Originally Posted by
Dani
Maybe, I still don't see where autotrim was to blame in AF447's case. It did autotrim what the PF ordered. Autotrim has no artificial intelligence to know what's in a pilot's mind. It stupidly does what it has been told: When you pull for a long time it trims this position to zero force. That's the definition of trim. What's wrong about this function on AF447? If only he would have pushed the stick as long as he pulled it, autotrim would have ordered THS back, this I'm pretty sure.
Totally agree with you, Dani.
When I proposed that the (nose up) auto-trim should be inhibited if S/W in ON, it's not because I felt the auto-trim performed wrongly.
I see this feature (inhibition) as a supplementary "protection" against a
worse upset. History teaches us that, sometimes, pilots do the wrong thing, specifically they pull when (nearly) stalled. No pilot in his right mind will do that, but still that happen.
On the other hand, I've never heard of a pilot "taking the time" to trim up (manually) his aircraft while "fighting" an upset, worsening it.
Then:
Provided the aircraft knows its current AoA (and knows it's too high), I think the aircraf should inhibit the NU autotrim.
With a 100% proficient crew, who reacts as soon as the S/W gets ON, this feature is of no use, I agree.
But with a less proficient crew, who for whatever reason delays his recovery actions (namely: nose down stick, idle power), the inhibition of the nose up auto-trim has 3 advantages IMO:
1] when, hopefully, the crew asseses that he's stalled and engage the correct recovery, the aircraft will be "less" stalled (lesser AoA) than it would have been with the "help" of a more NU THS. The recovery should then be quicker. That means higher, too. Perhaps, the difference in height will be enough to prevent the crash.
2] being "frozen", the THS won't (silencely) change the aerodynamics of the aircraft at a time when the crew may be trying to assess "what's going on". By not "discretely" changing a parameter (and subsequently the aircraft behavior), the inhibition of the THS may help the crew to asses just that.
3] in the event of a further command law reversion (i.e. alternate law to direct law, or alternate law to abnormal attitude law) which will totally disable the auto-trim, the crew will not have to think "hey, I did the wrong thing just earlier, and pulled my aircraft into a stall, let's push the stick
and not forget to unwind the trim because auto-trim was available as I pulled, but is no more active now that I push because the aircraft switched to direct law".
All those reasons are tiny holes in the cheese, and of course are less important (IMO) than a crew performing the right action when confronted to a stall (warning). Nonetheless, clogging those tiny holes may perhaps save the day, one time. Then if there is no crucial disadvantage to do so, why not implement it?
Cheers
AZR
PS: By the way, I just learned that USE MAN PITCH TRIM is not displayed in abnormal attitude law (in an A320 at last). That too is a "flaw" IMO.