Originally Posted by
HazelNuts39
I wonder if the system knows enough. It is latched in Alternate law because it doesn't trust airspeed anymore. Can it really trust AoA (one of the three is not functioning properly)? I would like to understand why, when the system reconfigures to Alternate law, the High-AoA protection reconfigures from an AoA reference to an airspeed reference. Is that because it doesn't trust AoA? Finally, the AoA value is zero when IAS<60kt. The system also 'knows' attitude and vertical speed. Is that enough?
Two levels of Alt law. If speeds have
not failed, then it trusts those to keep some protections in place (Alt1). If speeds
have failed, then it will latch the lower level of Alt law, with no protections in place.
Quite possibly it's this way round because airspeed was considered one of the more reliable air-data parameters...
Is attitude and vertical speed enough to warn on ? - I'm not sure. VS is coming, I think, from the same airdata that's dubious. What if there's static port failure ? If anything. I'd want to go to just the inertials - accelerometers should be plenty reliable enough over short time frame - and engine data only. Something like "if pointy end up, and engine power up and IRU says we going down, must be still stalled, or wings fallen off".
Need to be careful though that we're introducing a lot of complexity, and hence additional failure risk, into a very simple system (currently more or less "is aoa > threshold"). Stall warning failure is not good either.