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Under the traditional mechanical control system, which is still used on light aircraft, the pilot adjusts the aerodynamic control surfaces via a system of levers, wires, and pushrods. Often, servo-tabs and aerodynamic balances are used to reduce the amount of force required to operate the controls.
On larger or higher performance aircraft, the control forces the pilot must apply increase rapidly and even with servo-tabs it is usually necessary to power the control system. This operates much as an automotive power steering system does: it converts the pilot's control inputs into a hydraulic impulse that is transferred via a system of small pipes to servos that drive the control surfaces. (Alternatively, the system may be pnuematic or use electric motors, but the principle remains the same.) 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, which 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 a measure of 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 many modern fighters, the aircraft is designed to be aerodynamically unstable and cannot be controlled without computer monitoring.
In 1988, Airbus was the first company to introduce fly-by-wire in a commercial aircraft, the A320. On June 26 1988 an A320 crashed at an airshow at Mulhouse-Habsheim Airport in France killing three passengers. The pilot claimed that the fly-by-wire system prevented him from flying safely, but the official inquiry returned a finding of pilot error. The A320 went on to be one of the most successful aircraft ever made. Many other modern large commercial aircraft now use fly-by-wire, other Airbus designs to begin with, and more recently newer models from Boeing.