From
page 13 post #260 - AF447 Thread No.3 and 2725 posts ago.
At 2 h 12 min 02, the PF said "I don’t have any more indications", and the PNF said "we have no valid indications". At that moment, the thrust levers were in the IDLE detent and the engines’ N1’s were at 55% the angle of attack decreased, the speeds became valid again and the stall warning sounded again.
My interpretation is that the aircraft was in a deep stall and passing through FL250 with RoD about 10,000 feet/min, and even though both engines N1's were 55%, the pitch-down inputs decreased the AoA and the airspeed data became valid. However, that IAS was low and the stall warning activated again but ceased with further pitch-up commands as the data again became invalid.
Without AoA information, neither pilot had any idea in what part of the stall regime they were, and seemed to react as if a return of the SW was indicating Vmo, hence the continuing pitch-up command. So failure at this time to persist with the pitch-down command effectively left them doomed, as even then with the time taken for the THS to readjust (if it was going to) and for the wings to start flying, meant that the chance of reducing the RoD to zero before FL0 was marginal.
It strikes me that no-one on the flight deck realized that the AoA data was only valid when IAS was greater than 60KTS. Even though they went looking for the FPV, it seems it was only after the SW stopped at 2:11:40 plus a few seconds, and therefore to no avail - see ACARS FPV messages timed 0212z. Their final chance to grab the FPV page was missed shortly after 2:12:02 when the stall warning sounded again - meaning valid airspeed!
Since the post above,
A33Zab has provided information on the No Calculated Data (NCD) and
PJ2 has provided a possible PFD image as a help to interpreting what the pilots were seeing at this time.
I have previously provided some updated timing for when the FPV page data was selected (between 02:11:55 and 02:11:59) and
A33Zab has posited that the Capt ordered the thrust to idle. I also suspect that the Capt didn't know that the aircraft had been to FL380 and was in no better position than those in the front seats in understanding the reason for the SW following ND commands.
All were confused by the continuing NU eliminating the SW, and the PF most of all as he apparently didn't realize that the initial couple of stall warnings were short excursions into high AOA.
A system designed to provide warning of an approach to a stall has implemented that warning on the basis that the normal means of avoiding the stall will be taken. If as part of the design criteria an increasing AoA and decreasing IAS were further considered, then the NCD case would have resulted in "
STALLED" in large red letters on the PFD.
Now stalling an Air Transport aircraft is what the SW set out to avoid, and the (NCD) lack of data on the PFDs resulted in the, "No valid indications" comment rather than, "We are stalled!"
If it is now possible to rationally explain some unforeseen bi-product of switching to ALT 2 LAW (following loss of airspeed data) that contributed to the "zoom climb" and inability of the crew to get the nose down, then all the A332 series aircraft could be at risk. I don't think that, and I guess that neither does Airbus Industries or the BEA. An aviation psychologist may provide an answer, but the simplistic approach is "action = reward", and that
false reward needs to be addressed in the software logic associated with the Stall Warning.
I am not advocating that the FD crew become the servants of computers, though if those same computers had been programed to "save" the aircraft from the "crew", then IMHO none of what we are second guessing would have happened. The crew are given the right to have the final say, and sadly in this accident their interpretation of events turned out to be wrong.
EDIT :: It is worth noting that no control surface systems fault warnings entered the the CMC and resulted in ACARS messages. There was plenty of time for them to be transmitted, and due to the manner in which the BEA produced their Note, it may be safe to assume that there were no control faults recorded by the FDR.