Editing Fly-by-wire (section)

=== Redundancy ===
The multiple redundant flight control computers continuously monitor each other's output. If one computer begins to give aberrant results for any reason, potentially including software or hardware failures or flawed input data, then the combined system is designed to exclude the results from that computer in deciding the appropriate actions for the flight controls. Depending on specific system details there may be the potential to reboot an aberrant flight control computer, or to reincorporate its inputs if they return to agreement. Complex logic exists to deal with multiple failures, which may prompt the system to revert to simpler back-up modes.<ref name=avihand/><ref name=airbus/>

In addition, most of the early digital fly-by-wire aircraft also had an analog electrical, mechanical, or hydraulic back-up flight control system. The [[Space Shuttle]] had, in addition to its redundant set of four [[digital computer]]s running its primary flight-control software, a fifth backup computer running a separately developed, reduced-function, software flight-control system – one that could be commanded to take over in the event that a fault ever affected all of the other four computers. This backup system served to reduce the risk of total flight control system failure ever happening because of a general-purpose flight software fault that had escaped notice in the other four computers.<ref name=suth/><ref name=avihand/>