HN39 and others,
I made the mistake, on April22 (Good Friday), of suggesting
here that the stall warning provided in Alternate Law is IAS-based. It is, as HN39 quickly pointed out, AoA based. (While I was out of circulation, that Part-1 thread was closed and locked, so am unable to edit the error.)
The fact that the stall warning is indeed AoA-based, as one would like, makes it all the more puzzling that as many of 9 of the 13 cases of unreliable airspeed indications leading to Alternate Law − analysed by the BEA in their Interim 2, 1.16.3 (the 13 best-recorded of the 36 incidents summarised in Appendix 7 of the same report) − involved stall warnings. Why puzzling? Because the same analysis seems to support the theory that these pitot problems lead to under-reading, not over-reading, airspeed indications. If the A/THR (not to mention the crew) is being led to believe that the IAS is falling when it isn’t, why in level flight would these 9 cases involve the aeroplane finding itself at a high-enough AoA to trigger a stall warning?
Extracts from the BEA report, referred to by HN39:
“The speed anomalies can be characterised by two distinct signatures:
- intermittent falls (peaks);
- fall followed by levelling off (continuous period).
[...] the lower speed limits recorded were below 100 knots.
[...]
“In the cases studied, it is notable that:
- the airplane
remained within its flight envelope...”
Although the ambiguous expression “peaks” (has something been lost in the translation?) might be interpreted as indicating an intermittent rise in indicated speed in some cases, I don’t think that is intended. The burden of the analysis seems to be of consistent under-reading of IAS. On the face of it, this and a high AoA in level flight seem to be mutually exclusive, but the BEA does not address that apparent inconsistency.
However, turbulence with a gross increase in normal acceleration (load factor) is one possible explanation.
Quote:
“Turbulence was always recorded and reported. The levels felt by the crew varied from slight to strong. The recorded amplitude in recorded normal [acceleration] went from 0.75g/1.2g to 0.2g/1.9g.”
Perhaps HN39 might be able to provide a graph of level-flight AoA versus load-factor, please?
Other inconsistencies of analysis results are apparent in the BEA report − always presented without comment. In this case of unreliable airspeed indications, the most striking vulnerability in the stall protection logic − if the BEA is telling us the whole story − is, as quoted by HN39:
“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.”
Given that an IAS of 100 kts − one of the figures recorded in the analysis − at FL350 is in the region of Mach 0.3, the inference is that a stall warning would not happen until an AoA of 10 degrees. On the other hand, in all cases studied, “
the airplane remained within its flight envelope...”
As HN39 puts it:
“the occurence of stall warning in the cases studied by BEA does not seem to match the higher threshold after loss of IAS.”