Why does Airbus suppress stall warnings in certain situations? I was reading about Air France 447 and came across the line:


  The stall warnings stopped, as all airspeed indications were now considered invalid by the aircraft's computer due to the high angle of attack.


In short, due to pilot error, the aircraft (A330) reached a 40 degree angle of attack while under Alternative Law (due to icing conditions) and stalled at 52 kts. Further confusion came from the stall warning suppressing itself after some period of time.


  Roughly 20 seconds later, at 02:12 UTC, the pilot decreased the aircraft's pitch slightly, airspeed indications became valid and the stall warning sounded again and sounded intermittently for the remaining duration of the flight, but stopped when the pilot increased the aircraft's nose-up pitch.


It would seem, if the aircraft was stalled, and the computers knew the angle of attack was still far greater than normal, that it's most reasonable to assume the aircraft would still be in a stall even after other sensor data became unreliable. Also, if no airspeed sensors are reading valid data... seems it's most reasonable to assume the aircraft is in a stall... or on the ground (but wheel load sensors can corroborate when that's the case) - not just give up and assume everything is fine.

It seems to me, that perhaps this stall warning is at least partly at fault for this accident.

Why did Airbus design the stall warning system to suppress warnings in this situation?
Stall warning was inhibited below 60 knots airspeed, in order to eliminate nuisance warnings during taxi and other situations. It is well below the stall speed of an A330 in cruise configuration:


landing speed with flaps extended is in order of 135 knots;
stall speed in landing configuration would be in order of magnitude of 120 knots;
stall speed in cruise configuration way above that.


It was indeed a very tragic circumstance, one of the factors preventing the flight crew from recovering from their situation. The solution however, perhaps in addition to enabling stall warnings for very low speeds in cruise configuration, is to train flight crews in recovering from a fully developed stall. During recovery, the load factor must remain under 2.5 g, the structural limit for an airliner.

This is now mandated by regulation authorities, and the first systems have been certified for training use at airlines. FAA has published 14 CFR part 60 Change 2 for upset recovery training. Alaska Airlines had the first Part 121 simulator certified for Full Stall Training under Part 60 Change 2, in April 2016. The simulator is a B737-800.