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Fly-by-wire describes a system where no direct mechanical connections exist between the steering instruments of an aircraft and the aerodynamically acting flaps or rudder. In smaller or older planes the pilot directly adjusts the aircraft's control surfaces via steel wires or pushrods. On heavier or faster planes the control forces the pilot must apply increase rapidly, and although there are techniques to overcome this (eg servo-tabs and aerodynamic balances on the control surfaces), most large or fast aircraft use powered controls.

With powered controls the pilot's control inputs are converted into a (usually) hydraulic signal that is transferred via the hydraulic system to a servo that drives the control surface. Feedback to the pilot is generated artificially. There is still, however, a direct (although not necessarily linear) relationship between the pilot's control inputs and the movement of the control surface.

In a fly-by-wire system the pilot's control inputs are fed to a computer which decides, by consulting the rules with which it has been programmed, what control surfaces to operate, and by how much. Normally the control surfaces will move as the pilot commands, but in some cases the fly-by-wire system may modify the response, depending on the particular circumstances. If the pilot's control input is considered unsafe it may not be carried out at all. In this respect fly-by-wire represents a major departure from all that has gone before, since it takes ultimate authority from the pilot and gives it to the system designers.

The first fly-by-wire aircraft were military jet fighters in which the system was intended to give the fighters greater maneuverability. In some fighter jets, the jet itself is aerodynamically unstable and cannot be controlled without computer monitoring.

On June 26 1988 an Airbus A320 (the first commercial aircraft to use fly-by-wire) crashed at an airshow at Mulhouse-Habsheim Airport in France killing three passengers. The pilot has claimed that the fly-by-wire system prevented him from flying safely, although the official report blames the pilot. Controversy continues over fly-by-wire but its use is becoming increasingly commonplace.