Originally Posted by BOAC
The stall warning 'quit' due to airspeed and not AoA reading, I believe? Simple solution - keep it functioning whatever the speed without WoW. Unless the AoA readings are 'borked' due to failure/icing/whatever, at least there will be a 'nudge' to the crew (did they really need it!!??) to see if alpha might be a touch on the silly side.
Since stall AOA varies with Mach No., and the ADRs requires >60 knots dynamic pressure to have a valid (i.e. 'acceptable' airspeed value), the aircraft had no way to calculate the stall AOA using the available data when the AOA got so high that the pitot tubes could not work. It could calculate the AOA, just not what the stall AOA was and thus, it could not provide a warning.
The programming was inadequate for this condition of being airborne with very high AOA.
There are a number of things that could have been done to determine how much stall AOA correction for Mach was necessary, but since it wasn't written into the code, it didn't happen.
If, for example, the aircraft could compare g to measured AOA, it should then have been possible to estimate IAS with sufficient accuracy to set flight control gains as well as estimating Mach number based on OAT and altitude.
In any case, when NCD was encountered while airborne, the stall warning AOA should have at least been set to the backup value of 8.6 degrees and not just turned off simply because the airspeed didn't make sense. See Hazlenut's post for additional data.
http://www.pprune.org/7210355-post943.html
I'll imagine that we will hear from Dozy on this.
