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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 25th Apr 2019, 04:10
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Originally Posted by Icarus2001
Get the trim somewhere near correct with the electric trim switch and then turn off the trim. As 737 Driver keeps saying, this is not test pilot territory. It is basic flying skills, trim away the pressure.
"Get the trim somewhere near correct with the electric trim switch and then turn off the trim." - Perfect wording to have put in that AD! - yet there is the "no problem here" attitude that prevented such a simple wording.

Instead it was something like revert to previous check lists and procedures. (but some of you will not know AoA disagree - just to complicate things a bit more)

I think that actually and technically once MCAS is switched off certain areas of flight within the flight envelope are actually in "test pilot territory" as certification standards are not meet.

Around post # 330 there seems a pilot has used the MAX sim and the impression is - basic flying skills are very much challenged in the simulated event.
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Old 25th Apr 2019, 06:43
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Originally Posted by Water pilot
I thought that the sensor had been replaced before the fatal flight, or am I mistaken?
Yes, you are correct.

My understanding is that there are three failed sensors, not necessarily for the same reason.
We don't know that. There was an inconclusive discussion over in Tech Log a few weeks ago - the consensus appears to be that there has so far been no information published on whether the removed sensor was found to be faulty on the bench or, if it was, what the precise nature of the problem was.

That, in itself, is very odd. If anyone knows more, please share.
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Old 25th Apr 2019, 11:08
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Originally Posted by DaveReidUK
Originally Posted by Water pilot
I thought that the sensor had been replaced before the fatal flight, or am I mistaken?
Yes, you are correct.
My understanding is that there are three failed sensors, not necessarily for the same reason.
We don't know that. There was an inconclusive discussion over in Tech Log a few weeks ago - the consensus appears to be that there has so far been no information published on whether the removed sensor was found to be faulty on the bench or, if it was, what the precise nature of the problem was.

That, in itself, is very odd. If anyone knows more, please share.
The sensor was replaced before the penultimate flight and was ~20 degrres off for both flights according to the preliminary report.
The original replacement was due to intermittent problems that could have been the sensor or other things such as flaky connector.
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Old 25th Apr 2019, 12:37
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Originally Posted by Water pilot
So they understood that a single failure of a sensor would cause the plane to pitch nose down shortly after takeoff and they did not feel that it was necessary to inform pilots of this quirk?
First, I'm not defending Boeing here, but I would like to add in some background regarding 737 system design that might shed some light how the Boeing design team might have overlooked the significance of the single sensor failure issue with MCAS.

The 737 has a number of warning and assist systems that activate when an approach to stall is detected. Determination of the approach to stall condition is accomplished by the Stall Management Yaw Damper (SYMD) computer and the Air Data Inertial Reference Unit (ADIRU) using inputs from various sensors to include the AOA vane and pitot-static system. (There is also a completely independent and uncompensated standby attitude/airspeed/altitude instrument on the center forward instrument panel, but it does not generate any warnings.)

The SYMD's and ADIRU's work independently, although divergent outputs can generate specific warnings like IAS Disagree and ALT Disagree. Each side can independently generate a stall signal. This is a conservative approach since it is better to receive a stall signal when you are not in a stall (false positive) than to receive none when you are (false negative).

When a stall signal is generated, a number of systems can activate to provide warning and assistance to the pilot in the subsequent stall recovery - Stick Shaker, Elevator Feel Shift, Speed Trim Stall ID function, Autoslats, and reduced Yaw Damper input. The important point here is that it only takes one stall signal (possibly erroneous) to activate these systems. While erroneous activation of these systems is an annoyance, they are not existential threats.

In terms of design philosophy, MCAS was no different than these other stall-related systems. If fact, in at least one Boeing document MCAS is referred to as a sub-function of the Speed Trim System (STS) and MCAS responds to a stall in almost the exact same manner as the STS does currently.

With one important difference, and that difference was crucial.

First of all, some of you might be surprised to learn that in a stall condition, the STS will trim the stab nose down at the exact same speed as MCAS and can do so continuously until it hits the stop. This is far more authority than MCAS ever had. Why is not a problem? Because the STS will respect the control column trim cutout switches. These are not the pedestal switches that are activated by the pilot. These limit switches are located at the base of the control column and prevent trimming opposite the direction of column displacement. That is, if a pilot is pulling back on the yoke, then nose down trim is inhibited. If someone is pushing forward on the yoke, nose up trim is inhibited. Thus the authority of STS is limited by the control inputs of the pilot.

Because MCAS was needed to activate during an accelerated stall (i.e. turning, nose low recovery), this system was designed to bypass the aft control column cutout switch. Someone at Boeing did not connect the dots and realize that by removing this important barrier, it now created a new threat in the case of an erroneous stall signal.

I'll speculate here that since the folks at Boeing apparently considered MCAS to merely be an additional function of the existing STS, no further education was required.

Obviously, they were wrong and now everyone is powerfully aware of the significance of this oversight.

Last edited by 737 Driver; 25th Apr 2019 at 13:12. Reason: added comment.
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Old 25th Apr 2019, 13:31
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Originally Posted by 737 Driver
First, I'm not defending Boeing here, but I would like to add in some background regarding 737 system design that might shed some light how the Boeing design team overlooked the significance of the single sensor failure issue with MCAS.

The 737 has a number of warning and assist systems that activate when an approach to stall is detected. Determination of the approach to stall condition is accomplished by the Stall Management Yaw Damper (SYMD) computer and the Air Data Inertial Reference Unit (ADIRU) using inputs from various sensors to include the AOA vane and pitot-static system. (There is also a completely independent and uncompensated standby attitude/airspeed/altitude instrument on the center forward instrument panel, but it does not generate any warnings.)

The SYMD's and ADIRU's work independently, although divergent outputs can generate specific warnings like IAS Disagree and ALT Disagree. Each side can independently generate a stall signal. This is a conservative approach since it is better to receive a stall signal when you are not in a stall (false positive) than to receive none when you are (false negative).

When a stall signal is generated, a number of systems can activate to provide warning and assistance to the pilot in the subsequent stall recovery - Stick Shaker, Elevator Feel Shift, Speed Trim Stall ID function, Autoslats, and reduced Yaw Damper input. The important point here is that it only takes one stall signal (possibly erroneous) to activate these systems. While erroneous activation of these systems is an annoyance, they are not existential threats.

In terms of design philosophy, MCAS was no different than these other stall-related systems. If fact, in at least one Boeing document MCAS is referred to as a sub-function of the Speed Trim System (STS) and MCAS responds to a stall in almost the exact same manner as the STS does currently.

With one important difference, and that difference was crucial.

First of all, some of you might be surprised to learn that in a stall condition, the STS will trim the stab nose down at the exact same speed as MCAS and can do so continuously until it hits the stop. This is far more authority than MCAS ever had. Why is not a problem? Because the STS will respect the control column trim cutout switches. These are not the pedestal switches that are activated by the pilot. These limit switches are located at the base of the control column and prevent trimming opposite the direction of column displacement. That is, if a pilot is pulling back on the yoke, then nose down trim is inhibited. If someone is pushing forward on the yoke, nose up trim is inhibited. Thus the authority of STS is limited by the control inputs of the pilot.

Because MCAS was needed to activate during an accelerated stall (i.e. turning, nose low recovery), this system was designed to bypass the aft control column cutout switch. Someone at Boeing did not connect the dots and realize that by removing this important barrier, it now created a new threat in the case of an erroneous stall signal.

I'll speculate here that since the folks at Boeing apparently considered MCAS to merely be an additional function of the existing STS, no further education was required.

Obviously, they were wrong.
Thanks for re-stating the background, and many important points. Much of that material has previously been posted posted on this forum, including the Lion Air thread (many months ago). An important point not often discussed, was whether any of those other systems were operating at the same time as MCAS and stick shaker. The answer is they were probably inhibited, based on the known FDR readouts, and the likely control column inputs.

Your version is plausible on many levels, but IMO fails at several hurdles. These may be due to differences in definitions:
- MCAS was never intended to be an anti-stall system (rather it has to do with longitudinal stability and stick forces).
- MCAS was designed to activate at a much lower AOA, independent of the stick shaker.
- MCAS was designed to activate under many areas of the flight envelope, independent of the stick forces and airspeed.
- Disabling the control column cutout switches was intentionally added to MCAS, but was not its sole function or scope.

It has been mentioned before, but there seems to be no clarity if MCAS was intended to continue operating during an actual stall/stick shaker event, or only during the initial high AOA condition preceding the stick shaker/stall warning. The new version will have no such scope, since it will be limited to a single increment per event.

N.B. The quoted text in my reply was copied before your post was edited.
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Old 25th Apr 2019, 13:48
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Originally Posted by GordonR_Cape
An important point not often discussed, was whether any of those other systems were operating at the same time as MCAS and stick shaker.
In the Lion Air preliminary accident reports, illumination of the FEEL DIFF PRESS warning was cited in the maintenance history, and the post-Lion Air emergency AD includes it as a symptom of the failed AOA. This would be an indication of the Elevator Feel Shift Module activation. The STS would not have been active with flaps up. We know about the stick shaker and MCAS. There are no published DFDR traces for Leading Edge Device position, so we don't know for sure if the Autoslats system was active, but hopefully that information will be included in future updates. The Ethiopian preliminary and at least one incident report I found in the Aviation Safety Reporting System database describes the presence of small yaw oscillations which would be expected if the yaw damper response was suppressed.
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Old 25th Apr 2019, 14:15
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Originally Posted by Icarus2001
No, the six minutes was the result. Had the PF used electric trim ANU they could have had as much time as they wanted to try a memory item, like turning off the trim system.
Some aircraft do not have a manual trim wheel, this one does. Get the trim somewhere near correct with the electric trim switch and then turn off the trim. As 737 Driver keeps saying, this is not test pilot territory. It is basic flying skills, trim away the pressure.
Valid in a normal out of trim situation. But when an automatic system is actively pushing trim nose down to that extent? Repeatedly? And the only way to fix the problem (MCAS) is to switch out the solution (elec trim) as well? Yes this is test pilot territory.
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Old 25th Apr 2019, 14:55
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Originally Posted by PerPurumTonantes
Valid in a normal out of trim situation. But when an automatic system is actively pushing trim nose down to that extent? Repeatedly? And the only way to fix the problem (MCAS) is to switch out the solution (elec trim) as well? Yes this is test pilot territory.
As far as defeating the MCAS input, the ONLY thing required of the Captain was to trim the aircraft normally. The yoke trim switch will trump MCAS every single time. He did not need the strength of his arms, he only need the strength of this left thumb.

Yes, MCAS will try again after a 5-second delay, but that motion would have been stopped as easily as the first - if the Captain had simply applied normal trim technique. He could have done this all day long, and MCAS never would have driven the stab to an unrecoverable position. At some point, after tiring of this back and forth, one of the pilots could then have disabled MCAS by using the trim cutout switches.

No, this is most definitely not test pilot territory. This isn't even commercial pilot territory. Trimming the elevator/stab to relieve control pressures is private pilot territory. Heck, we even expect this level of skill from a student pilot before they are cleared solo!

Set the pitch, set the power, check the performance, and trim. That is all that was required to get this aircraft safely away from the ground.
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Old 25th Apr 2019, 15:13
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So, its that piss easy is it ?.

So 4 professional pilots failed to press a little trim switch to keep the aircraft in trim and killed a few hundred people.

Really ?, there is something wrong with a system that causes 2 planes to crash like that, if the solution was soooooooooo simple.

Shame you wern't on the plane to instruct them all.
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Old 25th Apr 2019, 15:32
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Fly the damn aircraft...

Originally Posted by michaelbinary
So, its that piss easy is it ?.

So 4 professional pilots failed to press a little trim switch to keep the aircraft in trim and killed a few hundred people.
Yes, it really was that easy. As you say, they were all professional pilots. And what exactly should be our expectations of a professional pilot? I would think that being able to fly their aircraft (and not rely on the automation to do it for them) in the presence of a distraction would be a pretty minimum requirement.

The first and most important step of any aircraft emergency is FLY THE AIRCRAFT. Set the pitch, set the power, check the performance, trim the aircraft. Pretty basic stuff.

Sadly, this has not been the first time, and probably won't be the last time, that one of the primary causes of a fatal accident was because at least one of the pilots was not FLYING THE AIRCRAFT.

As to the question of WHY four professional pilots failed to revert to these basics of airmanship, that should be a major topic of the accident investigation.

Last edited by 737 Driver; 25th Apr 2019 at 15:44. Reason: added comment
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Old 25th Apr 2019, 15:45
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Easy to criticize from the comfort and safety of your arm chair with the help hindsight, no urgency to say or do anything.
Seen lots of people like you.

Nothing takes away the fact that 2 aircraft crashed and killed everybody when there was essentially nothing wrong with them, and 4 pilots all made the same simple mistake of not using the electric trim tab to normalise the trim.

That stinks. Any common sense would tell you, that stinks.

If that is really the case which I dont think it is, Boeing have screwed up big time and deserve to be sued into the ground.


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Old 25th Apr 2019, 15:51
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Originally Posted by michaelbinary
Easy to criticize from the comfort and safety of your arm chair with the help hindsight, no urgency to say or do anything.
So you're perfectly okay with letting major lapses in airmanship slide even though hundreds a people might die sometime in the future because the deficiency wasn't ever addressed?

Boeing needs to fix their problem, but the professional pilot corp can just continue to whistle past the graveyard when their is an obvious issue with crew training and proficiency?

Last edited by 737 Driver; 25th Apr 2019 at 15:54. Reason: added comment
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Old 25th Apr 2019, 15:56
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Originally Posted by michaelbinary
So, its that piss easy is it ?.

So 4 professional pilots failed to press a little trim switch to keep the aircraft in trim and killed a few hundred people.

Really ?, there is something wrong with a system that causes 2 planes to crash like that, if the solution was soooooooooo simple.
Actually it was that easy.

Equally, leaving the flaps at flaps 1, would have suppressed all MCAS operation, and again they would have all the time their fuel load allowed to "figure it out".

You probably won't like this, but it's clear there is a generation of pilots currently responsible for hundreds of lives per flight, that know real well how to select the autopilot. Much else seems a bit of a challenge. Worry, much.

- GY
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Old 25th Apr 2019, 16:13
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Well neither of you were there, and neither of you know exactly what did or didnt happen or what the pilots did or didnt do to fix the issues.

I dont beleive the situation was as simple as you would like to make out.

You have the benefit of sitting there smugly, saying, I would do this or that, or they should have done this or that.
They were presented with a situation that they were not trained to respond to, or even told it was there or what it did, such was Boeing's arrogance.
There was probably confusion, lack of information as to what exactly was happening, and therefore confusion of what to do to correct it, and at the same time not making the issue whatever it was, worse.

If you are unlucky you may get the chance one day to prove to the world just how wonderful you both are.
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Old 25th Apr 2019, 16:15
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Originally Posted by 737 Driver
As far as defeating the MCAS input, the ONLY thing required of the Captain was to trim the aircraft normally. The yoke trim switch will trump MCAS every single time. He did not need the strength of his arms, he only need the strength of this left thumb.
We do not know that for sure, since nobody has provided any definitive proof, and the pilots that tried clearly failed. It is clearly in Boeing's interest to imply that there was pilot error, but an objective crash investigation may find differently. It may turn out that some weird software glitch, or wiring error, or activation of speed trim (in addition to MCAS), may have prevented the pilots from raising the nose.
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Old 25th Apr 2019, 16:30
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Originally Posted by GordonR_Cape
We do not know that for sure, since nobody has provided any definitive proof, and the pilots that tried clearly failed. It is clearly in Boeing's interest to imply that there was pilot error, but an objective crash investigation may find differently. It may turn out that some weird software glitch, or wiring error, or activation of speed trim (in addition to MCAS), may have prevented the pilots from raising the nose.
I'm not sure what level of definitive proof you are looking for, however, if you look at the DFDR output in the ET302 preliminary report, you will see that the Captain was making stab trim inputs and the stab was indeed moving. If you look closely at about the 5:40:25 mark, you will even see a pilot nose up trim input that stopped and reversed the ongoing MCAS input. This is strong evidence that the system was working as designed. As previously mentioned, Speed Trim was not active after the flaps were retracted, nor were there any indications on the DFDR trace of any automatic trim inputs other than MCAS. The Elevator Feel Shift module may have been active thus increasing elevator control pressures, but that would have only accentuated the need to trim off the control pressures.

As I keep saying, there were multiple causes to these accidents. Design issues, oversight issues, maintenance issues, and training issues. However, there were also crew performance issues. They all need to be addressed.

.
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Old 25th Apr 2019, 16:33
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Originally Posted by GarageYears
Actually it was that easy.

Equally, leaving the flaps at flaps 1, would have suppressed all MCAS operation, and again they would have all the time their fuel load allowed to "figure it out".

You probably won't like this, but it's clear there is a generation of pilots currently responsible for hundreds of lives per flight, that know real well how to select the autopilot. Much else seems a bit of a challenge. Worry, much.
- GY
Clearly, Boeing and their Good Ol' Boys are culpable here, but, [FWIW=0] I have completely lost faith in the supposed competence of the unseen pilots behind that door, and I'm not going to fly anymore unless it's an absolute necessity! Too much of a gamble. Apologies to the 100s of thousands of you who ARE competent and responsible.
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Old 25th Apr 2019, 16:58
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Originally Posted by michaelbinary
Well neither of you were there, and neither of you know exactly what did or didnt happen or what the pilots did or didnt do to fix the issues.

I dont beleive the situation was as simple as you would like to make out.
There are lots of comments on this and other threads about how Boeing screwed up, how the FAA screwed up, how the airlines screwed up, and how maintenance screwed up without much in the way of dissenting voices. We expect them to fix their mistakes, don't we? Well how about the professional pilot corps? Are we willing to acknowledge that there were deficiencies on the part of the crew?

Listen, I get it. A lot of us here are pilots, and we hate to contemplate that someone within our group, for whatever reasons, fell short of the expectations of a professional pilot. It is easy to point fingers across the fence, much harder to look in the mirror. However, the first step in solving a problem is acknowledging a problem exist. Issues with automation dependency, pilot proficiency and deterioration of basic flying skills has been a constant presence in our industry for quite some time. I don't care what airline you fly for or how many hours you have, you have likely encountered someone (perhaps even yourself) who was no longer comfortable with turning off the automation and hand-flying in other than day VMC when there were no other distractions. This is a problem, and it needs to be addressed.

We do not control our training departments, but we do control how we approach our flying duties. If you are a professional pilot and you are honestly telling yourself that you could not have done any better given the MAX failed AOA scenario, then you owe it to yourself and your passengers to do something about it. Yes, we all now know the details about MCAS and how to deal with it. That's not what I'm talking about. The next malfunction you have may be equally unique and initially confusing.

If you are not comfortable with your hand-flying skills, then do something about it. Turn off the magic when you can. Hand fly the aircraft all the way to level off and all the way down. Captains, encourage your FO's to do the same. Automation is not always your friend, and sometimes it is your enemy. Start making note of pitch and power setting at different phases of flight. Pull out the manuals more often. Know your aircraft's memory items and limitations. Read incident reports and consider what you would do differently. Make training events in the sim count. If your sim instructor has the discretion, ask to see something different or something you haven't seen in awhile. Set a high standard for yourself and your crew. Yes, you can get by with much less effort for the same amount of pay 99.9% of the time. However, that last 0.1% can quite literally be the difference between life and death.

All this advice isn't anything you haven't already heard before. Just do it.
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Old 25th Apr 2019, 17:07
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Originally Posted by GarageYears
Actually it was that easy.

Equally, leaving the flaps at flaps 1, would have suppressed all MCAS operation, and again they would have all the time their fuel load allowed to "figure it out".

You probably won't like this, but it's clear there is a generation of pilots currently responsible for hundreds of lives per flight, that know real well how to select the autopilot. Much else seems a bit of a challenge. Worry, much.

- GY
Yes, I agree. It SHOULD have been easy, but for some unknown reason, it was not.

There are at least three things that could have saved them:

1) leaving the flaps extended to stop MCAS operation
2) using electric trim to stop MCAS operation
3) using the stab trim cutout switches to stop MCAS operation

They did NONE of these things CORRECTLY. We need to find out why.

Hopefully the final report will give us the answer.
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Old 25th Apr 2019, 17:11
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Originally Posted by 737 Driver
I'm not sure what level of definitive proof you are looking for, however, if you look at the DFDR output in the ET302 preliminary report, you will see that the Captain was making stab trim inputs and the stab was indeed moving. If you look closely at about the 5:40:25 mark, you will even see a pilot nose up trim input that stopped and reversed the ongoing MCAS input. This is strong evidence that the system was working as designed. As previously mentioned, Speed Trim was not active after the flaps were retracted, nor were there any indications on the DFDR trace of any automatic trim inputs other than MCAS. The Elevator Feel Shift module may have been active thus increasing elevator control pressures, but that would have only accentuated the need to trim off the control pressures.

As I keep saying, there were multiple causes to these accidents. Design issues, oversight issues, maintenance issues, and training issues. However, there were also crew performance issues. They all need to be addressed.

.
I certainly did look at the ET 302 FDR readout, before posting my comment. The question is why did the pilot stop nose up trim inputs at 2.5 degrees? There were still considerable control column forces shown on the FDR. Why would any pilot only half trim out those forces, unless they were stopped by software or hardware? My question remains unanswered...




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