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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 9th Apr 2019, 17:01
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Originally Posted by dlen
Could they just have switched autopilot on?
AP disengage when AoA disagree.
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Old 9th Apr 2019, 18:15
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trim drift

Another possible actor in this drama.

When boeing says to hold the trim wheel, when one study how the system is build and when one studies the data trace (and the misterious ND changes during multiple periods with no ND input).

I believe with sufficient pull and sufficient speed the trim motor clutch will slip and the trim wheels will creep ND.

If you blip the yoke switch and see the trim wheel not moving at a good pace and then you release and you see it slowly moving BACKWARDS, the whiskey tango foxtrot moment may be enough to make you forget about the pedestal switches.

Just another disturbing thought.
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Old 9th Apr 2019, 18:20
  #3743 (permalink)  
 
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Originally Posted by dlen
Wouldn't it be possible to just measure the nose up tendency - be it caused by engine nacelle lift or thrust at low speed - by evaluating both pitch change rate and control column position, and apply ND trim accordingly?
Pitch and AOA are not the same. Pitch and AOA can change independently during maneuvers, which is why tracking pitch changes will not help determine rates of AOA changes, nor will it help determine the absolute value of AOA. In order to do this properly, you need a full blown computer system that integrates all of these sensor inputs, plus GPS, airspeed, etc. Those are fitted in an A350 or B787, and their price is more than twice that of a B737.

Originally Posted by derjodel
AP disengage when AoA disagree.
Someone should write a thread showing all of the things that do not work when there is AOA disagree. IMO it would also simplify checklists if there were a single comprehensive one for AOA disagree, instead of parts of other checklists for procedures which bear some relevance to the actual flight conditions.

Edit: @ecto1 IMO Boeing needs to produce some hard data about how the trim system actually operates under real world conditions, not how manuals written in 1968 say it should work.
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Old 9th Apr 2019, 18:26
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Originally Posted by ecto1
Another possible actor in this drama.

When boeing says to hold the trim wheel, when one study how the system is build and when one studies the data trace (and the misterious ND changes during multiple periods with no ND input).

I believe with sufficient pull and sufficient speed the trim motor clutch will slip and the trim wheels will creep ND.

If you blip the yoke switch and see the trim wheel not moving at a good pace and then you release and you see it slowly moving BACKWARDS, the whiskey tango foxtrot moment may be enough to make you forget about the pedestal switches.

Just another disturbing thought.
There is no evidence that anything like this happened. I suppose with enough time you could sit and make up all sorts of possible problems, but let's stick to the evidence as we have it.

- GY

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Old 9th Apr 2019, 19:10
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I wonder if Boeing would not be better off taking their losses with MCAS and the trim system kludge by designing something appropriate for the narrow stick feel purpose, even if it means the certification of new hardware. A further software attempt is unlikely to impress and the equivalent of recertification and pilot sim training will be required anyway.
Loss of face, loss of money and admission of guilt are all water under the bridge.
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Old 9th Apr 2019, 19:22
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[QUOTE=Lost in Saigon;10443422]Just about everything you have read in the media is wrong. MCAS is not stall protection. MCAS does not counter the additional thrust of the more powerful engines. It is only there because the larger engine nacelles of the B737 MAX cause an aerodynamic pitch up moment at high angles of attack that did not meet FAA longitudinal stability and stick force certification standards.

The easiest fix was to automatically apply a little nose down trim at high angles of attack. (Quote)

I agree with all in the first paragraph but believe it needs to be expanded a little bit.

The B737 sat low on its undercarriage.For fuel efficiency the MAX was powered with much larger engines which had to be slung more forward than the old version. In addition the nose gear was raised by eight inches. This therefore meant that in order to satisfy the longitudinal stability requirements for certification, some fix was necessary. This then gave birth to MCAS. A quick fix through electronics control technology. The only point of its application naturally became the point of maximum effect, namely the horizontal stab. Accordingly the whole thing is one that addresses the original design concept, everything else would appear to be an attempt to overcome/control this in built longitudinal instability issue.
With it now comes an additional instrument, the AoA indicator wired into the whole system of automation. This piece of instrumentation did have its rightful place in the cockpit of a fighter jet, with its all moving horizontal stab, but does it really also have a place in a civil transport aircraft. Especially as a command function. After all to what is known so far about this particular event is that it was the stubborn stab that took the aircraft down despite all the crews efforts to encourage it away from its determined course.

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Old 9th Apr 2019, 19:25
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AoA Failure Rate

I hope the investigators will look into the AoA failure rate pre MAX. How many have been pulled for repair or replacement? Birds and ramp rash did not begin with the MAX.

Pre MAX, an AoA failure seems to produce "only" stick shaker and IAS disagree which every 737 crew handled successfully (until ET and JT). I suspect there's been a considerable number.
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Old 9th Apr 2019, 19:31
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Originally Posted by ecto1
Another possible actor in this drama.

When boeing says to hold the trim wheel, when one study how the system is build and when one studies the data trace (and the misterious ND changes during multiple periods with no ND input).

I believe with sufficient pull and sufficient speed the trim motor clutch will slip and the trim wheels will creep ND.

If you blip the yoke switch and see the trim wheel not moving at a good pace and then you release and you see it slowly moving BACKWARDS, the whiskey tango foxtrot moment may be enough to make you forget about the pedestal switches.

Just another disturbing thought.
A better explanation for the slight drift down while the electrical trim was disabled is that the pilots were attempting to move the wheel by rocking it but were not able to.
The deleted video clearly show how this might happen.
Just because the prelim report has no mention of activity during this time does not mean it was absent, we do not have a CVR transcript just a few excerpts.

The jackscrew cannot back drive the manual trim wheel for the same reason a screw jack holding a car will not slowly lower the car.
.
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Old 9th Apr 2019, 19:39
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Originally Posted by dlen
Could they just have switched autopilot on?
That's what they tried to do, but it didn't engage.
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Old 9th Apr 2019, 20:16
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Originally Posted by MurphyWasRight
A better explanation for the slight drift down while the electrical trim was disabled is that the pilots were attempting to move the wheel by rocking it but were not able to.
The deleted video clearly show how this might happen.
Just because the prelim report has no mention of activity during this time does not mean it was absent, we do not have a CVR transcript just a few excerpts.

The jackscrew cannot back drive the manual trim wheel for the same reason a screw jack holding a car will not slowly lower the car.
.
That's called irreversibility. A screw Jack is irreversible. A ball recirculation screw Jack is reversible. The steering of a truck as an example of the latter.

And to the poster who suggested to stick to what happened and not to what may have happened, I think he is right and I would try to post only probable theories, not merely possible ones. The trim drift is bugging me inmensely but it is probably irrelevant and the rocking explanation also fits.
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Old 9th Apr 2019, 20:21
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And yet...it's the MD95, also known as the Boeing 717, that never has had a fatality..... McDonnellDouglas made a great plane...
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Old 9th Apr 2019, 20:30
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Originally Posted by GordonR_Cape
Avionista



I sincerely hope that Boeing engineers are doing this as we speak, otherwise they won't have a leg to stand on if this ever comes to trial. Saying that it was designed and calculated to do X and Y back in 1968, is not going to impress a jury...
Unless a jury is composed of very competent pilots, then I doubt a layman will lead to a real prosecution.
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Old 9th Apr 2019, 20:47
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Only 156 produced though . . .
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Old 9th Apr 2019, 20:59
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“Just because the prelim report has no mention of activity during this time does not mean it was absent, we do not have a CVR transcript just a few excerpts.”

True. There is no mention of the gear up call.
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Old 9th Apr 2019, 21:16
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Originally Posted by ecto1
That's called irreversibility. A screw Jack is irreversible. A ball recirculation screw Jack is reversible. The steering of a truck as an example of the latter.

And to the poster who suggested to stick to what happened and not to what may have happened, I think he is right and I would try to post only probable theories, not merely possible ones. The trim drift is bugging me inmensely but it is probably irrelevant and the rocking explanation also fits.
Not sure context of your comment, whether just to illustrate the difference or suggest 737 uses recirculating ball screw.

To be clear the 737 does not use a ball recirculation screw Jack, it uses a acme nut and threaded rod hence irreversible .
I am not an aircraft mechanic (engineer across the pond) so I could be wrong but a quick search seems to confirm my understanding.
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Old 9th Apr 2019, 21:28
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Originally Posted by Chronus
With it now comes an additional instrument, the AoA indicator wired into the whole system of automation. This piece of instrumentation did have its rightful place in the cockpit of a fighter jet, with its all moving horizontal stab, but does it really also have a place in a civil transport aircraft. Especially as a command function. After all to what is known so far about this particular event is that it was the stubborn stab that took the aircraft down despite all the crews efforts to encourage it away from its determined course.
The AoA vane (not an indicator) is not new with MCAS. All airliners use AoA vanes.
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Old 9th Apr 2019, 21:42
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Originally Posted by Brosa
That's what they tried to do, but it didn't engage.

It was engaged in 05:39:20 with AP CMD A for 30 seconds [off about 05:39:50], or i was wrong?

Since it engaged, the pilot confidence to told the FO retracting the flaps. 05:39:45




FDR ET AVJ
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Old 9th Apr 2019, 21:56
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Originally Posted by yanrair

As you accelerate the plane pitches up due to lift being in front of cg and you need to trim nose down to fly level. And vice Versa. Lesson 1 of type rating course on sim covers this.
I know that, thank you

At low speed and with a limited amount of AND untrim, increasing speed may reduce or cancel aft column required, due to static stability of the aircraft. But in the case of ET302, even at 500 kt IAS and still both pilots pulling as much as they can, the aircraft has a -10° AoA and -2g ! No hope to reach any in-trim airspeed..

What I mean is that manual trimming was impossible due to loads on elevator (which vary as the square of airspeed) ; that the only thing that could have save the day is to slow down at a speed where loads would have permitted manual trimming.

Instead, they let full power up to VMO and above... I could not imagine why, now I know that it was what was recommended by Boeing...

Last edited by deltafox44; 9th Apr 2019 at 22:20.
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Old 9th Apr 2019, 22:07
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Originally Posted by derjodel
AP disengage when AoA disagree.
No. AP has been ON for 33 seconds with AOA disagree since the rotation. It disconnected only when the captain decided to maintain runway heading while the heading selector induced a right turn.

I have read that, depending of software version, AP would disconnect after 5 minutes of stick shaker, but I don't know it is the case for ET301
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Old 9th Apr 2019, 22:25
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Acme nuts and threaded rods CAN be reversible, especially in a vibrating environment, as has been realised too late too many times.
But probably irrelevant in this case.
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