Originally Posted by
DozyWannabe
The problem (as you state below regarding another point, and as I think I said in my original reply) is with false positives.
Well, what would be a pilot's choice. I ask the question:
Pilots, do you prefer:
a] an aircraft that may (auto)trim nose up when it's (near) stalled
b] an aircraft that may not (auto)trim nose up when such trim
is good, because the said aircraft suffers a "failure" with AoA measurement?
My (unqualified) answer is that 'a' seems more dangerous than 'b'. Then I choose 'b'.
Originally Posted by
DozyWannabe
About complexity: Yes, such a feature would add one more logical test.
See airtren's post - it's a lot more involved than that, going right back to the specification and trying to define potential knock-on effects of the change.
Dozy, I'm not trying to describe what it takes to implement a feature, I'm limiting myself on the "logical sheme", as I think this is all what is of interest to airmen (I can be wrong, there, and stand to be corrected).
So basically, I was agreeing to what you said: yes, one more "test" to do before allowing the autotrim to do its job
is adding complexity to the system.
Having by no mean done any of a serious study (from a manufacturer perspective) on such an implementation, I won't comment on the "how much more complexity".
Originally Posted by
DozyWannabe
The protections are a separate subsystem entirely from the annunciation/warning subsystem. There is no overarching logic connecting them, which makes implementing such a change considerably harder.
Huh? What's the point? Are you saying that:
- inhibiting NU autotrim when a protection is ON
is far more simple than
- inhibiting NU autotrim when a warning is ON
??
I don't understand. Protection and warning share a source: The actual AoA. I don't see where is the "more complex/difficult" thing.
By the way, IIRC (*) on the A310 (older system), there is no "hard" protection. But when the aircraft "senses" a too large AoA, I believe it enters a specific mode which "unwind" the trim by x degrees.
(*) Ref is an east-european A310 (TAROM?) incident on approach to ORY, years back.
Originally Posted by
DozyWannabe
I can see where you're coming from, I just think that a hard limit on autotrim under certain flight control circumstances would make more sense.
I'm not sure to follow you, there. Could you elaborate, please?
Originally Posted by
DozyWannabe
With an eager and switched-on crew, sure that'd work. But this crew in the wee hours appeared not to notice a Stall Warning that was blaring in their ears for nearly a minute. By the time the AoA values became invalid, the situation was pretty grim - what chance they'd notice the warning you suggest?
I don't know why AF447 crew didn't react to the S/W. If we assume they "fail to notice it", then no, the warning I suggest would probably not have had any use. Except, perhaps, with the CPT who returned just ~ the time the S/W came OFF...
Originally Posted by
DozyWannabe
I think that design decision is a compromise - i.e. after an abnormal situation like that, would the crew notice that FDs were available and switch them back on again? I think the idea there was that they'd bring the FD back once the data was good, it would then be up to the crew to either use, disregard or disable them.
Yep. Would like to know what pilots would prefer?