Safety of powered controls

For safety in the event of power failure, duplicate, triplicate or even quadruple systems are provided on aircraft with power controls. Davies (1971) gives an excellent diagram showing the complex arrangement used in the Boeing 747.

On very large aircraft there may be as many as four separate systems, each driven by a separate engine, and each with an auxiliary air-turbine driven backup supply. Most surfaces are powered by more than one system, and there may be alternative surfaces for each function. Unfortunately, no system is totally fail-safe and one fatal accident occurred when a fin was lost, because all four systems were used to operate rudder surfaces. Although the aircraft was flown successfully for some time with no fin, a total loss of hydraulic fluid finally resulted in disaster. Such fluid leakage can be prevented by means of limiter valves, which seal when excessive fluid flow occurs.

Mechanical devices are not necessarily any safer, however, and many accidents have been caused by control cables breaking or jamming. Hydraulic tubing and electrical wiring have the added advantages that they can follow much more tortuous and convenient paths.