Dozy: Is that because you can't see any instance when it would be useful or because it goes against the methods you were taught flying aircraft with more "conventional" controls?
Autotrim is useful in ensuring that the aircraft remains trimmed when in autopilot so that when the autopilot is disconnected, either by the flight control system or by the pilot, he is not dumped into an out-of-trim situation. However, as I have said several times before, when I am hand-flying, I don’t want the system changing the trim. I want to do that myself. Trimming is not an arduous task. In fact, it is a completely natural act and it is second nature to a properly trained pilot. It would be even easier if the AB side sticks were equipped with “coolie hat” switches. That is my personal opinion as a pilot. You have told us that you are not a pilot so I believe that you have little basis for an opinion as to how you would want to fly an airplane. Apparently you are qualified to offer opinions on how software should be written to meet certain criteria specified by the users (the pilots), but it is those users who are best qualified to set the criteria in the first place. You have stated that there are good and sufficient reasons for the design of the software in the hand-flying mode, but
AFAIK you have not offered any reasons why that design is preferable to my reasons for disconnecting autotrim when hand-flying. As a user, I believe that my preferences should take precedence over yours. I am open to reasoned arguments on that subject.
Dozy: IMO (for what that's worth) there's nothing wrong with the autotrim setup as long as the way it works is taught properly.
That’s your opinion as a non-pilot. It seems to be the consensus on this forum that the pilots of AF447 were not taught properly. It is my opinion, as a pilot, that because the autotrim continued to follow the inexplicable nose-up inputs by the PF, stall recovery would have been made more difficult, had they ever attempted stall recovery.
Dozy: If you look at the traces the trim moves minimally under automatic control. What caused it to deviate so strongly was a series of inputs that trended towards nose-up, the majority of which were around half the stick's rearward travel limit, that were sustained for the best part of 45 seconds. What caused it to continue the movement to the stops was a full nose-up deflection that lasted between a further 30-40 seconds - that's more than 1 minute and 20 seconds of nose-up input at a deflection that ranges from halfway to the stops - at cruise level!
I fully agree that the full nose-up trim situation was the direct result of the inexplicable nose-up inputs, but you need to understand the proper technique for hand-flying in turbulence, a technique which has stood the test of time over many decades. As I have previously posted, the technique for flying in turbulence is to first be properly trimmed before entering the turbulence
and not to change the trim, allowing the aircraft’s designed positive longitudinal stability to keep you flying properly. That is essentially what the stated procedure is all about
: “Fly pitch and power”. Presumably, AF447 was properly trimmed and the power properly set before the autopilot and autothrottle dropped out so they really didn’t need to do anything except take a deep breath and monitor what proper pitch and power was doing for them. If the dynamics of the weather take you off the desired altitude, you simply make small inputs to return to that altitude but you don’t change the trim because you want that proper trim setting to help you minimize excursions from the desired flight parameters. If, as a non-pilot, you think you have a better technique for flying in turbulence, please post that on the forum for the pilots here to consider.
Sometime back, someone here offered that the pilots of AF447 only needed to let go of the controls and the aircraft would have returned to normal flight on its own. While that is true with conventional flight control systems and a properly trimmed aircraft (for cruise), it was not true with AF447 because the autotrim had changed the pitch trim from a stable, level-flight setting. With a century of manned flight under our belts, the consensus of pilots is that aircraft should exhibit positive longitudinal stability. But the AirBus flight control system changes that, overrides the natural aerodynamic stability of the airframe and introduces neutral longitudinal stability when hand-flying. As a result, when a pilot moves the stick in pitch, the nose stays right there and will stay there even if the pilot lets go. It is my opinion as a pilot that an aircraft that exhibits neutral longitudinal stability is more difficult to fly properly than with positive longitudinal stability. If you have a different opinion, it will have to be as a non-pilot.
Dozy: The trim doesn't move in any noticeable way as long as the inputs are relevant to the flight regime. The PNF notices that the PF's lateral inputs seem to be extreme, and admonishes his counterpart. He then further upbraids the PF for commanding a climb when it is unnecessary. This is why the BEA recommend training for manual aircraft handling at altitude, because in this case the sidestick inputs are repeatedly way beyond what is reasonable at that altitude and airspeed.
It is my opinion that the pilots of AF447 were so poorly trained that they really didn’t know what had happened to the trim. For AB pilots, trim seems to be out of their notice in any situation except takeoff where they set it and forget it. Actually I believe that even if the AF447 pilots had known what had happened to the pitch trim, they would not have understood the consequences. They clearly didn’t understand much about flying an aircraft.
Dozy, it you have relevant arguments on why autotrim should not drop out with the autopilot, I will be glad to consider them.
There are other subjects which would be appropriate for separate threads such as the lack of tactile feedback in the side sticks, and whether or not it is appropriate to use the autopilot in heavy turbulence. I did not bring these up at this point because I wanted only to address your opposition to having the autotrim drop out when the autopilot drops out.