Originally Posted by
DJ77
@DozyWannabe.
DW, I understand your point that in ALT2 the computers are missing some handles but to lose s/w would require triple AoA sensor failure or triple ADR failure or double FWC failure (and btw the UAS check list asks to respect stall warnings) so I still cannot see in what way preventing up-trim based on s/w AoA could be harmful.
S/W
can be spurious, particularly in uAS, but also, I believe, with one AOA broken (it would be outvoted by the other two for the PRIMS, but would still trigger SW - think Perpignan but other way round).
Stall warning is a
warning to the pilots to asses the situtation and take action. It is not a certainty based on which the plane itself should take action or restrict the pilots action - the FBW Bus has all that in normal law, but if you get as far as s/w then this has already failed. Or in other words, if you get a s/w on a bus, the plane is
already uncertain of its data.
The logic running through is that automation doesn't
act (or restrict pilot actions) on known-bad data - but it may still
warn pilots. Sadly in this case, I think the pilots believed the stall warning to be spurious due to uAS, and possibly believed they were overspeed and acted to correct that instead.