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The ill fated MAX of recent news had 2 angle of attack vanes, with the MCAS computer picking the worst one.
It seems a "software patch" may not be the only improvement option available. Sensible engineering may suggest that a minimum of three, possibly 5, or even 7 vanes would enable writing a subroutine to reject "outliers" and use an Average AOA within an acceptable Standard Deviation.
Could more AOA sensor vanes, at various locations on the aircraft, be added to this design?
This depends very much on what the AOA sensors are used for.
Traditionally, AOA sensors are only used for stall warning and stall barrier (i.e. stick pusher). In this case, you can get away with only two AOA sensors. A sensible design would trigger the pilot/copilot side stall warning with the onside (or opposite side) AOA sensor, which would confer the largest availability on stall warning. This is sensible because traditional stall warning does not effect flight controls and a nuissance stall warning can be inhibited manually by the flight crew.
Stick pusher, on the other hand, should require more prudence. By regulation, the activation of the stick pusher must be beyond the stall warning activation; and the airplane is designed to be stall-free if appropriate actions are taken from the recognition of stall warning activation. Therefore, the use of stick pusher is beyond the operational envelope of the airplane. A sensible design would be to require both AOA sensors to be functional and agree for the stick pusher to activate.
For FBW or even partial FBW airplanes, AOA information is often directly in the loop of flight controls for gain scheduling and envelope protection (e.g. MCAS). This becomes a completely different problem. By regulation, continued safe flight and landing must be possible without exceptional pilot skills following any single point failure in any flight control related system.
This would require at least three AOA sensors for these airplanes. It's not enough to know that the AOA measurements disagree, but it is also necessary to detect which measurement is erroneous. With three sensors, if a single one fails and disagrees with the other two, you can shut off the failed sensor and continue flying as if nothing happened. Of course, the airplane would not be dispatchable until that failed sensor has been serviced once landed.
Some airplanes (e.g. A220) have four primary AOA sensors. But this is more for dispatch availability (MMEL) than for safety. Note that the A220 has two redundant AOA vanes on-top of the primary AOA sensors (SmartProbes) for Direct Mode operation.
