Xcitation makes a telling observation about the progression of events in the AF447 cockpit.
IMHO the most perplexing of all the holes in the swiss cheese is the total absence of discussion given to the stall warnings by all 3 pilots. Was it a Titanic belief in the un-stallable nature of the airbus that the warning must be a glitch not even worthy of discussion?
Have you ever had a wasp in your vehicle as you were driving? Darned distracting, wasn't it? In fact, this type hazard is so distracting that drivers experience a very high frequency of traffic accidents during these events because they cannot properly prioritize hazards and give their full attention to driving.
I have a personal theory (that is only a theory) that all 3 pilots were rapidly sent past their maximum sensory processing thresholds and thus they were filtering out all background noises. The development of this process in all 3 crewmembers seems to have followed a triggering sequence.
Bonin started the sequence by encountering roll PIO which dominated his full attention in a manner similar to the wasp phenomena that I mentioned at the beginning. His control of the aircraft was clearly deficient.
Bonin's flying greatly concerned Robert but since he felt constrained by the Captain's briefing, he did not give relief to his concerns by assuming control, but instead monitored the gent (big wasp) in the other seat who was effectively trying to kill him at the moment and concentrated on calling the Captain.
By the time Captain Dubois arrived on the flight deck, the situation had progressed to the point that the aircraft was not properly following the controls. Both the copilots loudly made exclamations that the aircraft was not under control. In effect, he had walked into a cockpit full of wasps. Apparently nothing in his background had prepared him for unraveling such a mess.
I know that things are never supposed to get this far out of hand in an airline cockpit, but it happened in real life. A triggering event followed by a triggered sequence is one way to explain how something like AF447 happened.
They never mentally heard the stall warning because there was no relief from the mental pressure of the event. The hazard kept increasing as the altitude unwound. There was no clear AHA moment.