Extreme Out Of Trim Flight
#11
Gets Weekends Off
Thread Starter
Joined APC: Jan 2006
Posts: 222
100 lbs of pull on the control column is going to tell any competent pilot that he's going to have to trim like crazy to get it back into a normal range. He's not going to give a couple of clicks of trim, have the control forces go to 99 lbs, shrug his shoulders and give up because the problem wasn't instantly corrected. This whole "feedback loop" discussion is nonsense.
https://assets.documentcloud.org/doc...X-Ethiopia.pdf
I am not going to label them as competent or incompetent - all I know is that they sure knew how to trim and did it all the time under normal circumstances. Then when the forces were too great and the feed back loop non-existent they all of a sudden could not do it.
Same report, page 11, quote:
At 05:40:27, the Captain advised the First-Officer to trim up with him.
Why on earth would you want someone to click on their thumb switches with you unless you believe the trim on your side is not working? And why does he think it is not working? Surely because the feed back loop is distorted - the correction is small, he does not feel it and he gives up. This is my interpretation, you are welcome to disagree.
#12
Rather than build some complex feedback loop theory, isn't it more likely that they simply weren't familiar with the flight regime they were in and failed to take appropriate action?
I also fail to understand why you chose to open your comments with the statement that "everyone" is deficient. You appear to think this feedback loop myth is waiting to bite us all. What data are you using to make these broad generalizations?
Last edited by Adlerdriver; 04-11-2019 at 06:26 PM.
#13
#14
An interesting similar mishap....
#15
Disinterested Third Party
Joined APC: Jun 2012
Posts: 6,017
The control wheel nose-up trim attempt at 5:43:11 occurred as pilot-action. This is very significant, because at 5:40:35, nearly three minutes prior, the crew had already used the stab trim cutoff switches. Once the stab trim cutoff switches have been moved to the cutoff position, stab trim may not e re-engaged.
The recording of pilot electric trim input at 5:43:11, with subsequent increase from 2.1 to 2.3 units, is evidence that the crew re-engaged the pitch trim motors.
The pitch trim then moved nose down from 2.3 units to 1.0 unit. THIS WOULD NOT HAVE OCCURRED IF THE CREW HAD NOT RE-ESTABLISHED PITCH TRIM MOTOR CAPABILITY BY RESTORING THE POSITION OF THE STAB TRIM SWITCHES.
Read that again in case you didn't get it.
Once the stab trim motors have been cut off, the stab trim motor cutoff switches must NOT be restored. The crew restored them, thus enabling the trim to run away again.
At the same time, they continued to make the trim force problem WORSE by accelerating.
Clearly they had one mission with unwanted trim motion: cutoff the stab trim motors and leave them cutoff. That singular action would have prevented this mishap, had they been left in the cutoff position, and had the crew not continued to accelerate.
As stated previously, it's possible to make a salvageable situation unsalvagable, and that's exactly what the crew did.
Why on earth would you want someone to click on their thumb switches with you unless you believe the trim on your side is not working? And why does he think it is not working? Surely because the feed back loop is distorted - the correction is small, he does not feel it and he gives up. This is my interpretation, you are welcome to disagree.
The captain made several requests of the F/O. Trim with me. Pitch with me. Pitch with me.
The crew didn't trim in the wrong direction. The issue of a "feedback loop" is irrelevant. The controls were heavy nose down, due to an out of trim condition. They made an attempt to restore it, with minor success, but the let the trim run nose down again. They attempted to retrim, cut it off, and would have been required to maintain back pressure based on an out of trim condition, but it was manageable and flyable at that point. Only two things could have made it worse: re-engage the stab trim motors, and increase airspeed.
The crew did both. And it killed them.
The 737 Max didn't misfire, nor was it poorly designed. First and foremost, a sensor failure occurred: the crew recognized the sensor failure, and already had notification in their flight manual of the potential complications of an AoA failure; this notification had been in their flight manuals for several months.
The procedure for unwanted stab trim, regardless of the cause, is the same. It's fully applicable here. It's not new. It's been in play for decades. It's spelled out in the report, in case you don't know it. The single most important element of that procedure is a memory item: stab trim cutout switches CUTOUT.
Once those switches are placed in cutout, they are not to be restored. The crew restored stab trim operation, knowing that stab trim motion was nose down, and that it was uncommanded.
This was not a faulty design or aircraft: this was a faulty pilot action, and a fatal one. Accelerating beyond Vmo in the aircraft sealed it. No chance of recovery, and they rode it into the ground with the overspeed clackers going off the entire time.
No, if a man robs a liquor store with a S&W handgun and shoots the clerk, it doesn't matter if he fumbled the safety at the time, and it doesn't really matter if the pistol has a mechanical problem and discharges without his finger on the trigger: it's his action, robbing that store, that killed the clerk, and it won't help him a bit to try to pawn it off on the pistol.
A pilot in command has the ultimate responsibility for the safe outcome of the flight. He knows that malfunctions can occur in the aircraft; it's quite literally all we train to do, is handle them. Ultimately, however, our first job is to fly the aircraft, and we have procedures to do this; the procedure was violated in this case, and it was that violation that allowed the trim to decrease further nose down, and on top of that, the crew allowed the aircraft to continue to accelerate, thus increasing the nose-down force as the flight diverged farther and farther from it's trimmed speed. Additionally, leaving power in and flying it beyond it's maximum operating speed, entirely out of the operating envelope, eventually prevented any possibility of recovery.
If you want to pick nits about a pistol analogy, the pilots were robbing the liquor store, and held the pistol on the clerk. They engaged the safety (stab trim cutoff switches), and at that stage, there was no chance of shooting the clerk...not until they disengaged the safety (restored the stab trim motors), and set the ball rolling. Everything they did thereafter only sealed the fate, and that's pilot action, not a manufacturer failure.
Last edited by JohnBurke; 04-11-2019 at 09:11 PM.
#16
Gets Weekends Off
Thread Starter
Joined APC: Jan 2006
Posts: 222
You understand neither the report, nor the aircraft, nor the system.
The control wheel nose-up trim attempt at 5:43:11 occurred as pilot-action. This is very significant, because at 5:40:35, nearly three minutes prior, the crew had already used the stab trim cutoff switches. Once the stab trim cutoff switches have been moved to the cutoff position, stab trim may not e re-engaged.
The recording of pilot electric trim input at 5:43:11, with subsequent increase from 2.1 to 2.3 units, is evidence that the crew re-engaged the pitch trim motors.
The pitch trim then moved nose down from 2.3 units to 1.0 unit. THIS WOULD NOT HAVE OCCURRED IF THE CREW HAD NOT RE-ESTABLISHED PITCH TRIM MOTOR CAPABILITY BY RESTORING THE POSITION OF THE STAB TRIM SWITCHES.
Read that again in case you didn't get it.
Once the stab trim motors have been cut off, the stab trim motor cutoff switches must NOT be restored. The crew restored them, thus enabling the trim to run away again.
At the same time, they continued to make the trim force problem WORSE by accelerating.
The control wheel nose-up trim attempt at 5:43:11 occurred as pilot-action. This is very significant, because at 5:40:35, nearly three minutes prior, the crew had already used the stab trim cutoff switches. Once the stab trim cutoff switches have been moved to the cutoff position, stab trim may not e re-engaged.
The recording of pilot electric trim input at 5:43:11, with subsequent increase from 2.1 to 2.3 units, is evidence that the crew re-engaged the pitch trim motors.
The pitch trim then moved nose down from 2.3 units to 1.0 unit. THIS WOULD NOT HAVE OCCURRED IF THE CREW HAD NOT RE-ESTABLISHED PITCH TRIM MOTOR CAPABILITY BY RESTORING THE POSITION OF THE STAB TRIM SWITCHES.
Read that again in case you didn't get it.
Once the stab trim motors have been cut off, the stab trim motor cutoff switches must NOT be restored. The crew restored them, thus enabling the trim to run away again.
At the same time, they continued to make the trim force problem WORSE by accelerating.
I am not sure where the disconnect is.
If we put aside whether they should or should not deviate from a procedure, whether they should or should not exceed Vmo, and everything else they may or may not have done right - at some point they found themselves with electric stab trim restored. Right or wrong, they did it. We can analyze and debate what they did right and wrong prior to that point, but lets put that aside, as it detracts from my main point - which is that after the electric trim was restored, at that point one would assume they would aggressively trim with the thumb switches (and in the process override MCAS), instead they barely did anything - two clicks at around 5:43:11 and 5:43:15. That in my mind is incredible. Then the MCAS commands nose down trim at what looks like 5:43:20 for about six seconds and shortly after they lose control.
The captain made several requests of the F/O. Trim with me. Pitch with me. Pitch with me.
The crew didn't trim in the wrong direction. The issue of a "feedback loop" is irrelevant. The controls were heavy nose down, due to an out of trim condition. They made an attempt to restore it, with minor success, but the let the trim run nose down again. They attempted to retrim, cut it off, and would have been required to maintain back pressure based on an out of trim condition, but it was manageable and flyable at that point. Only two things could have made it worse: re-engage the stab trim motors, and increase airspeed.
The crew didn't trim in the wrong direction. The issue of a "feedback loop" is irrelevant. The controls were heavy nose down, due to an out of trim condition. They made an attempt to restore it, with minor success, but the let the trim run nose down again. They attempted to retrim, cut it off, and would have been required to maintain back pressure based on an out of trim condition, but it was manageable and flyable at that point. Only two things could have made it worse: re-engage the stab trim motors, and increase airspeed.
By the way, when they cut off the stab trim motors, they were so out of trim that it is not a given, based on what we know, that the plane was flyable, no matter the speed. You can hold the pressure for so long until your muscles give out.
Last edited by sgrd0q; 04-11-2019 at 11:02 PM.
#17
Disinterested Third Party
Joined APC: Jun 2012
Posts: 6,017
I am assuming they kept accelerating and probably realized they could not maintain level flight in that configuration due to the control column pressure, and they could not move the trim wheel manually, again presumably because of the aerodynamic forces involved at that speed.
They didn't "find themselves" with trim restored. That sounds too much like a trumpian alternate fact.
They restored trim. They'd put the genie in the bottle, and they let it out. It cost them their lives.
Right or wrong, they did it. We can analyze and debate what they did right and wrong prior to that point, but lets put that aside, as it detracts from my main point - which is that after the electric trim was restored, at that point one would assume they would aggressively trim with the thumb switches (and in the process override MCAS), instead they barely did anything - two clicks at around 5:43:11 and 5:43:15. That in my mind is incredible. Then the MCAS commands nose down trim at what looks like 5:43:20 for about six seconds and shortly after they lose control.
#18
But the speed does matter. It’s the main reason the aircraft became uncontrollable. Had they stayed in the flight envelope, any trim setting, even an extreme one would have still been flyable. It wasn’t until that trim setting was combined with a speed well beyond limits that the aircraft was un-flyable. So saying “no matter the speed” as if the trim position alone doomed the aircraft makes absolutely no sense.
#19
Gets Weekends Off
Thread Starter
Joined APC: Jan 2006
Posts: 222
But the speed does matter. It’s the main reason the aircraft became uncontrollable. Had they stayed in the flight envelope, any trim setting, even an extreme one would have still been flyable. It wasn’t until that trim setting was combined with a speed well beyond limits that the aircraft was un-flyable. So saying “no matter the speed” as if the trim position alone doomed the aircraft makes absolutely no sense.
I should add that we are going off on a tangent here. They didn't even try to slow down, so this is all highly hypothetical. My initial point was about trimming or failure to trim in extreme conditions.
#20
You do realize that with normal hydraulic pressure (as was the case here), the control column forces are artificially created by a feel computer and the flight controls are still hydraulically actuated, right? The pilots aren't actually working against the control forces created by airflow over the control surfaces.
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