flight control syste1

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FLIGHT CONTROL SYSTEM

&LAWS

FLIGHT CONTROLS

Fly-by-wire(FBW) is a system that replaces theconventionalmanual flight controlsof an aircraft withanelectronicinterface. The movements of flight controlsare converted to electronic signals transmitted by wires(hence the fly-by-wire term), and flight control computers
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determine how to move theactuatorsat each controlsurface to provide the ordered response. The fly-by-wiresystem also allows automatic signals sent by the aircraft's

computers to perform functions without the pilot's input, asin systems that automatically help stabilize the aircraft.

Development

Mechanical and hydro-mechanicalflight controlsystemsare relatively heavy and require careful routing offlight control cables through the aircraft by systems ofpulleys, cranks, tension cables and hydraulic pipes. Bothsystems often require redundant backup to deal withfailures, which increases weight. Both have limited abilityto compensate for changingaerodynamicconditions.Dangerous characteristics such asstalling,spinning

andpilot-induced oscillation(PIO), which depend mainlyon the stability and structure of the aircraft concernedrather than the control system itself, can still occur withthese systems.

The term "fly-by-wire" implies a purely electrically signaledcontrol system. It is used in the general sense ofcomputer-configured controls, where a computer system is

interposed between the operator and the final controlactuators or surfaces. This modifies the manual inputs ofthe pilot in accordance with control parameters.

Side-sticks,centre sticks,or conventional flightcontrolyokescan be used to fly FBW aircraft
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Basic operation

Command

Fly-by wire systems are quite complex, but their operationcan be explained in simple terms. When a pilot moves thecontrol column (orsidestick), a signal is sent to acomputer (analogous to moving agame controller) thesignal is sent through multiple wires (channels) to ensurethat the signal reaches the computer. A 'Triplex' is when

there are three channels being used. In an Analog system,the computer receives the signals, performs a calculation(adds the signal voltages and divides by the number ofsignals received to find themean averagevoltage) andadds another channel. These four 'Quadruplex' signals arethen sent to the control surfaceactuator,and the surfacebegins to move.Potentiometersin the actuator send asignal back to the computer (usually a negative voltage)reporting the position of the actuator. When the actuatorreaches the desired position, the two signals (incomingand outgoing) cancel each other out and the actuatorstops moving (completing afeedbackloop). In a Digital FlyBy Wire Flight Control System complex software interpretsdigital signals from the pilots control input sensors andperforms calculations based on the Flight Control Laws

programmed into the Flight Control Computers and inputfrom the Air Data Inertial Reference Units and othersensors. The computer then commands the flight controlsurfaces to adopt a configuration that will achieve thedesired flight path.
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Automatic stability systems

Fly-by-wire control systems allow aircraft computers toperform tasks without pilot input. Automatic stabilitysystems operate in this way.Gyroscopesfittedwithsensorsare mounted in an aircraft to sensemovement changes in thepitch, roll and yaw axes.Any

movement (from straight and level flight for example)results in signals to the computer, which automaticallymoves control actuators to stabilize the aircraft.
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Safety and redundancy

Aircraft systems may be quadruplexed (four independentchannels) to prevent loss of signals in the case of failure ofone or even two channels. High performance aircraft that

have FBW controls (also called CCVs or Control-Configured Vehicles) may be deliberately designed tohave low or even negative stability in some flight regimes,the rapid-reacting CCV controls compensating for the lackof natural stability.
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Pre-flight safety checks of a fly-by-wire system are oftenperformed usingBuilt-In Test Equipment(BITE). Onprogramming the system, either by

thepilotorgroundcrew,a number of control movementsteps are automatically performed. Any failure will beindicated to the crews.

Some aircraft, thePanavia Tornadofor example, retain avery basic hydro-mechanical backup system for limitedflight control capability on losing electrical power, in thecase of the Tornado this allows rudimentary control ofthestabilatorsonly for pitch and roll axis movements.

Facts

The FBW system replaces heavy mechanical control cablesfound in traditional mechanical flight control systems,with electrical signals generated by a computer and thentransmitted through wires to the final control

actuators. FBW system controls the aircraft ailerons,elevators, rudder, flaperons, spoilers, and horizontalstabilizers.

Features of Fly-By-

Wire Benefits

Replacement of heavy

mechanical system by

lightweight electricalwires

Reduced fuel costs,

increased passenger

capacity, simplifiedmaintenance

Electrical control

signals

Improved control

signal response time

facilitates precise
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aircraft handling

Auto stabilizers Reduction in pilot

fatigue, improved

flight efficiency

Airbus/Boeing

Airbus and Boeing commercial airplanes differ in theirapproaches in using fly-by-wire systems. In Airbus

airliners, the flight-envelope control system always retainsultimate flight control when flying under normal law, and itwill not permit the pilots to fly outside these performancelimits unless flying under alternate law. However, in theevent of multiple failures of redundant computers, the

A320 does have a mechanical back-up system for its pitchtrim and its rudder. TheA340-600has a purely electrical

(not electronic) back-up rudder control system, andbeginning with the new A380 airliner, all flight-controlsystems have back-up systems that are purely electricalthrough the use of a so-called "three-axis Backup ControlModule" (BCM)

With theBoeing 777model airliners, the two pilots cancompletely override the computerized flight-control system

to permit the aircraft to be flown beyond its usual flight-control envelope during emergencies. Airbus's strategy,which began with theAirbus A320,has been continued onsubsequent Airbus airliners.
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To explain better flight control we take some aircraft andthere implemented flight control .

Fly-by-Wire Principles

On aircraft of the A300 and A310 type, the pilot orders aretransmitted to the servo-controls by an

arrangement of mechanical components (rods, cables,pulleys, etc.). In addition, specific computers and

actuators driving the mechanical linkages restore the pilotfeels on the controls and transmit the autopilot

commands


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The term fly-by-wire has been adopted to describe the use of

electrical rather than mechanical signalling

of the pilots commands to the flying control actuators. One can

imagine a basic form of fly-by-wire inwhich an airplane retained conventional pilots control columns

and wheels, hydraulic actuators (but

electrically controlled), and artificial feel as experienced in the

1970s with the Concorde program. The

fly-by-wire system would simply provide electrical signals to

the control actuators that were directly

proportional to the angular displacement of the pilots controls,

without any form of enhancement.In fact, the design of the A320, A321, A330, and A340 flight

control systems takes advantage of the

potential of fly-by-wire for the incorporation of control laws that

provide extensive stability augmentation

and flight envelope limiting [Favre, 1993]. The positioning of

the control surfaces is no longer a simple

reflection of the pilots control inputs and conversely, the naturalaerodynamic characteristics of the

aircraft are not fed back directly to the pilot

The sidesticks, now part of a modern cockpit design with a large

visual access to instrument panels,

can be considered as the natural issue of fly-by-wire, since the

mechanical transmissions with pulleys,cables, and linkages can be suppressed with their associated

backlash and friction.

The induced roll characteristics of the rudder provide sufficient

roll maneuverability of design a


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mechanical back-up on the rudder alone for lateral control. This

permitted the retention of the advantages

of the sidestick design, now rid of the higher efforts required to

drive mechanical linkages to theroll surfaces.

Looking for minimum drag leads us to minimize the negative lift

of the horizontal tail plane and

consequently diminishes the aircraft longitudinal stability. It was

estimated for the Airbus family that

no significant gain could be expected with rear center-of-gravity

positions beyond a certain limit.

This allowed us to design a system with a mechanical back-uprequiring no additional artificial

stabilization.

These choices were obviously fundamental to establish the now-

classical architecture of the Airbus flyby-

wire systems , namely a set of five full-authority digital

computers controlling

the three pitch, yaw, and roll axes and completed by amechanical back-up on the trimmable horizontal

stabilizer and on the rudder. (Two additional computers as part

of the auto pilot system are in charge of

rudder control in the case of A320 and A321 aircraft.)

Of course, a fly-by-wire system relies on the power systems

energizing the actuators to move the control

surfaces and on the computer system to transmit the pilot

controls. The energy used to pressurize the

servo-controls is provided by a set of three hydraulic circuits,

one of which is sufficient to control the

aircraft. One of the three circuits can be pressurized by a Ram

air turbine, which automatically extends


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in case of an all-engine flame-out.

The electrical power is normally supplied by two segregated

networks, each driven by one or two

generators, depending on the number of engines. In case of lossof the normal electrical generation, an

emergency generator supplies power to a limited number of fly-

by-wire computers (among others).

These computers can also be powered by the two batteries.


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Main System Features

Computer Arrangement

RedundancyThe five fly-by-wire computers are simultaneously active. They

are in charge of control law computation

as a function of the pilot inputs as well as individual actuatorcontrol, thus avoiding specific actuator

control electronics. The system incorporates sufficient

redundancies to provide the nominal performance


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and safety levels with one failed computer, while it is still

possible to fly the aircraft safely with one single

computer active.

As a control surface runaway may affect the aircraft safety(elevators in particular), each computer is

divided into two physically separated channels . The first one,

the control channel, is permanently

monitored by the second one, the monitor channel. In case of

disagreement between control and

monitor, the computer affected by the failure is passivated,

while the computer with the next highest priority

takes control. The repartition of computers, servo-controls,hydraulic circuit, and electrical bus bars and

priorities between the computers are dictated by the safety

analysis including the engine burst analysis.

DissimilarityDespite the nonrecurring costs induced by dissimilarity, it is

fundamental that the five computers all be

of different natures to avoid common mode failures. Thesefailures could lead to the total loss of the

electrical flight control system.

Consequently, two types of computers may be distinguished:

2 ELAC (elevator and aileron computers) and 3 SEC (spoiler

and elevator computers) on A320/A321

3 FCPC (flight control primary computers) and 2 FCSC (flight

control secondary computers) on

A330/A340.

Taking the 320 as an example, the ELACs are produced by

Thomson-CSF around 68010 microprocessors


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and the SECs are produced in cooperation by

SFENA/Aerospatiale with a hardware based on the 80186

microprocessor. We therefore have two different design and

manufacturing teams with different microprocessors(and associated circuits), different computer architectures, and

different functional specifications.

At the software level, the architecture of the system leads to the

use of four software packages

(ELAC control channel, ELAC monitor channel, SEC control

channel, and SEC monitor channel) when,

functionally, one would suffice.


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Serve-Control ArrangementAilerons and elevators can be positioned by two servo-controls

in parallel. As it is possible to lose control

of one surface, a damping mode was integrated into each servo-

control to prevent flutter in this failure

case. Generally, one servo-control is active and the other one isdamped. In case of loss of electrical

control, the elevator actuators are centered by a mechanical

feedback to increase the horizontal stabilizer

efficiency.


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as gusts or engine failure due to a very strong spin stability,

unlike conventional aircraft. Aircraft control

through objectives significantly reduces the crew workload; the

fly-by-wire system acts as the inner loopof an autopilot system, while the pilot represents the outer loop

in charge of objective management.

Finally, protections forbidding potentially dangerous excursions

out of the normal flight domain can

be integrated in the system . The main advantage of such

protections is to allow the pilot

to react rapidly without hesitation, since he knows that this

action will not result in a critical situation.


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Computer ArchitectureEach computer can be considered as being two different and

independent computers placed side by side. These two (sub)computers have different functions and are

placed adjacent to each

other to make aircraft maintenance easier. Both command and

monitoring channels of the computer are

simultaneously active or simultaneously passive, ready to take

control.

Each channel includes one or more processors, their associated

memories, input/output circuits, apower supply unit, and specific software. When the results of

these two channels diverge significantly,

the links between the computer and the exterior world are cut by

the channel or channels which detected

the failure. The system is designed so that the computer outputs

are then in a dependable state (signal

interrupt via relays). Failure detection is mainly achieved bycomparing the difference between the control

and monitoring commands with a predetermined threshold. As a

result, all consequences of a single

computer fault are detected and passivated, which prevents the

resulting error from propagating outside

of the computer. This detection method is completed by

permanently monitoring the program sequencing

and the program correct execution.

Flight control computers must be robust. In particular, they must

be especially protected against

overvoltages and undervoltages, electromagnetic aggressions,

and indirect effects of lightning. They are


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cooled by a ventilation system but must operate correctly even if

ventilation is lost.

Failure Detection and ReconfigurationFlight Control LawsThe control laws implemented in the flight control system

computers have full authority and must be elaborated

as a function of consolidated information provided by at least

two independent sources in agreement.

Consequently, the availability of control laws using aircraft

feedback (the so-called normal laws) is

closely related to the availability of the sensors. The Airbusaircraft fly-by-wire systems use the information

of three air data and inertial reference units (ADIRUs), as well

as specific accelerometers and rate

gyros. Moreover, in the case of the longitudinal normal law,

analytical redundancy is used to validate

the pitch rate information when provided by a single inertial

reference unit. The load factor is estimatedthrough the pitch rate information and compared to the available

accelerometric measurements in order

to validate the IRS data.

After double or triple failures, when it becomes impossible to

compare the data of independent sources,

the normal control laws are reconfigured into laws of the direct

type where the control surface deflection

is proportional to the stick input. To enhance the dissimilarity,

the more sophisticated control laws with

aircraft feedback (the normal laws) are integrated in one type of

computer, while the other type of

computer incorporates the direct laws only.


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Actuator Control and MonitorThe general idea is to compare the actual surface position to the

theoretical surface position computed

by the monitoring channel. When needed, the control andmonitor channels use dedicated sensors to

perform these comparisons. Specific sensors are installed on the

servovalve spools to provide an early

detection capability for the elevators. Both channels can make

the actuator passive. A detected runaway

will result in the servo-control deactivation or computer

passivation, depending on the failure source.

Comparison and RobustnessSpecific variables are permanently compared in the two

channels. The difference between the results

of the control and monitoring channels are compared with a

threshold. This must be confirmed before

the computer is disconnected. The confirmation consists of

checking that the detected failure lasts for

a sufficiently long period of time. The detection parameters(threshold, temporization) must be

sufficiently wide to avoid unwanted disconnections and

sufficiently tight so that undetected failures

are toleratedby the computers environment (the aircraft). More

precisely, all systems tolerance (most

notably sensor inaccuracy, rigging tolerances, computer

asynchronism) are taken into account to

prevent undue failure detection, and errors which are not

detectable (within the signal and timing

thresholds) are assessed in respect to their handling quality and

structural loads effect.

Latent Failures


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Certain failures may remain masked a long time after their

occurrence. A typical case is a monitoring

channel affected by a failure resulting in a passive state and

detected only when the monitored channelitself fails. Tests are conducted periodically so that the

probability of the occurrence of an undesirable

a computer runs its self-test and tests its peripherals during the

energization of the aircraft, and therefore

at least once a day.

ReconfigurationAs soon as the active computer interrupts its operation relative

to any function (control law or actuatorcontrol), one of the standby computers almost instantly changes

to active mode with no or limited jerk

on the control surfaces. Typically, duplex computers are

designed so that they permanently transmit

healthy signals which are interrupted as soon as the functional

outputs (to an actuator, for example)

are lost.

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