Hi DW, being evasive?
Originally Posted by RetiredF4
there is no situation in cruise, where autotrim is improving a situation in ALT2 like that encountered by AF 447.
Reply by DW
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).
I described a flight phase as opposite to another flight phase (like one being close to the ground) where autotrim could help in Alt2 even at the extremes of the flight envelope (i´m not saying it would). My above statement stays. Should have explained to you, looks like.
Originally Posted by RetiredF4
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.
Reply by DW
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. .
Is it of relevance to this discussion? You may accept, that I look at it from an operators point of view. If the system is able to prevent trimming into a stall in ALT1 and Direct Law, it should be able to design one, that can do the same in ALT2.
But you did not answer my question.
Originally Posted by RetiredF4
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)
Reply by DW
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.).
You don´t need to explain how it is done, the technical aspect does not matter to the operator. Graceful would be logical, but this degradation is not.
- all Pete knows is that he mustn't let it through past a certain point. ).
is exactly my point. Let Pete know, that autotrimming into a stall is no good idea, like Pete knows in Alt1 at Vprot and in direct law. Implement it, however you do it.
Originally Posted by RetiredF4
That should straighten up the manual trim option once and for all. It´s even worse, because the functioning autotrim in ALT 2 prevents manual trim in its original sense of implementation.
Reply by DW
What do you mean by "original sense of implementation"?
In any case, using manual trim and then holding on to the wheel will prevent the autotrim from re-engaging.
The original sense of trim is to aleviate loads on the system after a flightpath change / change of loadfactor has been achieved (long term).
FCPC will always try to hold a load factor demand, by using elevators in short term and autotrim in longterm. How to disable the elevators during manual trim? With SS, I know, but if properly used manual trim would not have been necessary in the first place.
But if the system would have reverted to manual trim like in ALT2 at Vprot or like in Direct LAW, THS would not have been trimmed Full NU by the FCPC via SS inputs.
Originally Posted by RetiredF4
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.
Reply by DW
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.
You know very well that i´m fully aware about the functioning and the necessity of autotrim and do not question it. I even find it a clever and well thought out system. But the situation developing in ALT2 with AF447 was not being expected somehow and needs to be addressed and changed. It works in Alt1 at Vprot, why not do the same in Alt2? Nobody seems to be concerned to hinder autotrim in Direct Law, but you explain it would be difficult for the crew if autotrim would be hindered in Alt2 when predesigned values (aoa, speed, Trim value, take whatever would suit yourself) are reached?
Originally Posted by RetiredF4
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.
Reply by DW
The only way you could possibly know that is with a Ouija board.
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.
You are going flying? Pull on the stick and start trimming while pulling (hope you have your chute with you. No need to use a Ouija board.
Overall design has to follow function, not vice versa.