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Ethiopian Prelim Report

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Ethiopian Prelim Report

Old 5th Apr 2019, 04:37
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Ethiopian Prelim Report

Hi all
Have had a look at the report, if I've missed anything/misunderstood very happy to be corrected.

It appears thigs were "contained" until the last 30 seconds when it all fell apart, attitude of -40 degrees was reached and recovery impossible.

Thing I can't work out is that previously the STAB TRIM switches were moved to cutoff and verified no electric trim was available, also manual trimming was attempted but unsuccessful (high aerodynamic forces?)

In the last 30 seconds there was electric manual trim followed by severe automatic nose down trim - with the STAB TRIM switches in cutoff?

Can only think of 3 options:
- STAB TRIM override switch engaged..but that only allows manual trim in direction opposite to control column forces?
Equally did not see any mention of the override being engaged;
- STAB TRIM switches were returned to normal position/operation;
- an electrical fault that allowed the system to be energised regardless of the STAB TRIM switches position.

Thoughts appreciated.

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Old 5th Apr 2019, 05:07
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Salute!

I am getting old and need a magnifying glass to look at the data traces.

So can anyone else note more differences than I do on the data traces than for the Lion flights we have? Times, magnitude, etc.

Gums asks...
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Old 5th Apr 2019, 05:09
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Agree most likely scenario is they returned the STAB TRIM switches to normal but would have expected some sort of verbal discussion/confirmation which appears lacking - even if just a couple of words to indicate intent.
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Old 5th Apr 2019, 06:35
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Originally Posted by gums View Post
Salute!
need a magnifying glass to look at the data traces.
Gums asks...
I have found best quality is obtained by downloading PDF and then using say Acrobat Reader. You can zoom in to your hearts content.

http://www.ecaa.gov.et/documents/204...8ET-AVJ%29.pdf

Looks like this-


Maximise window to ensure full size. This is just a small corner of the chart for illustrative purposes.
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Old 5th Apr 2019, 09:45
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Originally Posted by gums View Post
I am getting old and need a magnifying glass to look at the data traces.
Click on this graphic for the trace chart at its native resolution (i.e. you won't get any more detail by zooming in):

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Old 5th Apr 2019, 09:49
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Second chart, likewise:



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Old 5th Apr 2019, 11:49
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galdian

It appears things were "contained" until the last 30 seconds when it all fell apart, attitude of -40 degrees was reached and recovery impossible.
My thoughts are that things were 'contained' only in the sense that if you fall out of a 100 story building, nothing bad happens until you reach the ground.

Once the speed passed 250kts, that was a road from which it was very difficult to return. Reducing thrust would have produced a nose down pitch, at an altitude when that was least desirable.

It is not clear the exact stage at which that point-of-no-return was reached, but IMO no pilot on earth could have landed that plane safely, from the conditions it was at 30 seconds prior to them re-engaging the trim cutoff switches.

Edit: Perhaps some weird combination of elevator roll, speed brakes, or something else may have helped. Someone with a lot of free time on a simulator, might be able answer that question.
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Old 5th Apr 2019, 11:56
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The report mentions in passing that autothrottle was left engaged throughout but fails to acknowledge it should have been disengaged. It avoids the question of whether the pilots switched the stab cut-out back on even though the data shows that must have been the case. The Press Conference "was almost entirely focused on vindicating the actions of the pilots" according to the admittedly partisan Seattle Times - it is subjective but FWIW I agree.

Nonetheless it is abundantly clear the MCAS failed (as if we didn't know that by now). But it is a matter of fact that contrary to the Ethiopian Transport Minister's statement, the crew did NOT follow correctly the Boeing procedures and the Non Normal Checklist. The autothrottle was never disengaged (a significant oversight). Although the stab cut-out was belatedly used the trim was later re-engaged contrary to the Boeing advice to "ensure the CUTOUT switches remain at CUTOUT for the remainder of the flight".

Last edited by oggers; 5th Apr 2019 at 12:24.
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Old 5th Apr 2019, 12:25
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Originally Posted by oggers View Post
The report mentions in passing that autothrottle was left engaged throughout but fails to acknowledge it should have been disengaged. It avoids the question of whether the pilots switched the stab cut-out back on even though the data shows that must have been the case. The Press Conference "was almost entirely focused on vindicating the actions of the pilots" according to the admittedly non-paretisan Seattle Times - it is subjective but FWIW I agree.

Nonetheless it is abundantly clear the MCAS failed (as if we didn't know that by now). But it is a matter of fact that contrary to the Ethiopian Transport Minister's statement, the crew did NOT follow correctly the Boeing procedures and the Non Normal Checklist. The autothrottle was never disengaged (a significant oversight). Although the stab cut-out was belatedly used the trim was later re-engaged contrary to the Boeing advice to "ensure the CUTOUT switches remain at CUTOUT for the remainder of the flight".
The preliminary report is a statement of facts, not an attribution of blame. The press conference is an entirely different matter, and your points are probably valid.
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Old 5th Apr 2019, 12:40
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Plenty of Monday-morning quarterbacking there, Oggers. "Didn't follow the NNC". They were airborne for only a few minutes before diving into the ground at over 400KIAS! Cut them a bit of slack!
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Old 5th Apr 2019, 12:49
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Looks like the L AoA error was +75 degrees in this case. That is three times more than in the Lion-Air accident, so not a binary multiple. However 75 degrees could be the Full Scale Deflection, so could still be a data line problem. Also it contained no signal component, just an FSD steady reading.
.
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Old 5th Apr 2019, 13:54
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The elephant in the living room is the source of the erroneous angle of attack inputs. How did two brand new airplanes experience nearly the same angle of attack input faults within six months of each other, particularly when one of them had the AoA vane changed halfway through the four day maintenance history? Is this a hardware problem? Or are we dealing with some form of bad code within the systems somewhere?

As Dave Davies once wrote regarding a runaway stabilizer, “if this should occur at high speed the airplane is bound to be in severe trouble; the only hope is to get the speed off.” Certainly reducing power would have added a nose down pitching moment, but getting rid of the speed would have taken the bite out of the stabilizer, reduced the elevator required and allowed the trim to be moved manually (if that was a problem).

We don’t really know whether the trim could be moved manually or not. It took the FO 8 seconds to conclude that “it’s not working”. I don’t know how hard he tried, or if he even understood how much force might be required. There may be more information within the CVR transcript that we don’t have.

However, EASA Equivalent Safety Finding B-05 discusses the limits of the yoke trim switches, stating that “Simulation has demonstrated that the thumb switch trim does not have enough authority to completely trim the aircraft longitudinally in certain corners of the flight envelope, e.g. gear up/flaps up, aft center of gravity, near Vmo/Mmo corner, and gear down/flaps up, at speeds above 230 kts. (italics added). It then states that “The trim wheel can be used to trim the airplane throughout the entire flight envelope. “

Boeing has always stated in their FCTMs that “excessive” airloads may require the efforts of both pilots, and that in “extreme” cases those airloads may have to be relieved aerodynamically. I don’t know what ‘excessive” airloads are or what kind of case is an “extreme” case, but based on the EASA document as well as the absence in the QRH of any such discussion, I am assuming that I can operate the trim wheel manually in all corners of the envelope. This will be an interesting discussion next month in recurrent training.

It appears to me that they were climbing with the master trim selected off. I don’t know why the captain said “The pitch is not enough”…of course, I don’t really know what he said at all, only how it was presented in the public release. But it seems to me that they were stable and climbing, so one option would have been to just hang on and get some altitude before trying to sort things out.

The other option would have been to turn the thing back on and mash the trim switches until you got a pretty good nose up trim established, then shut it off again. You’d have to understand how the system works, but we’ve had those discussions for years in regard to using some parts of the trim system to reset a runaway stabilizer if it was necessary.

The Lion Air captain trimmed nose up against the MCAS 21 times; the Lion Air FO only tried this twice. The preceding Lion Air crew never came close to the ground, so presumably they trimmed nose up against the MCAS quite a lot, until they shut it off and finally left it off. The Ethiopian captain appears to have trimmed nose up against the MCAS at least 5 times, but when the system was turned back on, he only tried a couple of short nose up inputs. I wonder if he even knew it was back on. For the life of me, I cannot understand why either the Lion Air FO or the Ethiopian captain did not simply mash those yoke switches and hold them there until they relieved the column force, and then mash them again when the MCAS started running the trim nose down. They are hand flying the thing…they certainly would feel the trim start to change.

But like I said, the real elephant in the living room is the source of the erroneous angle of attack data. The airplane isn’t going back to flight status until that gets sorted.



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Old 5th Apr 2019, 14:36
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Plenty of Monday-morning quarterbacking there, Oggers. "Didn't follow the NNC". They were airborne for only a few minutes before diving into the ground at over 400KIAS! Cut them a bit of slack!
Call it what you will, you will never understand this accident if you do not examine all the facts. But my point is really just the Ethiopian Transport Minister claims the crew carried out the Boeing procedure, whilst the interim report shows unequivocally that they did not.
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Old 5th Apr 2019, 15:14
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Mansfield, turning your ‘elephant’ around, consider the actual threat to safe flight. The aircraft was not flyable because of the trimmed condition; the abnormal trimmed state.

AoA failure in previous variant 737 other aircraft types can be managed. In the Max, MCAS with AoA contributed to the trimmed condition, but so too could other trim failures - continuous runaway (not this accident).

The Boeing argument is that the abnormal drill for trim runaway would mitigate the MCAS induced trimmed condition. The mounting evidence is that this is not so, particularly considering crew’s inability to understand the abnormal trim motion - the initial MCAS trim movement was exactly as designed; and thereafter the physical impossibility to move the trim wheel manually.
The obverse is if the trim runaway drill will mitigate a ‘trim runaway’, (no MCAS or AoA involvement), particular with larger trim offsets and / or higher air speeds; does the MAX differ from the NG in this respect.

There are differences between the two trim failure initiators, but each depend on human perception, understanding the situation, choice of action, and acting - which may or may not work.

Managing the abnormal trim state depends on an assumed human ability; that the crew will be able to identify and mitigate the threat.
Thus the fundamental question is if reliance on human ability in this instance has been stretched too far; highly questionable for mental workload and situation assessment (+ no training, no published system description), even more so for the impracticality of crew procedures( + none / hastily issued inadequate drill).

‘It is very difficult to change the human behaviour, but you can change the conditions in which the human works’ - James Reason.
In changing the conditions of work, Boeing must consider all possible contributions to the real threat - all trim malfunctions, all abnormal trimmed states.


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Old 5th Apr 2019, 17:02
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Originally Posted by oggers View Post
Call it what you will, you will never understand this accident if you do not examine all the facts. But my point is really just the Ethiopian Transport Minister claims the crew carried out the Boeing procedure, whilst the interim report shows unequivocally that they did not.
In fact, their adherence to 'normal' procedures immediately after takeoff may well have set themselves up for the fatal endgame:
They got the stick shaker almost immediately after takeoff, but he tried to engage the autopilot 4 times while it was still shaking!

They retracted the flaps while the stick was still shaking!

They never brought the engines back from TO thrust.

If they had left the flaps in the TO position while troubleshooting, they could have trimmed the airplane normally throughout the flight. Depending on autothrottle mode, it may have brought the thrust back to maintain the flaps limit speed (228 kt?).

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Old 5th Apr 2019, 17:20
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Translate what she said to "they did all they could as per the manufacturers' suggestion, given the fear of losing their dear-lives and the myriad of physiological and psychological issues they were experiencing during the roller coaster ride". Is that better?
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Old 5th Apr 2019, 17:28
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Intruder.
Speed limit for Flap 5 is 250kts.
My go-to speed is 230 as that gives me safe flight in most conditions in the 737-800.
So
Level change , speed 230, leave F5 AT off and climb to say 15 000 feet, ca 7000 feet above field?
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Old 5th Apr 2019, 18:50
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Originally Posted by Takwis View Post
I think they turned the switches back on, as a last resort, after the inability to turn the trim wheels against the high aerodynamic loads. Like many, here, I wish they would have pulled back the throttles, somewhere along the line, but totally understand that they had a lot of other confusing and distracting things to deal with, and not a lot of time.

Until recently, I did not realize that the trim wheel was smaller, the damper larger, and therefore the mechanical advantage reduced from what I was used to. It is truly no longer a 737. MAX kludge.
The trim wheel on the 737 MAX is identical to the trim wheel on the 737 NG. It has the same part number and they are interchangeable.
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Old 5th Apr 2019, 18:51
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Mansfield,
Further to your views on EASA #13, see Ethiopian airliner down in Africa

The safety case considered normal trim, at or near trim to maintain trim. The accident involved restoring trim from an extreme position where the combined air loads of tail / elevator prevented operation of the trim wheel.
If so, then the basis of the certification is flawed.
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Old 5th Apr 2019, 19:16
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Aerodynamics and pitch 737 MAX

Hello all.

I'm not in the industry but have rudiments of aero.
So here is my silly question.

If the engines generate lift far forward pitched up this tends to assist further pitch up since the centre of lift moves far in front of the centre of gravity.

If the pitch becomes nose down, at some negative AOA the engines will similarly move the centre of negative lift forwards the air strikes the top of the nacelle. And tend to push nose down.

Is this a reasonable analysis?

David
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