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Commercial aircraft have mechanical fallbacks for all controls, and emergency protocols for if some controls don't work (for example, using spoilers to turn if

Commercial aircraft have mechanical fallbacks for all controls, and emergency protocols for if some controls don't work (for example, using spoilers to turn if ailerons are inoperative). Many military jets have pure software control, as they are aerodynamically unstable and are unable to be flown safely by hand if the fly-by-wire system goes down; however, the procedure for a system failure in a military jet is usually "eject!", which is not feasible for passenger aircraft.

WalterBright 9y ago
The Boeing 757 had 3 independent hydraulic systems, with 3 actuators per surface. Any two could overpower the 3rd. Any software in the loop was dual, written on different hardware using different algorithms, with a comparator that would physically disconnect both if they disagreed. The pilot could also physically disconnect them via circuit breakers. There were also various mechanisms to prevent the surfaces from going too far (cannot move full travel at 500 mph, it would rip the airplane apart). This is all very serious stuff, and was worked over by a lot of people imagining every perverse thing that could happen, and going through the list of things that had happened, to ensure it is safe. The track record of the 757 in service shows how effective this is: https://en.wikipedia.org/wiki/Category:Accidents_and_incidents_involving_the_Boeing_757 https://en.wikipedia.org/wiki/Category:Accidents_and_inciden... None of them resulted in recommendations for design changes.