News Ethiopian 737 crashed on way to Kenya, 157 people on-board

dbeachy1

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Linguofreak

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I highly doubt that they actually followed the procedures for MCAS runaway without actually disabling it, as that involves cutting power to the jackscrew motor and driving the trim by hand. It is already known that they tried the procedures to deal with an autopilot runaway, which didn't work because the issue was not the autopilot, and MCAS kept reactivating. If they ever did get to the procedure for dealing with MCAS runaway (or general trim motor runaway not caused by the autopilot, which is the same), I think it more likely that they attempted it late enough that they could not return to neutral trim by hand before impact than that MCAS somehow drove the trim without power to the jackscrew motor. That would require something like a short in the jackscrew motor wiring (and a tendency for the aircraft to develop such shorts), which probably wouldn't be MAX-specific and would likely have shown up on other 737 models.
 

dbeachy1

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More details surfaced in this article on CNN:

cnn.com said:
Recognizing a problem with the automatic trim, the pilots followed emergency procedures and turned off the system. Instead, the pilots tried to use the backup manual trim wheel to adjust the trim, but the airplane was traveling too fast and the manual trim wheel would have been physically impossible to operate.

So it appears the pilots did indeed disable the MCAS, but it was then impossible to correct the excessive trim before crashing into the ground, which I would argue in itself is another fatal design flaw. From my perspective, this whole MCAS system is a hack Boeing added to push the MAX out quickly so pilots would not require retraining on the MAX's inherently different flight characteristics due to its larger engines that sit farther forward under the wings. :(
 

MaverickSawyer

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From my perspective, this whole MCAS system is a hack Boeing added to push the MAX out quickly so pilots would not require retraining on the MAX's inherently different flight characteristics due to its larger engines that sit farther forward under the wings. :(

I'm beginning to feel it's worse: the 737 MAX as a whole feels like an attempt to counter the A320neo family as quickly and cheaply as possible, rather than producing a significantly redesigned 737 (new wings, new engines, revised landing gear, assorted aerodynamic improvements) or, better yet, a completely new, clean-sheet design that would have been superior in every way to the 737 and the A320. But nope, they decided to go for the low-hanging fruit... and it looks like not only was that fruit rotten, they had to jump to get it, and now they've rolled their ankle on landing. :dry:
 

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From my perspective, this whole MCAS system is a hack Boeing added to push the MAX out quickly so pilots would not require retraining on the MAX's inherently different flight characteristics due to its larger engines that sit farther forward under the wings

:)

Actually, somewhere else I described it as 'the kind of thing I would add to a virtual aircraft as a hack to quickly correct some flaw, not the kind of thing one would add to a craft that really flies'

So yeah, it does seem a rather faith-based system...
 

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I highly doubt that they actually followed the procedures...


Thats what the BEA investigators say. The crew followed the Boeing manuals to the letter and the problems kept on persisting. Then they stopped following the procedures and the aircraft crashed.


In another article, they reproduced the Air Lion crash in a 737Max simulator and found out that the crew had only 20 seconds to correctly identify the problem and react before crash was inevitable.
 

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The Ethiopian Airlines plane that crashed last month nosedived several times before it hit the ground, a preliminary report has said.
Pilots "repeatedly" followed procedures recommended by Boeing before the crash, according to the first official report into the disaster.
Boeing's boss has admitted for the first time a failure in the jet's anti-stall system was a factor in the crash.

https://www.bbc.co.uk/news/business-47812225
 

APDAF

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Seems to me like the plane is unfit to fly.
Boeing should replace all current 373 MAX planes in service once they have completely fixed the problem.
 

Wolf

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Unfortunately the preliminary report of the ethiopian AIIB is not very clear about the pilots actions on the stab trim system. The report says the FIrst Officer called “stab trim cotuot” twice (action to disconnect the 2 electrical stab trim systems) and states the pilots correctly followed the related procedure/checklist. Problem is later in the flight and before impact there have been further electric trim inputs from both the pilots (nose up) and the MCAS (nose down). These inputs could not happen had BOTH system been deactivated according to the checklist. So the info in the report seems contradictory with the FDR and probably misled the media.
From what I read in the report it seems that they disconnected the autopilot trim system only and left the manual trim system active (thus the MCAS was not disengaged)
 

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Thats what the BEA investigators say. The crew followed the Boeing manuals to the letter and the problems kept on persisting. Then they stopped following the procedures and the aircraft crashed.

Note that my exact words were "I highly doubt that they actually followed the procedures for MCAS runaway without actually disabling it". See my commentary on the ECAA report below.


More details surfaced in this article on CNN:

So it appears the pilots did indeed disable the MCAS, but it was then impossible to correct the excessive trim before crashing into the ground, which I would argue in itself is another fatal design flaw.

So, looking at the ECAA's actual report:

ECAA report said:
At 05:40:28 Manual electric trim in the ANU direction was recorded and the stabilizer reversed
moving in the ANU direction and then the trim reached 2.3 units.
At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and First-
Officer confirmed stab trim cut-out.
At 05:40:41, approximately five seconds after the end of the ANU stabilizer motion, a third instance
of AND automatic trim command occurred without any corresponding motion of the stabilizer,
which is consistent with the stabilizer trim cutout switches were in the ‘’cutout’’ position

(emphasis mine)

So the crew cut out the stabilizer trim within about 40 seconds of the issue first occurring, and the MCAS subsequently activated without result because trim was cutout, however:

From 05:40:42 to 05:43:11 (about two and a half minutes), the stabilizer position gradually moved
in the AND direction from 2.3 units to 2.1 units. During this time, aft force was applied to the
control columns which remained aft of neutral position.

This is somewhat concerning. It does not seem to have to do with MCAS. The fact that it was just 0.2 degrees in about 2.5 minutes makes me think this may have to do with the stabilizer being driven by aerodynamic forces with no corrections from automatic or manual trim inputs. (?)

The left indicated airspeed increased from
approximately 305 kt to approximately 340 kt (VMO). The right indicated airspeed was
approximately 20-25 kt higher than the left.
The data indicates that aft force was applied to both columns simultaneously several times
throughout the remainder of the recording.
At 05:41:20, the right overspeed clacker was recorded on CVR. It remained active until the end of
the recording.

I just want to note that they began receiving overspeed warnings from the air data sensors on the good side at this point, and the overspeed condition was never corrected. More on this later.

At 05:41:21, the selected altitude was changed from 32000 ft to 14000 ft.

I find this concerning, this time regarding crew behavior rather than aircraft performance: Why were they making inputs to the autopilot in a trim runaway situation, in particular, when they had already cutout the trim motor? At best, it was wasting time and energy that they needed to deal with the problem at hand.

At 05:41:32, the left overspeed warning activated and was active intermittently until the end of the
recording.

At this point even the air data sensors on the bad side are recording overspeed.

At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied
that the trim was not working and asked if he could try it manually.

This is where the Ethiopian government's insistence that there was no pilot error rings as hollow as Boeing / the FAA's insistence that there was no problem with the aircraft just after the accident.

The First Officer ought to have known that automatic trim was not going to work, as he had been the one to suggest stab-trim cutout. The checklist calls "Stabilizer: Trim manually" and "Anticipate trim requirements" at this point, but this is the first point at which manual trim is mentioned, about a minute after they cutout the trim motor, and the aircraft is definitely not in the desired trim. Their airspeed has been increasing all the while, which will make manual trimming harder.

The Captain told him to try. At
05:41:54, the First-Officer replied that it is not working.

No mention is made here of the captain attempting to assist, or of any cabin crew being called forward to assist.

At 05:42:30, ATC instructed ET-302 to turn right heading 260 degrees and the First-Officer
acknowledged.
At 05:42:43, the selected heading was changed to 262 degrees.

Once again, why are they making autopilot inputs when the checklist for this issue says "Do not re-engage the autopilot."? (Emphasis in original)

At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,400 2 ft, two
momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in
the ANU direction from 2.1 units to 2.3 units.
At 05:43:20, approximately five seconds after the last manual electric trim input, an AND automatic
trim command occurred and the stabilizer moved in the AND direction from 2.3 to 1.0 unit in
approximately 5 seconds. The aircraft began pitching nose down. Additional simultaneous aft
column force was applied, but the nose down pitch continues, eventually reaching 40° nose down.
The stabilizer position varied between 1.1 and 0.8 units for the remainder of the recording.

(emphasis mine)

So at this point they make electric trim inputs, and the aircraft responds, which indicates that they took the stab-trim cutout switches out of cutout. MCAS then activates and at that point they're screwed. The Boeing documentation attached to the report says that the switches are to remain in cutout for the remainder of the flight.

Returning to your comments:

dbeachy1 said:
So it appears the pilots did indeed disable the MCAS, but it was then impossible to correct the excessive trim before crashing into the ground, which I would argue in itself is another fatal design flaw.

They were operating over Vmo from the time they disabled the trim motor to the end of flight, so I wouldn't say it's necessarily a fault with the aircraft that it was difficult to operate the manual trim in those conditions. They failed to do anything, as far as the report directly mentions, to bring their airspeed under control. There is some indication in the N1 graphs shown that they may have taken some throttle off in the middle of the flight, and a definite indication that they reduced throttle as the aircraft entered its final dive, but they gained 5000 feet between disabling the trim motor and reenabling it, so they probably had some margin for further throttle reduction without the loss of pitch-up moment from the thrust preventing them from recovering. They could also have deployed the speedbrake, which would have given them a pitch up moment, but there is no indication that they did so.

Also, whatever the airspeed, in the end the issue was not that "it was impossible to correct the excess trim before crashing into the ground", but that they reenabled the pitch trim motor and MCAS activated again.

From my perspective, this whole MCAS system is a hack Boeing added to push the MAX out quickly so pilots would not require retraining on the MAX's inherently different flight characteristics due to its larger engines that sit farther forward under the wings. :(

I'm not altogether opposed to such systems, but that there was no retraining, even on the operation of the system itself, is a major blunder, and that it only used one set of air-data sensors is a huge problem.
 
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Wolf

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Just a side note about your questions on why they bothered selecting altitudes and headings in the MCP during such a mess. These actions are not only commands for the autoflight system, they also give inputs to the pilot for his altitude and heading ajdustments while he is flying manually (the altitude-speed-heading commands on the MCP are reflected on the primary flight display and navigation display). This is routinely done regardless the fact that the autpilot is engeged or not.
I agree with you that in such a situtation these actions were not a priority and maybe also distracting
 

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Unfortunately the preliminary report of the ethiopian AIIB is not very clear about the pilots actions on the stab trim system. The report says the FIrst Officer called “stab trim cotuot” twice (action to disconnect the 2 electrical stab trim systems) and states the pilots correctly followed the related procedure/checklist. Problem is later in the flight and before impact there have been further electric trim inputs from both the pilots (nose up) and the MCAS (nose down). These inputs could not happen had BOTH system been deactivated according to the checklist. So the info in the report seems contradictory with the FDR and probably misled the media.
From what I read in the report it seems that they disconnected the autopilot trim system only and left the manual trim system active (thus the MCAS was not disengaged)

You posted this while I was writing my post. The thing I noticed, and stated in that post, is that one MCAS activation was supressed due to the trim motor being off, and the report seems to indicate that there was at least one attempt to use the manual trim switches (as opposed to manually operating the trim wheel) that was supressed by the trim motor being off. The crew then tried trimming with the trim wheel and was unable to move it. After this, they seem to have reactivated the trim motor.
 

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I find it much more interesting in the report that there had been oscillations that started 6 seconds after autopilot activation and kept on persisting after the autopilot was disable (About at 5:39:28). You also see that in the flight data - when the left AOA jumped to +74°, roll became suddenly instable. Did maybe the stick shaker cause it?

That looks like a worse control issue than just the MCAS, which did not interfere at that point, the first automatic AND happened at 5:40:00 for 9 long seconds. The pilot corrected by the trim switches on the stick, 5 seconds later the MCAS activated again. That is consistent to the description of MCAS.

Stab trim cut off happened at 5:40:35.

But then, stabilizer position dropped slowly over 2.5 minutes by 0.2 units to nose down, while crew was keeping nose up by force on both control columns. Did mechanic trim not work at all?

That would explain the discussion at 5:41:46 - the mechanic trim wheels were not operating. There is also no attempt to use the electric trim switches before and during that discussion.

Still, I wonder why they did not react to the overspeed warnings - the whole situation looks like an information overflow, with ice warnings also appearing along the way.


At 5:43:04, the overspeed condition made controlling pitch by both control columns impossible. At that point they activated electric trim again, because mechanic trim had no effect. That allowed to trim nose up again.

At 5:43:20, MCAS overruled the pilots again.


You can also see that well in the recorded data. Also very horrible IMHO: The AOA sensor divergence was sudden and happened in flight. It did not build up. The sensor suddenly decided to report nonsense, fully independent of the other.
 
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Can we safely say the plane killed everyone?
 

Urwumpe

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Does somebody know how much stick force the crew needed in the final 3 minutes with increasing air speed? I wonder if it might be another distracting factor there.
 

Wolf

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Can we safely say the plane killed everyone?

At this stage it wouldn’t be reasonable to say this. I think we need to wait til the final report will come out since there are still questions that need to be answered on what exatly happened. And usually there are different contributing factors in aviaton accidents and there also relationships between one another. I do not recall of reading one single report where it was stated that the cause was one (be it the airplane, the pilots or any other factor)
 

Urwumpe

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The Concorde accident likely is the closest you can ever get to a single point of failure.

And even that report had multiple events leading to the metal part on the runway...
 

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But then, stabilizer position dropped slowly over 2.5 minutes by 0.2 units to nose down, while crew was keeping nose up by force on both control columns. Did mechanic trim not work at all?

That would explain the discussion at 5:41:46 - the mechanic trim wheels were not operating. There is also no attempt to use the electric trim switches before and during that discussion.

My understanding is that full manual trimming can be quite stiff due to aerodynamic forces and sometimes requires both crew members to apply force to the trim wheel, and AFAIK, that applies at more typical flight speeds. At or above Vmo, it would be all the worse.

There *does* seem to be an attempt to use the electric trim switches in the discussion at 5:41:46:

1)The captain asked if trim was working.
2)The first officer stated it was not.
3)The first officer offered to try using manual trim.
4)The captain gave him the goahead.
5)The first officer reported that manual operation of the trim wheel was not working.

From the fact that the first officer presented manual operation of the trim wheel as an alternative option under 3), it would seem that his statement that trim was not working at 2) was regarding something other than brute-force manual trim, i.e, operation of the electric trim switches.

Then again, he ought to have known that the electric trim switches would not work, because he had (correctly) *suggested and executed* the task of actuating the stab-trim cutout switches, which disabled all trimming except by muscle power operating directly on the trim wheel.

I will note here that distinguishing between the trim switches and direct operation of the trim wheel in this context is a bit confusing. We have three types of trim input that are relevant here:

1) Fully automatic trim inputs from MCAS, executed by the trim motor.
2) Pilot-commanded trim inputs executed by the trim motor.
3) Direct muscle-operation of trim by the pilot.

2) and 3) are both "manual" in as opposed to 1) being "automatic", in that the decision to change trim is taken by the pilot, not by an aircraft system, but 3) can also be considered manual as opposed to 2) being "powered", in that the trim change is executed by the pilot applying muscle power to the trim system. Considering that all three had a bearing on the outcome of the flight, this has the potential to create confusion as to whether "manual" trim refers to 2) or 3).

In any case, the whole exchange above took place over eight seconds, which to me indicates that the First Officer likely tried briefly to move the trim wheel and gave up, and neither asked for nor received assistance from the captain in moving it, which may well have been required.

Still, I wonder why they did not react to the overspeed warnings - the whole situation looks like an information overflow, with ice warnings also appearing along the way.

It certainly does.

At 5:43:04, the overspeed condition made controlling pitch by both control columns impossible. At that point they activated electric trim again, because mechanic trim had no effect. That allowed to trim nose up again.

I'm not sure that it actually wasn't possible for them to control pitch. They did not seem to actually be descending yet at this point, and while up-pitch authority was reduced, they were banked for a turn, and could have regained some vertical authority by rolling wings-level. Also, the graphs in the report show control column position, and when MCAS puts them into their final noseover, they put in a noticeable amount of extra aft-stick over what they had been using prior to that point, which indicates that they had at least some strength margin to pull back further on the stick.

---------- Post added at 14:57 ---------- Previous post was at 14:44 ----------

The Concorde accident likely is the closest you can ever get to a single point of failure.

And even that report had multiple events leading to the metal part on the runway...

Actually, the rudder hardovers Boeing had on the 737 back in the 90s are what come to mind for me. As the rudder was hardly used at cruise, the hydraulic servos in the rudder cooled down to ambient temperature, while the remainder of the hydraulic system remained hot. On approach, with the rudder being used more, hot hydraulic fluid was injected into the cold servos, and thermal expansion caused components to jam in such a way that the rudder either went full-travel to one side, or gave a reversed response to the rudder pedals. It was uncertain at the time, but I think that history has showed that crashes were pretty much single-cause results of that defect.

But even then, while it's a fairly clear-cut single-point-of-failure case *now*, contemporary reports were written before the problem was understood, and listed multiple potential causes.
 

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1) Fully automatic trim inputs from MCAS, executed by the trim motor.
2) Pilot-commanded trim inputs executed by the trim motor.
3) Direct muscle-operation of trim by the pilot.

Also important is the plot of the recorder data there:

1 and 2 can be seen and distinguished in the recorder data, while 3 acts directly on the stabilizer and can only be seen as stabilizer position change.

When the crew discussed the trim, there had been no attempts to use manual electric trim. But the aircraft was already at VMO, so its really possible that mechanic trim was only slowly possible.

But then: Why did the crew then trim nose down mechanically? Did they not know in which direction they have to move the wheel? (There was a minimal nose-down trim movement of the stabilizer position in that period of time)
 

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Also important is the plot of the recorder data there:

1 and 2 can be seen and distinguished in the recorder data, while 3 acts directly on the stabilizer and can only be seen as stabilizer position change.

Well, 1) at least is visible in FDR data, whether or not the cutout switches prevent it from operating, so I'd think 2) would be, but the discussion that led to the attempt at 3) seems to indicate that 2) was attempted right before the First Officer suggested 3). If it was, then apparently the cutout switches prevented it from being recorded as well as from being executed, but that isn't consistent with 1) being disabled by the cutout switches, but still recorded.


When the crew discussed the trim, there had been no attempts to use manual electric trim. But the aircraft was already at VMO, so its really possible that mechanic trim was only slowly possible.

But then: Why did the crew then trim nose down mechanically? Did they not know in which direction they have to move the wheel? (There was a minimal nose-down trim movement of the stabilizer position in that period of time)

The report indicates that that movement was gradual, and I'm inclined to think that it may have been slippage due to aerodynamic forces rather than any electrically- or muscle-driven movement.

My gut feeling regarding muscle trim, given that the First Officer suggested it and then stated it wasn't possible in such a brief window of time, is that the he briefly tried it, but had never been given a proper appreciation of the force required, and possibly didn't have the right arm positioning to apply full force (which I've seen mentioned as a factor in muscle trimming an airliner), and thus concluded that it was jammed and gave up.
 
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