HN39, re
Originally Posted by post #6 [URL
http://www.pprune.org/tech-log/493472-af-447-thread-no-10-a.html#post7370832]I[/URL] agree that inhibiting autotrim would not be a good idea. But how about limiting the range of THS settings it trims to as suggested in my post here? That would retain autotrim for trimming nose-down in response to sidestick demand, just as it does in normal law above alpha-prot. EDIT:: InAF447 the THS was at about 3.1 degrees NU at stall warning 2. The effect of limiting the THS to that value is described here.
HN39, AZR, first, thank you for your responses and links and for your reminder re the trim value at stall warning 2 for AF447.
As the discussion had turned to autotrim, elevator and their behaviours I was curious to see if the Perpignan accident which was an intentional reduction of speed to test low-speed flight and the alpha response, offered any new understanding.
As expected the THS moved to trim NU as the stick was held back and speed reduced. The flight data shows that the THS reached the NU limit at the same time that the stall warning occurred, (I still don't know if a stall warning stops the autotrim on the A320 or not. I suspect not). Accordingto the BEA Report, stick inputs at the initial stall were insufficient to cause the THS setting to move ND. The aircraft reacted to the developing stall and rolled, with sideslip causing a disparity between the airspeeds ADRs 1 & 2. This caused the FAC then ELAC to reject all three ADRs and the flight control laws reverted to direct law, requiring manual input to the THS which was annunciated on the two PFDs.
This is a case in which the THS would not have moved towards the NU position with forward stick if autotrim were stopped with the stall warning.
The case may be "academic" in some eyes due to the fact that there was no time for forward stick but it serves as a sufficient counter-example to indicate that the idea needs careful thought. If Bonin had placed the stick full-forward and held it, we know that there was a likely chance for recovery. But there is a great deal behind any such pilot's decision to take such action.
So AZR I agree with you regarding not stopping the autotrim at a stall warning and I'm merely adding here something to the recent discussions on THS and elevator behaviours which I think hasn't been considered, (unless I've missed someone else's comment on this).
I agree with you HN39 regarding limiting the autotrim in some form. My thought is, as the Perpignan accident serves to illustrate, at what point and for what reason should the autotrim be stopped in its motion (either way), and then re-engaged again?
It IS re-engaged in one circumstance: The abnormal attitudes law, in which autotrim is stopped but re-engaged once the abnormal attitude limits are no longer exceeded and the aircraft is recovered, (AMM).
TTex600; Re
One needs to place one's self in Bonin/Roberts exact shoes before judgement.
I don't think anyone who knows this business is "judging" this crew. Judging means one believes it can't happen to them and most of us know better. Many have said, "There but for the grace of god, etc"but in a way that isn't the case either because thirty-odd other crews WERE in Bonin's and Roberts' shoes and nothing untoward occurred. Why?
This is a performance accident. There is nothing about the airplane that caused an absence of SOPs, CRM, a disciplined cockpit and a disagreement between crew members which went unaddressed and unresolved, (PM taking control, PF taking it back after PM says get the nose down, etc). This absence has been noted by the BEA and much earlier by many here including me. That isn't judging the crew, that is asking why, when SOPs, CRM etc are what we do, they didn't respond as expected of an airline transport crew. Training, checking, standards, a professional culture are intended to ensure the performance of these fundamental and critical tasks but here that did not occur. Why? Startle effect has been noted for many years now. No one is immune of course, but the difference in the character of response is what solid training, knowledge, memory items and disciplined CRM is about. For lack of a better word, a "fear" has crept into the checking and standards regimes partly due to risks of liability but also through other, more subtle reasons, that prevents some required bluntness and otherwise clear address regarding crew performances. I would never return to some of the means I've seen to "ensure" crews do their job but the cockpit isn't a democracy, either.
If the airplane let them down, even in part and after they were well into serious trouble, then we need to ask the question, would another random crew similarly be "let down"? If so, how, when and why?