Fly-By-Wire
Fly-by-wire is
the most advanced flight control system in which computers process the flight
control inputs made by the pilot or autopilot and send corresponding electrical
signals to the flight control surface actuators. This modern technology
replaces direct mechanical linkage.
Fly-by-wire
(FBW) is a method that uses an electronic interface in place of an aircraft's
traditional manual flight controls. Flight control computers make the decisions
on how to operate the actuators at each control surface to deliver the desired
reaction after converting the flight controls' movements into electronic
signals that are delivered across wires.
All current
passenger aeroplanes use an electrical/ electronic flight control technology
known as "fly-by-wire" (FBW). In 1988, the Airbus A320 became the
first commercial passenger aircraft to feature FBW.
Fly-By-Wire Operation
Improved fully
fly-by-wire systems use a closed feedback loop to combine various combinations
of rudder, elevator, aileron, flaps, and engine controls depending on the
situation. These systems interpret the pilot's control inputs as a desired
outcome and calculate the control surface positions necessary to achieve that
outcome. The pilot may just be aware that the aircraft is responding as
predicted and may not be fully aware of all the control outputs influencing the
outcome. Without the pilot's input, the fly-by-wire computers stabilise the
aircraft, modify the flying characteristics, and stop the pilot from operating
outside the safe performance envelope of the aircraft.
Fly-by-wire
systems operate flight control surfaces (ailerons, rudders, etc.) utilising
electrical wire connections driving motors rather than mechanical linkages
controlling hydraulic actuators. Computers at the core of the system translate
pilot commands into electrical signals that are sent to the motors, servos, and
actuators that power the control surfaces.
The lack of
"feel" the pilot gets from the system is one issue. Concerns over the
dependability of FBW systems and the effects of computer or electrical failure
are another. Due of this, the majority of FBW systems have backup mechanical or
hydraulic devices in addition to redundant computers.
How FWB Works?
The error
control principle is applied, in which a control surface's location (the output
signal) is continuously measured and "fed back" to its flight control
computer (FCC). The computer analyses the difference between the current
control surface position and the ostensibly desired control surface position
indicated by the command when a command input (the input signal) is made by the
pilot or autopilot, and an appropriate corrective signal is then electrically
sent to the control surface. The FCC controls the system by comparing output
and input signals, and feedback compensation serves as error control. Until
output equals input, any difference between the two constitutes a directive to
the flight control surface.
The signal flow from the FCC to the control surface is known as the forward path in an FBW system, whereas the signal path from the control surface to the FCC is known as the feedback loop or path. To produce the required aircraft response, gain is the amplification or attenuation that is applied to the forward signal. Signals or motion that occur at an unwelcomely frequent interval can be blocked by a filter.
The ability to
use the flight control system (FCS) to lessen sensitivity to changes in
fundamental aircraft stability characteristics or outside disturbances is a
benefit of a feedback system like this. Feedback control systems include the
autopilot, a stability augmentation system (SAS), and a control augmentation
system (CAS).
In an SAS, a
damper function that typically has minimal gain or authority over a control
surface is created in the feedback loop. A CAS is used in the forward path to
provide consistent reaction under a wide range of flight situations. It is
high-authority "power steering." Prior to fly-by-wire, the CAS and
SAS principles were utilised separately in military aircraft; when combined
into an FCS, they can operate with higher accuracy and flexibility. Through the
use of CAS gains that are configured as functions of airspeed, mach,
center-of-gravity position, and configuration, consistent aircraft response is
accomplished over a wide range of flying envelopes.
System Redundancy
The strategy
with commercial aircraft often controlled entirely by FBW is to offer
redundancy for the FCCs and sensors by adding more of them, as opposed to
providing a conventional FCS as a backup. Triplex FCSs have typically been used
in civil aircraft FBW designs, as is the case with the Boeing 777 and Airbus
A340, both of which feature limited mechanical backup to provide a period of
"survivability" during cruise to address any electrical issues. It
should be expected that any duplex FBW systems will include a complete
mechanical backup.
An FCS is
referred to as working in normal law when all components are functional.
Limited failures typically result in auto reversion to a computed, but
degraded, FCS mode. The lowest level of FBW backup mode, known as Direct Law,
often uses analogue electronic signals that don't go through FCCs and instead
go straight to the flight control actuators. Direct Law allows for fixed gains
that are intended to deliver adequate control forces proportional to control
surface deflection but does not allow for feedback control. The chosen gain may
give various gains for cruise and landing, switched, for example, by the flap
selector, or it may optimize control forces for the landing configuration.
Flight Envelope Protection
The FBW aircraft
can use feedback control of airspeed, Mach Number, attitude, and angle of
attack to keep itself inside its certified flight envelope. In order to
accomplish this, two different approaches have been used: the Airbus "hard
limits" approach, where the control laws have complete authority unless
the pilot chooses Direct Law; or the Boeing "soft limits" approach,
where the pilot can override Flight Envelope Protection and thus retains
ultimate control over the operation of the aircraft.
Fly-by-Wire Advantages
Fly-by-wire has
a number of benefits, including the possibility to save weight by doing away
with cables, pulleys, and rods, as well as increased safety, dependability, and
manoeuvrability.
Automatic
control can be implemented thanks to computer control, which also lessens the
pilot's workload. A large decrease in aircraft weight, which lowers fuel
consumption and aids in lowering unfavourable CO2 emissions, is another
important benefit of FBW. A plane can be flown more accurately and consistently
inside its "flight protection envelope" with the aid of computer
control. As a result, the crew can handle emergency situations without
endangering the aircraft or going beyond the bounds of safe flying.
FADEC
Since the
introduction of FADEC (Full Authority Digital Engine Control) engines, the
operation of the engines' autothrottles and flight control systems has been
fully integrated. Other systems like autostabilization, navigation, radar, and
weapons systems are all integrated with the flight control systems aboard contemporary
military aircraft. FADEC enables the aircraft to operate at its peak capability
without worrying about engine failure, damage to the aircraft, or a heavy pilot
burden.
Integration
improves flight safety and economy in the civil sector. Flight envelope
protection safeguards Airbus fly-by-wire aircraft from risky circumstances like
low-speed stall or overstressing. As a result, in certain circumstances, the
flight control system automatically instructs the engines to boost thrust. The
flight control systems precisely adjust the throttles and fuel tank
configurations in economy cruise modes. FADEC lessens the rudder drag required
to counteract the sideways flying caused by uneven engine thrust. To improve
the aircraft's centre of gravity during cruise flight, fuel is moved between
the main (wing and centre fuselage) tanks and a fuel tank in the horizontal
stabiliser on the A330/A340 family of aircraft. Instead of using dragumig
adjustments in the elevators, the fuel management controls properly trim the aircraft's
centre of gravity with fuel weight.
Conclusion
FBW technology
has enabled aircraft producers to create "families" of remarkably
similar aircraft. For instance, the 107-seat A318 and the 555-seat A380 from
Airbus/EADS have similar flight deck layouts and handling qualities. As a
result, crew training and conversion is quicker, easier, and significantly less
expensive. Pilots can simultaneously keep up-to-date on multiple aircraft
types.
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