Lack of stall signal was the reason for the crash
My two cents...
.... once the plane stalled, why the pilot acted as he did:
The plane is losing height at 10912 ft/min. The pilot have to determine if the plane is:
1) Diving (then apply UP elevator)
2) Falling down in deep stall (then apply DOWN elevator)
The pilot knows the stall warning is the most important warning there it is.
...BUT... THERE IS NO STALL WARNING !!! ... so the pilot assumes the PITCH indicator that points to nose up attitude is WRONG!
He has two mutually exclusive indications:
1) PITCH UP
2) LACK OF STALL WARNING
The pilot choose to believe that if there is no stall warning, there is NO STALL, the pitch indicator has gone bad and the plane is in deep dive. Therefore he will PULL UP. No other crew member corrects him, because they all think the same. If any one believed they are in sustained stall for 2 minutes, they will be crying "STALL, PUSH DOWN" and not be mostly quet.
At one moment, the pilot pushes down the stick, the plane starts to recover and .... THE STALL WARNING COMES ON !!! So, the pilot is warned that his action CAUSES STALL. Being in confused state due to conflicting information, he goes back to pull from deep dive and all other pilots agree to the end...
Here is what BEA report says:
During the following seconds, all of the recorded speeds became invalid and the stall warning stopped.
Note: When the measured speeds are below 60 kt, the measured angle of attack values are considered invalid and are not taken into account by the systems.
In my opinion, this is design problem that lead to the crash:
The fact is, during the 3 minutes descend while the plane was falling at 11000 fpm THE STALL WARNING WAS QUIET. If the stall warning was ON, the pilots would take the correct action to lower the trim, push down the stick and save the plane (if physically possible).
Here is the relevant text from BEA:
At 2 h 10 min 51 the stall warning was triggered again. The thrust levers were positioned in the TO/GA detent and the PF maintained nose-up inputs. The recorded angle of attack, of around 6 degrees at the triggering of the stall warning, continued to increase. The trimmable horizontal stabilizer (THS) passed from 3 to 13 degrees nose-up in about 1 minute and remained in the latter position until the end of the flight. Around fifteen seconds later, the speed displayed on the ISIS increased sharply towards 185 kt; it was then consistent with the other recorded speed. The PF continued to make nose-up inputs. The airplane’s altitude reached its maximum of about 38,000 ft, its pitch attitude and angle of attack being 16 degrees.
Note: The inconsistency between the speeds displayed on the left side and on the ISIS lasted a little less than one minute. At around 2 h 11 min 40 the Captain re-entered the cockpit. During the following seconds, all of the recorded speeds became invalid and the stall warning stopped.
Note: When the measured speeds are below 60 kt, the measured angle of attack values are considered invalid and are not taken into account by the systems. When they are below 30 kt, the speed values themselves are considered invalid.
The altitude was then about 35,000 ft, the angle of attack exceeded 40 degrees and the vertical speed was about -10,000 ft/min. The airplane’s pitch attitude did not exceed 15 degrees and the engines’ N1’s were close to 100%. The airplane was subject to roll oscillations that sometimes reached 40 degrees. The PF made an input on the sidestick to the left and nose-up stops, which lasted about 30 seconds.
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%. Around fifteen seconds later, the PF made pitch-down inputs. In the following moments, the angle of attack decreased, the speeds became valid again and thestall warning sounded again. At 2 h 13 min 32, the PF said "we’re going to arrive at level one hundred". About fifteen seconds later, simultaneous inputs by both pilots on the sidesticks were recorded and the PF said "go ahead you have the controls".
The angle of attack, when it was valid, always remained above 35 degrees.