Originally Posted by
mm43
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The AoA vane plus IAS provides the stall warning. Only problem is that the IAS must be greater than 60KTS for the combo to provide the SW. The SW stopped because the IAS had dropped too low, and when they attempted nose-down and got an IAS over 60KTS, the SW started again. ...
That 60 kts again --- why would a blocked pitot and open drain stick at this value? It seems to me that the IAS system outputs 60 kts (or is it 60.001?) when UAS detected, so that protections/warnings that are disabled at low speed are not disabled with UAS.
So if there's any airspeed indicated above or below precisely 60kts its a real live reading, (and if 2 out of 3 agree it is validated and used in protections/warnings) - is this documented?
You can build AoA indicators with pitot style pressure measurements using the fact that the pitot under-reads when not pointing directly into the air-stream, but it must need careful calibration - if simply varying as the cosine of the AoA AF447 would have been indicating nearer 100kts during the descent?