The BEA 2nd Interim Report describes the calculation of the Stall Warning threshold as follows (in paragraph 1.6.11.4 on page 46, english version):
In alternate or direct law, the angle-of-attack protections are no longer
available but a stall warning is triggered when the greatest of the valid angle-of
-attack values exceeds a certain threshold. In clean configuration, this threshold
depends,in particular, on the Mach value in such a way that it decreases when the
Mach increases. It is the highest of the valid Mach values that is used to
determine the threshold. If none of the three Mach values is valid, a Mach value
close to zero is used. For example, it is of the order of 10° at Mach 0.3 and of 4°
at Mach 0.8.
This information leaves me with several questions -
Firstly, the Stall Warning should occur when the AOA exceeds 4 deg at Mach=0.8, but does not occur until AOA exceeds 10 deg when airspeed is invalid. In that case, is the Stall Warning still timely enough to permit the pilot to avoid stalling?
Secondly, in the Air Caraibes incident in october 2008 Stall Warning occurred at AOAs of 4.48 deg and 4.31 deg, while the calculated threshold was 4.2 deg. This would correspond to the actual Machnunber around 0.8, but how could 0.8 have been the 'valid' Machnumber for the system while both PFDs were displaying Mach=0.3, in ALTERNATE law, ADR DISAGREE, FD 1&2 lost, etc.?
Finally, the A330 FCOM procedure UNRELIABLE SPEED INDICATION / ADR CHECK PROC states:
Rely on the stall warning that could be triggered in alternate or direct
law. It is not affected by unreliable speeds, because it is based on angle of
attack.
Was the author of this procedure unaware of how the system works as described by BEA?
Regards,
HN39