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.