What Garage Years has been asking for a while now is the value of the critical AoA (AoAc) which is used by the confusers to trigger the stall alarm in ALT law when airspeed (including Mach number) is unreliable.
Unfortunately, it is not documented in the A330 FCOM. Only those who have access to technical documents not publicly available could answer the query. Others and I can only speculate hopefully trying to use sound logic.
The Air-Caraibe incident report mentioned a critical AoA of 4.2 deg. Cross-checking with the data just posted by A33Zab, this corresponds exactly to M 0.80, the recommended turbulence Mach number i.e. the speed at which the ACA A330 was supposed to fly at the time. It don’t see here a confirmation that it was the AoAc really used but just an assumption.
Obviously, confusers cannot just cancel stall warnings on the basis that Mach number is unreliable. They have to know what to do in this case but different solutions are possible like:
- Use the last Mach value considered valid.
- Use the highest current indicated Mach among the 3 sources.
- Use a combination of the above.
- Use MMO instead.
- Your choice …
None of these possible solutions are ideal.