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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 9th Apr 2019, 05:33
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Originally Posted by gmx
Well, it was cut out by the pilots. They had the option to re-trim the aircraft using electric trim before hitting the cutout. I believe this "option" occurred to the crew close to the end when they re-enabled the electric trim but their attempts to re-trim with the column switches was unsuccessful (either through pilot error or malfunction). Had they done this earlier, right before they activated the STAB TRIM CUTOUT, they would have been in a very similar situation to the pre-accident LionAir flight -- a trimmed, stable aircraft, with AP disconnected, and stick shaker going off on PF side.

A couple of points, one made a few posts back.

Firstly, it seems the lessons from the pre-accident LionAir flight and the subsequent accident flight were not learned well enough. As far as we know, the MCAS issue that occurs as a result of PF-side erroneous AoA signal is completely survivable by the crew taking the correct course of action. It seems that the correct course of action wasn't spelled out clearly enough for the ET302 crew, or it was not drummed into them methodically enough. That's a further failing of the airline industry following the initial accident.

Secondly, it has been remarked here on numerous occasions that it is extremely rare for a transport crew to encounter a stick shaker alert. Had the pre-accident LionAir B737 been grounded following that flight, and had the ET302 crew leveled the aircraft before disabling the electric trim and subsequently managed to land, that would have been two flights with PF-side stick shaker active for the duration of those flights. That would suggest a systemic issue, but it does not appear to have received the same attention as the MCAS response.
Of course, they could have done so, this and that. But they elected to start a 30 degrees bank manual flown turn at 340 kIAS without the ability to pull or trim ANU. Whatever dictated them to turn.

With regard to lessons learnt from Lionair: an Emergency AD is a quite ultimate document. If that is not well thought through and written by lawyers there is not much room left.
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Old 9th Apr 2019, 06:12
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But they elected to start a 30 degrees bank manual flown turn at 340 kIAS without the ability to pull or trim ANU.
Why do you keep saying this? They did have the ability to trim nose up.
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Old 9th Apr 2019, 06:14
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Originally Posted by gmx
Well, [electrical trim] was cut out by the pilots. They had the option to re-trim the aircraft using electric trim before hitting the cutout..
Is that sure ? Can the electric trim work despite aerodynamic forces in all situations (overspeed, trim full nose down) ? Does it even work in more situations than manual trim ?
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Old 9th Apr 2019, 06:20
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Originally Posted by DaveReidUK
That appears to be true for manual (wheel) trim, but there's no evidence that it's the case for electric trim.
Dave and others,
At ultra high speed the stab can only be trimmed after releasing up-elevator. This is because of loading to the stab mechanism.
This is the reason for elevator release during upset training.
But if you are at tree top height you canít do that.
So trim, Manual or Electric is, as the man said, not possible under certain conditions - and those were the conditions encountered here.
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Old 9th Apr 2019, 06:34
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Originally Posted by Rananim
Re manual trim...have we had any engineering input as to just when manual trim no longer becomes available in the flight envelope?
We've had lots of discussion about that, albeit without any numbers.

Is it primarily speed dependent, stabilizer positon-dependent, yoke dependent, or a combination of all 3?
All of the above contribute to the load on the stab, which determines the force you have to overcome in getting the trim wheel to move.

Originally Posted by bill fly
Dave and others,
At ultra high speed the stab can only be trimmed after releasing up-elevator. This is because of loading to the stab mechanism.
This is the reason for elevator release during upset training.
But if you are at tree top height you can’t do that.
So trim, Manual or Electric is, as the man said, not possible under certain conditions - and those were the conditions encountered here.

With respect, we don't have any evidence of that being a factor here - there are no points in the FDR trace that show trim input without corresponding stab movement. Even the last two very brief ANU inputs appear to have had an effect, although the stab only moved by a fraction of a degree before the the inputs ceased.

Last edited by DaveReidUK; 9th Apr 2019 at 06:45. Reason: Edited to answer Bill's point too
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Old 9th Apr 2019, 06:43
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Well, [electrical trim] was cut out by the pilots. They had the option to re-trim the aircraft using electric trim before hitting the cutout..

Originally Posted by fgrieu
Is that sure ?
Yes, it is sure. We know this because MCAS drives the stabilizer using the electric motor, and MCAS dialed in another batch of nose-down trim AFTER the pilots re-enabled electric trim and blipped the column switches (which had negligible affect on stabilizer position). Therefore, the electric trim motor was able to drive the stabilizer with the stabilizer in its final position prior to stab trim cutout activation. As such, I think it is fair to conclude that the pilots decided against trimming back to neutral before activating the cutout (or were cognitively overwhelmed at the time and did not think to do so).

Last edited by gmx; 9th Apr 2019 at 06:49. Reason: provide additional context
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Old 9th Apr 2019, 06:49
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Therefore, the electric trim motor was able to drive the stabilizer with the stabilizer in its final position
It certainly was able to move the stabilizer trim nose down. I am not sure we can say it was able to trim nose up, against the aerodynamic force being applied to the jack screw, rather than with it.
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Old 9th Apr 2019, 06:54
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It was able to move trim nose up, although ever so slightly. The movement is there is you zoom into FDR trace. I suppose it trimmed slower against the load than along with the load, but some nose-up capability was there. I presume the motor has some overheat/overload protection. Does anyone know the reset criteria, if any, of the motor going back to operation after whatever protection is tripped?
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Old 9th Apr 2019, 06:57
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Originally Posted by gmx
Well, [electrical trim] was cut out by the pilots. They had the option to re-trim the aircraft using electric trim before hitting the cutout..



Yes, it is sure. We know this because MCAS drives the stabilizer using the electric motor, and MCAS dialed in another batch of nose-down trim AFTER the pilots re-enabled electric trim and blipped the column switches (which had negligible affect on stabilizer position).
That the electric motor drives the trim in one direction, does not mean that it is capable to drive it in the other direction if loaded. Think about tightening a vice.
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Old 9th Apr 2019, 07:32
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Originally Posted by TryingToLearn
Oh, just one question:
People claim that the manual trim may not be operable in certain flight conditions while the electric trim motor is more powerful.
On the other hand the manual states that in case CUTOUT does not work, one should grasp and hold the wheel (?against the motor?). Did I miss something?
Yes, you did miss something. To be honest, I too have missed that same thing.

I think all the check lists that say something about manually trimming with the wheel and/or hold/grasp the wheels will need to be rewritten.
And then, what will the new procedure for trimming with cut out switches set to cut out be? How to stop trim runaway when wheels supposedly cant be stopped by grabbing them?
I think there hides a lot of problems for Boeing in the manual trim wheel system.
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Old 9th Apr 2019, 07:41
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Originally Posted by AlexGG
I presume the motor has some overheat/overload protection.
Probably not. What would be the point? It is a vital pitch control surface. Why would you intentionally make it unavailable because it might fail at some later time? It is better to have it available, and damage it, rather than to make it unavailable and potentially lose the entire aircraft.

I don't think there will be more than an overcurrent circuit breaker, but that is mostly to protect the wiring.


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Old 9th Apr 2019, 08:05
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Originally Posted by Rananim
We can't see the wood for the trees.
This one was just a faulty sensor(not even UAS)..
If MCAS would activate alone,any crew would simply
counter-trim and cut off its electrical supply without much thought.
But combine it with a "confusing" UAS scenario and shakers and
warnings and bingo..you get a smoking hole in the ground...
Pilots have to be trained more on these UAS scenarios before they kill again.
Aeroperu,birgenAF447 and the 2 MAXs,and others.....
Wise words. Seems to me that the entire concept of "artificial feel" and "artificial buffet" (AKA stickshaker) was envisioned many decades ago assuming pilots had stick and rudder skills ingrained and whose muscle instincts would guide them to do the right thing if their hands were provided with the correct aerodynamic, albeit artificial clues.
Today, the artificial manual feedback stuff is degrading the primary flight control ergonomics, and distracting and stressing the crew. Instead of providing clues, it may even make pilots loose the trim switch with their thumb as a result of a hydraulically shaken column with massive artificial pull on top.
MCAS could then be seen as the ultimate perversion of that concept. Abuse a secondary flight control to create artificial stick force, give it a delay of 5 s so the pilot will not intuitively get the message, and give it so much authority, that it effectively overrides the primary control.
2 ct

Last edited by spornrad; 9th Apr 2019 at 12:52.
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Old 9th Apr 2019, 08:09
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Originally Posted by TryingToLearn
Oh, just one question:
People claim that the manual trim may not be operable in certain flight conditions while the electric trim motor is more powerful.
On the other hand the manual states that in case CUTOUT does not work, one should grasp and hold the wheel (?against the motor?). Did I miss something?
I don't think there is any dispute about the fact that grasping the trim wheel stops the stab moving. Reportedly, and depending on how positively you grab it, you may lose a bit of skin in the process.

Once the wheel is held, you are not fighting against the electric trim motor, which is disabled if resistance is detected at the cable drum.
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Old 9th Apr 2019, 08:12
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I never tried ( in the sim! ) grabbing and trying to hold the trim wheel during a runaway stab. Not sure it was possible without burning my hand or losing some fingers !

Was it possible?

Sorry, DR s post was being written as I was typing !

However, anyone actually have any first hand ( no pun intended ) experience of doing it ?
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Old 9th Apr 2019, 08:58
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Originally Posted by TryingToLearn
People claim that the manual trim may not be operable in certain flight conditions while the electric trim motor is more powerful.
On the other hand the manual states that in case CUTOUT does not work, one should grasp and hold the wheel (?against the motor?). Did I miss something?
There is a clutch that ensures that manual trim has priority on electric trim. So Yes, grasping the wheel should stop trim runaway.
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Old 9th Apr 2019, 09:01
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Originally Posted by bsieker
I don't think there will be more than an overcurrent circuit breaker, but that is mostly to protect the wiring.
Bernd
A motor which is forcibly stopped can pull a huge amount of current. Depends on motor type but for old fashioned non-electronic controls can be 20x full speed current. Of course we don't know the motor type or controlling electronics. Anyone?

Also DaveReid's post above about motors stopping if they meet resistance.

I can't see any reason for the 'blips' in nose up trim other than pilot trying it and it not working/ not appearing to work. Why else would you release the switch when it's the one thing you're focussing on, the one thing you know you need?
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Old 9th Apr 2019, 09:07
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The 737 manuals says that incase of extreme nose-down out-of-trim, one should INCREASE speed to relieve elevator load and permit manual trimming.

That's insane !

(see https://www.satcom.guru/2019/04/what...-on-et302.html)
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Old 9th Apr 2019, 09:13
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so why not have a separate cutout switch just for MCAS, keep the other two to cutout electric trim. But the new MCAS cutout switch would leave electric trim available.

I'm also unclear since the certification requirement was for force feedback on the stick during certain stall regimes why Boeing didn't just use some kind of motor/pneumatic on the stick to give this force, rather than connecting MCAS to control surfaces! Seems overkill, no pun intended. I mean the stick shaker no doubt uses a motor to shake the stick, it doesn't shake the ailerons in order to make the stick shake!

G
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Old 9th Apr 2019, 09:18
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Originally Posted by PerPurumTonantes
I can't see any reason for the 'blips' in nose up trim other than pilot trying it and it not working/ not appearing to work. Why else would you release the switch when it's the one thing you're focussing on, the one thing you know you need?
I see on good reason : at very high speeds a long trim command would induce huge vertical acceleration, therefore you have to use it with caution, using actions of one second or so.

What I can't see is why they did not continue as many times as needed to trim back
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Old 9th Apr 2019, 09:24
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Originally Posted by groundbum
so why not have a separate cutout switch just for MCAS, keep the other two to cutout electric trim. But the new MCAS cutout switch would leave electric trim available.

I'm also unclear since the certification requirement was for force feedback on the stick during certain stall regimes why Boeing didn't just use some kind of motor/pneumatic on the stick to give this force, rather than connecting MCAS to control surfaces! Seems overkill, no pun intended. I mean the stick shaker no doubt uses a motor to shake the stick, it doesn't shake the ailerons in order to make the stick shake!

G
There would be no force on the stick without the feel and centering system. It has been said that, for some reason, this system is not able to provide the extra force needed near stall. Of course it could have been modified to do so, but that mean a lot of money, whereas MCAS is just a software modification
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