Originally Posted by
Capn Bloggs
Consider AF447. The design team thought that an aeroplane could not be flying below 60KIAS, so they inhibited the stall warning... We know how that ended.
In hindsight that is probably a shortcoming of the risk and hazard assessment of the ADIRUs of the A330, yes. But the rationale for regarding all air data invalid at indicated air speed below 60 knots is that it was simply not known how the aircraft would behave in that regime. And it is sometimes a better idea (although that, too, requires careful evaluation and assessment) to flag all data as invalid (or not deliver it at all) rather than deliver data wich may be wrong.
As far as I recall the A3330 was not flight tested into a full stall, and very few other ways of flying that slow can be envisioned.
On the other hand it is not clear that the "reverse logic" (pitch down in a deep stall: stall warning comes on, raise the nose again: stall warning stops) of the stall warning at very high angles of attack and very low indicated air speeds was a causal factor in the accident: There are some indications that the crew did not only not react to the stall warning, but that they literally did not perceive it. It was lost to cognitive overload.
It could be possible to have an AOA of 75° and appreciate some assistance from the machine (MCAS) to keep the nose from getting there in the first place. The F up was making it reliant on only one sensor.
While the absolute value may be valid (although 75° is probably a bit of a stretch, even AF447 never even reached 50°. but the 25° or so from Lion Air is clearly in an attainable range), some things can also be deduced about the validity by evaluating rate of change, or cross-checking with other values, such as static and total pressures, vertical speed, pitch angle, acceleration, etc., even without cross-checking with the other-side AoA.
Bernd