Franzl,
Originally Posted by
RetiredF4
there is no situation in cruise, where autotrim is improving a situation in ALT2 like that encountered by AF 447.
The FCU has no concept of "cruise", as far as I am aware. It is a real-time processing system that does the job it was designed to do very well, but it is intentionally quite a simple beast in terms of design (because the simpler a system is, the less things there are to go wrong).
If you can think of one, let me know. As we know, autotrim is inhibited in ALt1 at V-prot anyway and in direct law as well.
You have to look at this from a systems perspective to understand. Autotrim as a system is never "inhibited" in any law other than Direct and Manual Trim Only - it is the *protections* that prohibit the aircraft from leaving the flight envelope by preventing any commands - either manual or automatic - from doing so, and if necessary providing corrective commands to keep the aircraft within the flight envelope. The protections are a completely discrete subsystem that is loosely-coupled to the others that make up the FCU system.
Concerning your graceful degradation explain this degradation with autotrim of the THS:
In Normal LAW autotrim (a rally nice feature)
In ALT1 Law autotrim prohibit at VC-prot (looks sound to protect from entering unknown territory)
In ALT2 Law autotrim (that is your part to explain)
In Direct LAW autotrim off (looks sound to protect from entering unknown territory)
Again, that is not correct if you look at the architecture as a whole. I'll repeat for clarity - Autotrim as a system is not "prohibited", nor "turned off" in any other mode or Law than Direct or Manual Trim mode. If the protections are active, then the autotrim commands will be treated as any other command that takes the aircraft out of the flight envelope and corrected accordingly. Think of it as two separate processes running alongside each other rather than as an integrated whole.
Put even more simply, imagine two people on either side of a wall that has a two-handled saw poking through it. The person on one side (let's call him Otto Trim) is told to push the saw forward and the person on the other side (who is physically stronger and called Pete Tection) is told to not let the saw through past, say, two-thirds of it's length. Pete will always stop the saw at the limit and will try to return it to the prescribed position if it goes past, but he is not explicitly aware that Otto's on the other side trying to push it because the wall is in the way, and as such does not interfere or communicate with Otto directly - all Pete knows is that he mustn't let it through past a certain point.
That should straighten up the manual trim option once and for all. Itīs even worse, because the functioning autotrim in ALT 2 prevents manual trim in its original sense of implementation.
What do you mean by "original sense of implementation"? Using manual trim to set the THS to a certain degree of pitch and letting go will cause the autotrim to try to hold that pitch, unless a demand is then made on the sidestick for which the autotrim will try to compensate. It doesn't "remember" what it was doing X number of seconds or minutes ago, it simply tries to hold the trim set by the autoflight or pilot commands.
In any case, using manual trim and then holding on to the wheel will prevent the autotrim from re-engaging.
There might be pilots insane enough to pull on the yoke or sidestick in conventional and FBW aircraft until the aircraft stalls, but no one would trim while pulling.
"No pilot would ever..." is an impossible statement to prove.
To understand that above statement let me explain normal trim behaviour. With the intention to climb the pilot uses the yoke to change the pitch, once that change is reached he uses the trim to get rid of the pressure on the yoke. The trim comes into play when the change is achieved and not in the timeframe, where the change takes place.
I know you're not intending to patronise me, but I am fully aware of how trim works in the conventional sense - I even did it once or twice in the Chippy. Here there is no tactile feedback, so "trimming to the pressure" is impossible, and performing the same thing visually using the ADI as reference would be physically exhausting on a day-in, day-out basis. This was part of the reason autotrim was developed because the flight control design was a completely new paradigm.
In case of AF447 without autotrim the pilot would never ever have tried to achieve the desired flight path change with manual trim, because that is not the way to do it.
The only way you could possibly know that is with a Ouija board. The last time he trimmed manually would have either been during type conversion training, recurrent training or even back when he was flying trainers - because the Airbus FBW flight control paradigm does not require it except in case of major failure.
If all goes to plan I'm going to be doing some exciting research this weekend and I'll be able to argue from a much surer footing. If it turns out I've been wrong about anything you guys will be the first to know.
@Organfreak - Those were not intended to be personal insults, but I must confess I was flabberghasted with the throwaway manner in which you described the Airbus system design and those who developed it. I emphatically request you find out who Gordon Corps was, what he accomplished and how he died - and if you have the time, have a browse through the Flight International archives relating to A320 development (in which he figures very prominently) before you put your foot in it again.