A specific aspect of the AF447 incident that has not been mentioned here is the fact that the incident aircraft's computers declared data from the Angle of Attack (AOA) sensors INVALID if the measured airspeed was LESS than 60 knots.
By itself, this is a sensible thing to do, since the AOA sensors are composed of a little vane that is free to rotate and align itself with the direction of airflow, connected to an angle resolver that measures the angle of the vane. If the airflow is not fast enough, there will not be enough force exerted to reliably and continually align the vane, thus the instantaneous angle of the vane may not conform to the angle of the airflow. To quote the BEA report:
> If the CAS [calibrated air speed] measurements for the three ADR [air data reference] are lower than 60 kt, the angle of attack values of the three ADR are invalid and the stall warning is then inoperative. This results from a logic stating that the airflow must be sufficient to ensure a valid measurement by the angle of attack sensors, especially to prevent spurious warnings.
However, the pilot flying had gotten the airspeed under that 60kt threshold, which inhibited the stall warning. When he reacted by pushing the control stick forwards (the correct reaction to a stall), the airspeed increased (a good thing, you need airspeed for lift) and the AOA decreased (also a good thing). However, the airspeed quickly increased over the 60kt threshold -- well before the AOA had a chance to decrease past its threshold. Therefore, the stall warning sounded again.
This does not fit into any mental model of how aircraft operate. A pilot knows, from their training and experience, that when you pull back on the stick, you end up stalling your plane (which leads the stall warning to sound); and to recover from a stall, you must push forwards on the stick, which unstalls the plane. After you're out of the stall, after you've recovered by pushing the stick, you expect the stall warning to be silent.
The crew were likely not aware that the stall warning cannot sound under 60kt airspeed, and could not, were not given the opportunity or information to realize that "stall warning NOT sounding" can signify that the plane is in fact, very seriously stalled.
The crew reacted with control inputs to shut that stall warning up, to avoid a stall, because "silent stall warning" is supposed to mean "not stalled". Acting on that stall warning, unaware of its treacherous reversed semantics -- sounding with the stick pushed forwards, and silenced when pulling back -- sent that crew to their doom:
> Until the end of the flight, the angle of attack values changed successively from valid to invalid. Each time that at least one value became valid again, the stall warning re-triggered and each time the angle of attack values were invalid, the warning stopped. Several nose-down inputs caused a drop in the pitch attitude and the angle of attack, whose values then became valid, such that a clear nose-down input resulted in the triggering of the stall warning. It appears that the PF reacted, on at least two occasions, with a nose-up input, whose consequences were an increase in angle of attack, a drop in measured speed and consequently stopping the stall warning. Until the end of the flight, no valid angle of attack value was less than 35°.
There was no AOA instrument on the flight deck either! The computers saw the AOA increase when the pilots pulled the stick back and saw the AOA decrease when the pushed forwards, but the pilots themselves never saw that, because they were not shown a single AOA number/readout. The information they were given that was derived from AOA (the stall warning's status) was completely and fatally misleading; to the point that the pilots never diagnosed a stall:
> The crew never formally identified the stall situation. Information on angle of attack is not directly accessible to pilots. The angle of attack in cruise is close to the stall warning trigger angle of attack in a law other thann normal law. Under these conditions, manual handling can bring the aeroplane to high angles of attack such as those encountered during the event. It is essential in order to ensure flight safety to reduce the angle of attack when a stall is imminent. Only a direct readout of the angle of attack could enable crews to rapidly identify the aerodynamic situation of the aeroplane and take the actions that may be required.
> Consequently, the BEA recommends that EASA and the FAA evaluate the relevance of requiring the presence of an angle of attack indicator directly accessible to pilots on board aeroplanes.