[inferiorhuman]

There’s a big difference between a system which must work and a system which must not go wrong. For example, the fly by wire system in an Airbus must work. A failed sensor must not disable the system. Thus, you need at least triple redundancy to keep functioning in the event of a failure.

Fly-by-wire Boeings still only have two alpha vanes. Go ahead, take a look at the next 777 or 787 you come across.

[mikeash]

Presumably the AoA sensors are not required for that system to function.

[Xixi]

AoA sensors are not required for Airbus FBW systems to function either. But they are required for the flight enveloppe protection system to function.