Originally Posted by
Bpalmer
My own A330 simulator trials of recovery after the stab trim was full nose up and the stall fully developed was that a prolonged nose down input did help—but only initially. However, as the angle of attack reduced and the speed increased, apparently the full nose up stab was more than the elevator could overcome and the airplane pitched up regardless of the nose-down stick position.
It's not clear if the autopitch trim was operational with questionable airspeed inputs, but despite high bank angles in an attempt to keep the nose low, recovery was not achieved until I reduced the stab trim manually. Once trim was reduced manually, pitch control resumed and recovery was possible.
Right - our experiment - linked here:
http://www.pprune.org/tech-log/46062...ml#post6793521
Originally Posted by me
So, I was talking about some research this weekend. I got in touch with an old Aero Engineering pal of mine from Uni and he managed to wangle us some spare sim time at his facility in the wee hours inbetween training sessions. What we had was an A320 sim rather than an A330, which comes with some key differences - the most obvious of which is the lack of Alternate 2, the nearest equivalent being Alternate without speed stability, and a different underlying architecture past a certain point.
Due to time constraints we could only run each experiment once, preceded by some familiarisation time handling the sim manually in Normal Law, albeit at low level, following the FDs around basic turns and level changes.
The first experiment involved setting the conditions to night IMC with CBs in the vicinity, having set the autoflight to take us to 35,000ft and hold us there. We had a friend of his who is a TRE sitting in the LHS to provide guidance and monitor what we were doing. He then failed the ADCs, leading to autopilot disconnect and a drop to Alternate (without speed stability) and we tried to follow through and maintain a 15 degree pitch angle. Things we noted:
I'd suspected it would involve considerable effort to hold the sidestick there for a significant amount of time, but I was genuinely surprised at just how much.
The zoom climb occurred exactly the way we expected
The Alternate Law (no speed stability) on the A320 seems to have a hard trim limit of 3 degrees nose up
It was definitely possible to hold the aircraft in the stall with 3 degrees of nose-up trim and full back stick, but it required effort
The aircraft wanted to nose down and recover itself, and with about 10 degrees of nose-down maintained with the sidestick at the moment we passed about 30,000ft, we managed to effect a recovery with the speed coming back up to a point where we could level out safely at about 20-25,000ft judging by the standby altimeter.
The second experiment was the same as the first, but as my pal had noted, the A320 has a hard limit of 3 degrees NU trim available via autotrim in the secondary Alternate Law. We tried again, this time winding in full nose-up trim manually just prior to the point of stall. This time:
The aircraft seemed more willing to hold pitch with the trim at full-up, but to hold it at 15 degrees still required considerable effort
We had to add a touch of rudder (on the TRE's advice) to control the roll.
Despite full nose-up trim, we elected to start a recovery as we came down through about 35,000ft this time, just to see if it was possible using sidestick only
Following the same 10 degree nose-down sidestick demand as before, the trim rolled forward with the sidestick demand, returning to around neutral within about 5-8 seconds, and we came out of the stall as before.
Based on this, as far as the A320 is concerned at least, recovery is possible using autotrim via sidestick only even when the trim has been manually wound fully nose-up. Given more time we'd have liked to see what happened attempting recovery at lower altitudes, but the general take-away seems to be that with sufficient forward sidestick demand it is possible to recover from stall even with trim forced to where it's not supposed to be.
Of course, these were purely technical experiments. Not only was this a sim session with only pride at risk, but we all knew what was coming and had a pretty good idea of how to get out of it. This does not and cannot compare to a situation where you're trying to get out of it for real, especially with the added handicap of limited manual flying experience.
Whether the A330 behaves differently I don't know, but I've called in my favours for now and am eternally grateful to the people who made it possible. Someone else is going to have to take that on.
As far as I know, a UAS condition similar to AF447 - i.e. equivalent to double simultaneous ADR fail with the third following - is *not* enough to take the aircraft out of Alternate Law and into Direct, which means that autotrim would stay on.
In your experiment, did you fail all 3 ADRs at the same time? This would put the aircraft into Direct Law and fail the autotrim - which might explain why your trim wheel didn't roll forward with sustained nose-down stick in the way ours did.
@Winnerhofer - they're pilots, not dogs. You can't force behaviour just by making things more difficult for them. The THS position would have been a moot point if the PF hadn't made significantly inappropriate pitch inputs for that phase of flight.