Fly-By-Wire | Fly-By-Wire System in aircraft



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.


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.


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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