Fly-by-wire
Fly-by-wire (FBW) is a system that replaces the conventional manual flight controls of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires, and flight control computers determine how to move the actuators at each control surface to provide the ordered response. Implementations either use mechanical flight control backup systems or else are fully electronic.[1]
For other uses, see Fly-by-wire (disambiguation).Improved fully fly-by-wire systems interpret the pilot's control inputs as a desired outcome and calculate the control surface positions required to achieve that outcome; this results in various combinations of rudder, elevator, aileron, flaps and engine controls in different situations using a closed feedback loop. The pilot may not be fully aware of all the control outputs acting to affect the outcome, only that the aircraft is reacting as expected. The fly-by-wire computers act to stabilize the aircraft and adjust the flying characteristics without the pilot's involvement, and to prevent the pilot from operating outside of the aircraft's safe performance envelope.[2][3]
Safety and redundancy[edit]
While traditional mechanical or hydraulic control systems usually fail gradually, the loss of all flight control computers immediately renders the aircraft uncontrollable. For this reason, most fly-by-wire systems incorporate either redundant computers (triplex, quadruplex etc.), some kind of mechanical or hydraulic backup or a combination of both. A "mixed" control system with mechanical backup feedbacks any rudder elevation directly to the pilot and therefore makes closed loop (feedback) systems senseless.[1]
Aircraft systems may be quadruplexed (four independent channels) to prevent loss of signals in the case of failure of one or even two channels. High performance aircraft that have fly-by-wire controls (also called CCVs or Control-Configured Vehicles) may be deliberately designed to have low or even negative stability in some flight regimes – rapid-reacting CCV controls can electronically stabilize the lack of natural stability.[3]
Pre-flight safety checks of a fly-by-wire system are often performed using built-in test equipment (BITE). A number of control movement steps can be automatically performed, reducing workload of the pilot or groundcrew and speeding up flight-checks.
Some aircraft, the Panavia Tornado for example, retain a very basic hydro-mechanical backup system for limited flight control capability on losing electrical power; in the case of the Tornado this allows rudimentary control of the stabilators only for pitch and roll axis movements.[8]
Analog systems[edit]
All "fly-by-wire" flight control systems eliminate the complexity, the fragility and the weight of the mechanical circuit of the hydromechanical or electromechanical flight control systems – each being replaced with electronic circuits. The control mechanisms in the cockpit now operate signal transducers, which in turn generate the appropriate electronic commands. These are next processed by an electronic controller—either an analog one, or (more modernly) a digital one. Aircraft and spacecraft autopilots are now part of the electronic controller.
The hydraulic circuits are similar except that mechanical servo valves are replaced with electrically controlled servo valves, operated by the electronic controller. This is the simplest and earliest configuration of an analog fly-by-wire flight control system. In this configuration, the flight control systems must simulate "feel". The electronic controller controls electrical feel devices that provide the appropriate "feel" forces on the manual controls. This was used in Concorde, the first production fly-by-wire airliner.[a]