AF447

[Hyperveloce]

Pitot freezing event & false stall alarms: imagination ?

-during the two Air Caraïbe, the crew got two series of stall warnings a few (~30s and ~40s for the 2nd incident) seconds after the deluge of fault reports & the loss of flight assistances and protections (severe turbulence penetration proc.: N1 fixed between 81 and 82%, Mach 0.80).
-during a flight between Paris and Antananarivo (flight AF 908), FL370, A/THR OFF, Mach 0.80, after a similar sequence of faults, the crew immediately faced stall warnings although the A/THR was TO/GA locked (-50kts of speed trend), and decided to initiate a descent (and a departure from the initial route to avoid traffic during the descent).
-flight AF 422 (FGLZT) Paris/Roissy-Bogota, entering a cirrus veil at FL370, severe turbulence, A/P manually disengaged, deployment of the airbrakes to avoid MMO, Mach 0.78 selected, airbrakes retracted and A/P reengaged. Then again, new brutal acceleration, A/P automatically OFF, many red echoes on the weather radar, ice is beginning to accrete on the windshield. Altitude is maintained +/- 200 ft. Many thunderbolts around, key phrase to the PNC in the cabin. Suddenly the airspeed rolls back in the red area, recovers its value, then again the two airspeed indications are lost on the PFD and stall alarms are sounded. The PF was about to implement the stall procedure when the airspeeds recovered.
Over 6 recent known cases (incl. the two cases investigated by the NTSB), 4 produced stall warnings a few seconds (40s max for the AC case) after the classic sequence of faults had occurred, one produced a late overspeed warning (Northwest flight ?). If these weren't false alarms, this would be a very strange combination of coincidences, how to explain that without obvious alterations of the flight parameters (or with a thrust increase to TO/GA), the planes initially in stabilized cruise flight were approaching a stall in a blink of an eye, in all these cases ?
Jeff
PS) when you read the account of these flights, you don't really get the feeling that the crew had "plenty of time" (say more than 15-20 s) to think about the stall alarms, and you don't get the feeling that these were "smooth" quiet flights when the sequence of problems occurred. In the case of the AF 422, the turbulences had increased just before the event (maintaining altitude +/- 200 ft) and most of the flights had disengaged their A/THR (turbulence penetration), AF 447 excepted. There was a large number of sudden & confusing events concentrated within 1 or 2 minutes.