Stall Warning
In normal law, given the protections available, the stall warning was designed to avoid its activation before those angle of attack protections and not to provide an additional protection. So, this warning is triggered in normal law as soon as the value of the angle of attack of one of the three sensors reaches 23 degrees in landing configuration, higher than the angle of attack associated with Vs1g
N.B.: For a given configuration, Vs1g is the lowest speed at which the aeroplane can develop lift equal to the weight of the aeroplane
In alternate anddirect law,this threshold is modied in order to allow the stall warning to be set off at a lower angle of attack than that associated with Vs1g
(from Air NZ - Airbus A320-232 ,page 30)
Do these tighter AoA thresholds (in alternate law, along with turbulences) explain why stall alarms (going hand in hand with the revertion to the alternate law) were frequently observed: in 9 cases over the 13 past Pitot incidents ? (analysed in the 2nd BEA interim report).
In the Air Caraïbe case, as far as I can remember, the stall warning was triggered for an AoA around 4.3°.
So we get:
-a variable meaning/sensitivity (thresholding) for the stall warnings (depending on the law/mode you are in),
-when the airspeeds become unreliable and when the ADR disagree (triggering the alternate 2 law), this tighter stall warning threshold seems to become a function of the airspeed (not in normal law),
-variable ways to handle the stall warnings (depending whether you read the ECAM messaging "risk of undue stall warnings" or the unreliable IAS C/L urging to consider seriously the stall alarms since they are based on the AoA)
-a stall warning supposedly independant from the airspeeds but inhibited when the IAS are no longer valid
