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Comment by bitwalker | Hacker News Reader

They can't agree though - angle of attack and airspeed are two different measurements. Together they can be used to detect a stall condition, but independently are insufficient. Furthermore, if either are incorrect or inoperable, then a stall may be misreported or not reported when it should be.

AoA is really the weak point in my opinion, the probe can get stuck or iced up; they always seemed extremely fragile to me for something so critical. Not to mention, redundancy is limited by the number of probes, of which there are only two on all the aircraft I've worked on, so if they ice up, that's it. IIRC that's what started the chain of events which brought down the Air France fight.

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Given the expense and size of airliners, it might be worth determining AoA via tracking minute dust particles with LIDAR. That should be highly reliable, with very direct speed measurement in airflow that isn't in any way disturbed by the aircraft. You also get airspeed.

Maybe there is an alternative to AoA. The whole point is to determine how air will be affecting surfaces such as the wing, so one could instead just put pressure transducers all over the aircraft. From that you can determine how close you are to stalling. You can even tell where on the aircraft a stall is imminent.

I edited my comment and removed the airspeed reference but what I meant it that MCAS is only supposed to activate at airspeeds approaching stall and high AoA (and some other things). In the Lion case the plane had a pair of AoA sensors in disagreement and the airspeed shouldn’t be near stall in level flight, should it? Yet the MCAS commands nose down anyway.

Define "airspeeds approaching stall". You can stall a wing at any airspeed, if the angle of attack is great enough. And being able to tell if your angle of attack is great enough is the point of the AoA sensors.

AoE sensors may well be in disagreement even without malfunction: local turbulence, flying in somebody's wake, etc

AoA sensors might disagree to a point, but 20 degrees difference is ... unusual.

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