and will employ thrust-
control or artificial intelligence systems to augment the mental vectoring, in preference to the deflection of aerodynamic control
capacity of such pilots and, subsequently, to replace entirely such surfaces, at least, in some particular phases of flight. Such an extension
human operators, is being actively studied but is not yet very far of normal control practice, but using unfamiliar control effectors,
advanced in practice. Even where such technology could be considered could result in the eventual provision of an emergency AFCS which
for military applications, it would find institutional resistance to its will allow a passenger aircraft, with serious failures of its control
early adoption for civil aviation, since it would be impossible to surfaces resulting in a degradation of its flying qualities to level III,
demonstrate the integrity of the flight control law, no matter how to be controlled through its engines sufficiently to achieve a safe
effective in normal circumstances, to the airworthiness authorities. landing.
Indeed, new control theories based on biological principles, such as
genetic algorithms, will be unlikely to find early application in flight
control systems for that reason. (In addition, the experience of the
flight control community with the biologic-based notion of adaptive 13.0 CONCLUSIONS
control would act as a powerful disincentive). The practice of relying on full-time operation of AFCS for the efficiency
In combat aircraft, it is most likely that future AFCS must increasingly and safety of the aircraft is an established one. A modern multiple-
involve the use and harmonisation of an extensive number of active redundant, safe, full-time integrated FBW AFCS is exemplified by
control surfaces in a sustained attempt to achieve full decoupling of those in use in the American F-16. The use of such types of AFCS
the six degrees-of-freedom of an aircraft's motion. The performance also enable aircraft like the Boeing 777 and Airbus A320 to provide
benefits of having achieved such decoupling, particularly in respect flight envelope limiting so that the pilot can safely use all the
of close integration of the AFCS with the weapons control systems, performance of which those aircraft are capable. However, the
manoeuvring the aircraft beyond stall angles of attack, manoeuvring complexity of such AFCS is now so great that it is difficult for any
during sustained supersonic cruise, etc., are of such compelling individual engineer to provide a single design: the task has to be
significance to combat success that such systems will become undertaken over an extended period by a design team of multi-
operational within the next ten years. The achievement of these disciplinary specialists. The concern which remains in accepting the
developments must necessarily involve the use of thrust vectoring, effectiveness of such systems relates to the uncertainty that can affect
and, possibly, canards. pilots in an emergency situation at some moment when their knowledge
These techniques, when slightly modified, will also find use in of the exact nature of the control actions being taken by the system
civil aviation to provide such aircraft with the precise control which will be necessarily imprecise, and may bring with it a fatal lack of
will be required when four-dimensional navigation becomes the normal confidence in the system. This uncertainty has been referred to as
practice in air traffic control. However, in future civil aviation aircraft, mode confusion or adverse aircraft-coupling. Its prevention is one of
it is likely that there will be fewer AFCS modes than at present. The the principal problems now facing the design of AFCS.
greatest disbenefit of present day AFCS is considered to be the automatic
transition which takes place from one AFCS mode to another. For
example, when considering vertical navigation, the AFCS maintains a
fixed pitch attitude in the vertical climb mode until the required REFERENCES
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control wheel when commanding, for example, a bank angle of
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greater than 35°, or when pulling back the wheel as the aircraft
accelerates below the minimum manoeuvre speed. The technical
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availability of alternative methods of generating input commands,
such as touch screen technology, or voice inputs. Such technology is
already being applied, to a limited extent, in military aircraft. Its
continued use and development will result in an increase of confidence
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The successful implementation of such systems could eventually
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https://doi.org/10.1017/S0001924000064976
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