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AF 447 Thread No. 7

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AF 447 Thread No. 7

Old 17th Nov 2011, 21:07
  #361 (permalink)  
 
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@Dani
What did i write?
I described exactly what you now say, but did otherwise in your statement. It does not trim for force, but for loadfactor.

In former non FBW systems it trims for force, trim eliminates the force on the yoke.

I hope, i didnīt run into my personal language barrier here.

franzl
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Old 17th Nov 2011, 21:17
  #362 (permalink)  
 
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Retired F4 done writ:
I hope, i didnīt run into my personal language barrier here.
Nope, I understood you perfectly the first time!
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Old 17th Nov 2011, 22:25
  #363 (permalink)  
 
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Once these characteristics are known, it would not be particularly difficult to model them (at least for the longitudinal motion), and to use them in a simulation to produce your statement.
HN39: Exactly.

We can only hope.

RetF4: The autotrim is not a pitch rate augmentation system, it is a follow-up. It does the same thing as any aircraft trim system and trims out dynamic pressure that would tend to alter the pilot's pitch command. Dynamic pressure has to change to result in an automatic trim change just as would occur in your F-4 (or mine) to clue you that you needed to input a pilot actuated trim change.

It does not trim for load factor, it trims for force (Q) which would alter the pilot commanded load factor or the existing flight condition load factor if the pilot was either not inputting a command (SS neutral) or unable to change the existing flight condition load factor with his command.
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Old 17th Nov 2011, 23:36
  #364 (permalink)  
 
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Originally Posted by Dani
Maybe, I still don't see where autotrim was to blame in AF447's case. It did autotrim what the PF ordered. Autotrim has no artificial intelligence to know what's in a pilot's mind. It stupidly does what it has been told: When you pull for a long time it trims this position to zero force. That's the definition of trim. What's wrong about this function on AF447? If only he would have pushed the stick as long as he pulled it, autotrim would have ordered THS back, this I'm pretty sure.
Totally agree with you, Dani.
When I proposed that the (nose up) auto-trim should be inhibited if S/W in ON, it's not because I felt the auto-trim performed wrongly.
I see this feature (inhibition) as a supplementary "protection" against a worse upset. History teaches us that, sometimes, pilots do the wrong thing, specifically they pull when (nearly) stalled. No pilot in his right mind will do that, but still that happen.
On the other hand, I've never heard of a pilot "taking the time" to trim up (manually) his aircraft while "fighting" an upset, worsening it.

Then:
Provided the aircraft knows its current AoA (and knows it's too high), I think the aircraf should inhibit the NU autotrim.
With a 100% proficient crew, who reacts as soon as the S/W gets ON, this feature is of no use, I agree.
But with a less proficient crew, who for whatever reason delays his recovery actions (namely: nose down stick, idle power), the inhibition of the nose up auto-trim has 3 advantages IMO:
1] when, hopefully, the crew asseses that he's stalled and engage the correct recovery, the aircraft will be "less" stalled (lesser AoA) than it would have been with the "help" of a more NU THS. The recovery should then be quicker. That means higher, too. Perhaps, the difference in height will be enough to prevent the crash.
2] being "frozen", the THS won't (silencely) change the aerodynamics of the aircraft at a time when the crew may be trying to assess "what's going on". By not "discretely" changing a parameter (and subsequently the aircraft behavior), the inhibition of the THS may help the crew to asses just that.
3] in the event of a further command law reversion (i.e. alternate law to direct law, or alternate law to abnormal attitude law) which will totally disable the auto-trim, the crew will not have to think "hey, I did the wrong thing just earlier, and pulled my aircraft into a stall, let's push the stick and not forget to unwind the trim because auto-trim was available as I pulled, but is no more active now that I push because the aircraft switched to direct law".

All those reasons are tiny holes in the cheese, and of course are less important (IMO) than a crew performing the right action when confronted to a stall (warning). Nonetheless, clogging those tiny holes may perhaps save the day, one time. Then if there is no crucial disadvantage to do so, why not implement it?

Cheers
AZR

PS: By the way, I just learned that USE MAN PITCH TRIM is not displayed in abnormal attitude law (in an A320 at last). That too is a "flaw" IMO.
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Old 18th Nov 2011, 00:02
  #365 (permalink)  
 
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The endless disputes regarding the niceties of G Load versus its application to Elevator and THS demand is getting rather tedious.


Remember - same in NORMAL and ALTERNATE LAWS.

Nowhere have I seen a mention of anything different in ALT2, therefore why would the THS not supplement the G Load demanded by the Elevator to maintain the pitch effectiveness of the Elevator?
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Old 18th Nov 2011, 00:23
  #366 (permalink)  
 
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AoA versus Gee, once again

Salute!

Decent explanation of the trim, OK.

OTOH, I don't think Retired thot the trim had anything to do with rates. And he can jump in here to clarify.

It is true that the trim acts to reduce the pilot input to maintain either an AoA ( older planes) or a gee ( at least two FBW systems I am familiar with). And the amount of elevator deflection or THS position will be adjusted accordingly. Am I good so far?

So am I off-base assuming that a constant gee command of, say 1.15 gee for a 30 deg bank turn with none of the "normal" laws in effect, would result in the THS gradually trimming to reduce the pilot's requirement to hold a bit of back stick?

Sounds fine to this old dinosaur.

But then I note that AoA protections ( I prefer "limits") are lost in ALT laws when there are problems with the ADR subsytems. So I can understand trimming by the system or manually by the pilot that could result in the jet exceeding the stall AoA.

I look at the Airbus protections and laws and am impressed by how many are related to attitude versus AoA or even gee. The pterodactyl FBW system I flew 15 years before the A320 was AoA dominant. At low AoA you could get to 9 gees, but as "q" decreased, you hit the AoA limit and the gee available reduced until it was one gee, So at 25 deg AoA we flew at one gee with stick all the way back, regardless of our trimmed gee. And I point out that we trimmed manually for gee using the collie hat or the trim wheel. So we could trim for zero gee and if we let go of the stick the jet would try to achieve zero gee ( neat feature to gain energy, called unloading). Our trim limits were about - 1.4 gee and + 3.4 gee. The Airbus doesn't work this way.

As far as the THS contributing to the prolonged stall? I would think it hurt, but was not the primary factor. As Doze discovered in the sim, the elevators had sufficient authority to get the nose down.

I strongly disagree with the loss of AoA "protections" when airspeed is FUBAR. If the initial pilots did not complain about the 60 knot value or even unreliable speed values, I understand. OTOH, I cannot understand why the "q" was not augmented by a simple WoW switch. After all, the jet is in "direct law" until liftoff, isn't it? And then switches to "normal" law. Or am I mis-reading the FCOM's and other manuals I now have courtesy of several here?

Lastly, and for those who have not flown to the limits and beyond... If the wing camber does not have a decent washout, then the wing stall will progress from outboard to inboard. This results in movement of the center of pressure forward and actually reduces dynamic stability more than static stability. Hence, it becomes harder to get the nose down. It also reduces aileron/spoiler effectiveness.

too much verbiage, and remember that I started as a dinosaur and then evolved into a bird, heh heh.
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Old 18th Nov 2011, 00:51
  #367 (permalink)  
 
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THS response ...

OK465,

My own understanding would tell me that RF4 is correct, but I could not prove you wrong either, maybe YOU are correct ?

How things are working in my opinion :
  • The input is full fwd stick in ALT LAW
  • The request is for a load factor below 1G
  • Both elevators try to honor that request with a down deflection
  • THS under autotrim command will try to neutralize that initial elev down deflection
  • As no change in the load factor takes place both elev quickly reach full deflection
  • THS keeps moving in a ND setting but at a slower rate still trying to neutralize elev deflection
  • Until effect eventually takes place

What I don't get in your view is that nothing can happen until trim is manually moved, and so in ALT LAW when autotrim is active and therefore USE MAN PITCH TRIM is NOT displayed. If you are correct and I did understand you well, there would be another serious question directed to Airbus.
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Old 18th Nov 2011, 01:24
  #368 (permalink)  
 
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Quote:Machinbird
Ninety-nine point nine nine nine percent of your flight time or more has been in Normal Law. Just because your regular autotrim has been so sweet doesn't mean she does not have an ugly sister.

Originally Posted by Dani
Maybe, I still don't see where autotrim was to blame in AF447's case. It did autotrim what the PF ordered. Autotrim has no artificial intelligence to know what's in a pilot's mind. It stupidly does what it has been told: When you pull for a long time it trims this position to zero force. That's the definition of trim. What's wrong about this function on AF447? If only he would have pushed the stick as long as he pulled it, autotrim would have ordered THS back, this I'm pretty sure.
Hi Dani, the reason I called Alt2 autotrim an ugly sister is that it deceptively behaves exactly like in Normal law, but has no limits other than physical ones. Nothing more.

When a pilot is mentally confused about the practical meaning of Alternate law, as the PF of AF447 appears to have been, then it is more dangerous if the trim does not stop when commanded a particular direction until it reaches the physical limits.

Think about how often you have seen your trim approaching the physical limits in normal operation (assuming you feel the need to monitor it). Almost never, and definitely not in cruise no matter where your cg is.

I agree that the PF pulled the nose up into the stall using the elevator, and the THS only slightly helped the stall entry since it was perhaps about 1 degree (above cruise setting)during its run for the upper limits.

I have seen evidence in another accident report (DC-8) where the crew brought their roll control inputs to a higher priority than pitch control although they were testing the stall warning when they got into trouble and should have known that the source of their roll control problems was excessive AOA. When the shiny side starts to point down, you airline guys tend to give that a real high priority, don't you?

I believe that the PF for AF447 did do something similar. He over-controlled the roll and trusted Mother Airbus to handle the AOA while he sorted things out, forgetting that he was in a different Law where Mother Airbus had passed all responsibilities to him. He almost got away with it, except that when he finally began to get the roll and then pitch under control, the stall warning sounded and a fresh round of wing flailing began as the TOGA thrust (and excessive aft stick) sent the aircraft out of the envelope. The heavy buffet that must have resulted was another thing that must have increased crew disorientation.

PF's eventual adoption of full aft stick was an attempt to damp the bobbing of the nose of the now stalled aircraft (In my estimation.). By this point, he was in full panic mode because the controls were not responding properly. Logical thought was an impossibility.
The THS was quietly running nose up while the crew tried to make sense of this strange land and had the effect of trapping the aircraft in a deep stall. If it had not run nose up, it is not certain that a recovery would have been effected during one of the attempts by the crew of nose down stick, but by being trimmed so far nose up, it became a virtual barrier to recognition of how to escape the stall.

Originally Posted by AZR
So autotrim only slightly helped putting the aircraft into a stall, but then effectively became a barrier to stall escape.
All those reasons are tiny holes in the cheese, and of course are less important (IMO) than a crew performing the right action when confronted to a stall (warning). Nonetheless, clogging those tiny holes may perhaps save the day, one time. Then if there is no crucial disadvantage to do so, why not implement it?
AZR, looks like we are in agreement.

MM43, this should answer your question. Earlier speculation regarding possible disabling of pitch trim when all airspeed inputs are invalid is another additional possibility, but is not at all proven/demonstrated.

Last edited by Machinbird; 18th Nov 2011 at 03:04. Reason: Address 2 additional posts.
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Old 18th Nov 2011, 01:54
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I mentioned an aspect of control input yesterday that I am not sure the majority of you picked up on. Once the aircraft is stalled, the control inputs do not have much influence on the aircraft, and sometimes act in a contrary fashion.

The elevators became fully biased 30 degrees nose up while the PF was attempting to control the nose bobble by holding full aft stick. Once the PF tried lowering the nose, he also continued to try to control the pitch (and roll) rates of the aircraft with the stick so the frequency of his control movements was much higher than the response frequency of the elevators. We have called this stirring mayonnaise and it is definitely a bad technique.

Now the A330 pitch and roll control system does not provide feedback to the cockpit flight controls (stick). If it was an older non-FBW aircraft, the stick would move in close relation to control surface position. The only way for a pilot to know the control surface position on the FBW Airbus would be to observe it on a cockpit readout (but PF wasn't looking there).

The aircraft's attitude indications on the PFD don't follow the stick any more.
Although PF is making significant nose down inputs, he is also making significant nose up inputs. To actually start moving the elevator from its full up position past neutral toward nose down requires sustained nose down inputs.

PF is thus clueless about the net effect of his total input on actual elevator position. We can see the average control inputs pretty easily on the DFDR readout, but the PF could not. The feedback channel he had used all his flying career was no longer available. He was operating open loop.
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Old 18th Nov 2011, 03:18
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Salute!

c'mon. 'bird.

Whatthehell are you talking about?

We haven't had "real" control surface feedback for 50 years.

And I don't understand all the rest of the last post.
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Old 18th Nov 2011, 03:44
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Following the autotrim arguments - it all seems a bit tenuous to say the least. A trail of clotted cream perhaps?
Reading the interim reports again one can find no suggestion of this as a possible factor. I would submit that the only reason the stall was entered into and continued was the PFs actions. We have a known shock reaction for pilots - pull back on the stick and then a series of psychological effects which continue the action. Reread the third interim report (I admit it is harrowing reading) and it is clear that there was panic and a lack of understanding not a problem with the autotrim. Remember autotrim can be turned off by moving the trim wheel. There is no evidence that the pilot even touched it. Once again we are pushed back to training and lack of professionalism.
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Old 18th Nov 2011, 03:57
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C'mon Gums. Didn't the stick move as you trimmed your F-101 from fast down to landing speed? Doesn't matter whether you are supplying the force to move the whole control surface, or just the force to move the input arm on a hydraulic control valve. That movement is feedback.

I have personal experience with the subject when a small electrical box hanging by its wire bundle began to block the pushrod leading to my stabilator control valve (mounted on the hydraulic ram). I could feel the contact when I flew slow (but not when fast). If that box had moved 1/4 inch further, I wouldn't have had enough nose up to put the flaps down and land back aboard ship.

The point is that on the A3330 as (Old Carthusian likes to remind us) your feedback path is visual from the PFD when hand flying the thing. But when the aircraft stalls, it basically stops following the stick, so you have no real idea of your control surface position without actually looking at the control position display. Does it make sense now?
Better yet, try looking at the control surface position (elevator) versus stick position plots at the end of the 3rd BEA report.
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Old 18th Nov 2011, 04:50
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Salute!

No problem, 'bird.

Until some of the dinosaurs here have flown the full FBW systems with all the limits and the coded limits by sfwe and firmwe and such, it's hard to explain.

Ya gotta "feel" it.

I was blessed by a system that didn't care about "autopilot" type limits such as attitude or roll angle. We had no limits on that. It was all gee and AoA and rate limits. Not "protections", but "limits". So we lived or died using the cards we were dealt. And the rules were simple. I don't see this with the Airbuss control logic. Sorry for all the folks here that fly the plane. But that's the way I see it.

I am disappointed by the lack of training concerning stall entry and recovery, mach buffet detection without the computers advising you, complicated control law reversion sequences, disregard of AoA when the speed sesors go south, and the beat goes on.

Make no mistake, all here, I do not absolve the 447 crew of major screwups. But I also feel that the basic design/implementation of the system should be very clear as to the absolute limits of the jet and not provide the crew with seemingly endless/annotated exceptions to the basic jet control laws they use for 99.9% of the time.

Sorry to become emotional, but I can sympathize with the crew to a point ( like a minute). Then I question their training and experience with critical flight dynamics.

To wit:

- flying with marginal delta between overspeed and stall at high altitude
- recognition of high mach buffet versus the effects of turbulence
- failure to realize that nose could be up, but the jet is stalled, and vertical velocity should provide an indication that something is awry

gotta go
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Old 18th Nov 2011, 05:41
  #374 (permalink)  
 
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Gums, when you compare what we learned about actual aircraft handling compared to what is presently being taught in the puppy mills, it is night and day. Our training had a price however. Not counting the cost in Jet fuel, and bent aircraft, some of our peers did not survive it,

The whole thing is a cost-benefits tradeoff, but it seems to this old coot that it has gone much too far and seriously needs to become more comprehensive and balanced.

A crew not recognizing a stall! That just should not happen. Particularly with the amount of time they had at their disposal to recover.
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Old 18th Nov 2011, 07:52
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feed back

Hi Machinbird,
Doesn't matter whether you are supplying the force to move the whole control surface, or just the force to move the input arm on a hydraulic control valve. That movement is feedback.
I agree.

The familiar feel of the control surface displacement with control yoke (or side stick) is completely absent on AI FBW. e.g. If you take off with a strong crosswind, the aircraft may roll rapidly shortly after lift off unless you make a roll input. That roll input could be tiny - but the control surface deflection would be large to satisfy the commanded roll direction. So if we have no idea how much the control surfaces are moving in response to the side stick inputs we are making, we have no way of knowing when we are getting close to control surface saturation (max deflection) - (unless we display the Flt/Ctrl systems page).

The crew probably never "felt" the stall - they only observed the effects of it then remained in denial and disbelief.

Q. How many Airbus crew have actually taken the simulator to the stall buffet in ALT LAW? (before AF 447).
How many of you have taken your conventional aircraft simulator beyond the stick shaker to the buffet?
Is the difference because it wasn't a requirement due to the low probability?
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Old 18th Nov 2011, 08:45
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@ Machinbird (re: #369)
Thanks
But I didn't write the first sentence in the quote you attribute to me ("So autotrim only slightly helped putting the aircraft into a stall, but then effectively became a barrier to stall escape.")
I'm not sure the autotrim was a barrier to stall escape (I understand "barrier" as something wich prevent you (strictly) to escape stall; THS NU will for sure delay the escape, but I'm not qualified enough to say it will prevent it, even if it stay full NU; in fact, my guess would be it prevents not)
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Old 18th Nov 2011, 09:38
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Originally Posted by rudderrudderrat
Q. How many Airbus crew have actually taken the simulator to the stall buffet in ALT LAW? (before AF 447).
How many of you have taken your conventional aircraft simulator beyond the stick shaker to the buffet?
Is the difference because it wasn't a requirement due to the low probability?
Valid points made by yourself and Machinbird - *however*...

At the risk of sounding like a broken record, we have the Stony Creek 727 crash in which all three crew members mistook the shaker for Mach buffet and pulled their aircraft straight into the ground as a result, convinced the whole time that they were in overspeed - ditto the Birgenair 757 Captain. Then we have BEA548 in which a constant stick-shaker and two stick *pushes* were incorrectly diagnosed as false by the crew (who may have effectively been down to the two S/Os at the time). It's a comforting thing to think that if all aircraft behaved as they did in the past that standards would not be slipping and situations like this would be correctly diagnosed, but the stats simply do not bear this out.

mm43's extract from what looks to be an Airbus manual of some kind (I'd love to know which one) contains the legend "Don't fight with the stick; If you feel you overcontrol, release the stick.". It's pretty much accepted that the FBW Airbus setup requires a slightly different technique to get the best out of it compared with more conventional aircraft, but the methods to do so seem pretty well nailed-down, though I'm sure that it takes a leap of faith to release the stick under certain conditions.

For what it's worth I think that airlines should be re-evaluating their training - not just to include manual handling at altitude, but also basic aeronautical refreshers on recognising stall, dealing with stall recovery. If it means having to buy a bunch of Cessnas to do so then they should, after all they're the ones saving money with all this new technology, aren't they?
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Old 18th Nov 2011, 10:14
  #378 (permalink)  
 
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"Don't fight with the stick; If you feel you overcontrol, release the stick.". ...it takes a leap of faith to release the stick under certain conditions.
This is an extract from Airbus publication about Unusual attitudes, stall avoidance and unreliable speed, issued in light of AF447.

You can read it also in "A320 family instructor support" by Airbus training und flight operations support division.

There is another widespread misconception that "releasing the stick" means taking off the hand of one's side stick.

It means merely that you shouldn't apply force to it anymore. Leave your hand where it is, you have a nice support there, be calm, see what the aircraft does, check for another input. That's how you should do it.
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Old 18th Nov 2011, 12:11
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Someone who hasn't been on the KoolAde diet should develop Machinbird's post #370 further.

The PF input began immediately with a Roll Left and Nose UP. His mayonnaise stirring looks bad, but it is as Mach has said, he wasn't getting input/response sequencing.

Once the a/c balked at responding to PF's first attempt to correct (read the report), he parted ways with the a/c. He became a HUNTER (attitude), not a gatherer. Well and good to condemn, but without the connection hand to craft, the flight path became independent of the cockpit's control.

PNF? He talks a good game, but he isn't in the mix, and I have doubts his results would have been different.

So, after taking control, the PF begins to handle an a/c that is not responding well. He lost the plot almost immediately. The entire CVR is a rehash of every Bus quirk known to man.

What's it doing? The posturers and imposters here are a bit full of themselves. Opinion? I doubt two of three crews could have recovered that flight. The Bus is not a pilot friendly a/c in the stink.

Unstick? And leave the a/c to meander about the sky? What a crock. Damned if you do, dead if you don't. Not a pleasant place, of a Summer Night in the Tropics.

I suppose the Bus Fans have no choice but to condemn this crew. The alternative is to repeat, "What's it doing Now?"
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Old 18th Nov 2011, 13:04
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As one who has spent my entire career working on, designing and programming simulators (admittedly my "bit" is the sound and comms bit, just so you get the full picture), any assumption regarding a specific edge-case operation should not be trusted on the sim until verified and checked against aircraft operation OR double-checked against aircraft data (assuming doing it on the plane is either dangerous or really difficult).
GY, thanks for that informed view.
What I can tell from my experiment is that the trim behaved as it did for AF447.
That was the main purpose of the experiment - I will try more when possible - Maybe he didn't trim down for another reason, we don't know, but he did not.

What I'm curious and nobody came with an answer yet, why the trim did stop short of the UP limit for AF447 ? (He did also in my experiment but a bit earlier around 12 deg)

That autotrim up under stall warning is an aberration.

Like Airbus and the concerned authorities not reacting as needed after the well documented Air Caraibes events is also an aberration. Place every 330/340 crew in a simulated exercice of UAS in CRZ configuration and we are suddenly so more 'clever' if needed.
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