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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 9th Apr 2019, 18:15
  #3761 (permalink)  
 
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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
  #3762 (permalink)  
 
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Originally Posted by dlen View Post
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 View Post
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
  #3763 (permalink)  
 
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Originally Posted by ecto1 View Post
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
  #3764 (permalink)  
 
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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
  #3765 (permalink)  
 
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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
  #3766 (permalink)  
 
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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
  #3767 (permalink)  
 
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Originally Posted by ecto1 View Post
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
  #3768 (permalink)  
 
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Originally Posted by dlen View Post
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
  #3769 (permalink)  
 
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Originally Posted by MurphyWasRight View Post
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
  #3770 (permalink)  
 
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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
  #3771 (permalink)  
 
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Originally Posted by GordonR_Cape View Post
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
  #3772 (permalink)  
 
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Only 156 produced though . . .
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Old 9th Apr 2019, 20:59
  #3773 (permalink)  
 
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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
  #3774 (permalink)  
 
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Originally Posted by ecto1 View Post
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
  #3775 (permalink)  
 
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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
  #3776 (permalink)  
 
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Originally Posted by Brosa View Post
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
  #3777 (permalink)  
 
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Originally Posted by yanrair View Post

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
  #3778 (permalink)  
 
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Originally Posted by derjodel View Post
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
  #3779 (permalink)  
 
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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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Old 9th Apr 2019, 22:36
  #3780 (permalink)  
 
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Aviation Week: Boeing Expands MCAS Demos To Speed Lifting Of 737 MAX Grounding

Aviation Week: Boeing Expands MCAS Demos To Speed Lifting Of 737 MAX Grounding

Aviation Week: Boeing Expands MCAS Demos To Speed Lifting Of 737 MAX Grounding

Guy Norris Apr 9, 2019

Pilot feedback to the proposed software changes to the Boeing 737 MAX Maneuvering Characteristics Augmentation System (MCAS) flight-control law is positive, says Boeing. After demonstrations, pilots believe the potential for further flight-control problems from the system are a “nonissue,” the airframer says.

However, despite the positive response from pilots to the upgraded control system and associated training package, Boeing is gearing up for a prolonged international effort to reinstate the grounded MAX fleet. The embattled company, which first unveiled the proposed MCAS changes to a group of certification authorities and airline pilots in Seattle on March 27, is embarking on a global campaign to convince regulators that the updates will be sufficient to enable the aircraft to return to service.

The campaign encompasses a series of simulator demonstrations and briefings at multiple training sites throughout Europe, Asia and Australia and comes as Boeing attempts to handle a situation unprecedented in its history. Because the MAX was grounded first by China and other authorities around the world days before the FAA followed suit, the company says it is imperative to build support for an international caucus of regulators willing to reauthorize the MAX to return to flight.

The FAA, which in past years would have taken a lead role in such an effort, is similarly shifting gears and is now working alongside a broader group of international regulators to adjudicate the case. The agency says it expects to receive Boeing’s final package of its software enhancement over the coming weeks. Meanwhile the FAA has set up a Joint Authorities Technical Review (JATR) to conduct a comprehensive review of the certification of the aircraft’s automated flight-control system. Chaired by former NTSB Chairman Christopher Hart, the JATR is comprised of a team from the FAA, NASA and international aviation authorities.

Boeing, which on April 5 signaled a 19% production slowdown of its 737 line to ease the growing logjam of undelivered aircraft, is also providing more details about the changes to the MCAS functionality contained in the new P12.1 flight-control computer (FCC) software load that will replace the existing P11.1 software. Based on pilot reaction to date, the company says it is confident its software upgrade is certifiable.

The briefings continue to emphasize that the MCAS, which was added to the speed-trim system to standardize handling qualities with those of the 737 Next Generation, is “not a stall-protection function and not a stall-prevention function,” says Mike Sinnett, Boeing Commercial Airplanes vice president of product development and future airplane development. “It is a handling-qualities function. There’s a misconception it is something other than that.“

Added to ensure a linear relationship between stick force per G, “speed trim is a function of airspeed, so if you’re going fast, it’s a low angle-of-attack (AOA), and if you’re going slow, it’s at higher AOA,” he notes. “The thing you are trying to avoid is a situation where you are pulling back and all of a sudden it gets easier, and you wind up overshooting and making the nose higher than you want it to be.”

Underscoring the difference between the speed-trim system on the 737 Next Generation (NG) and the MAX, Sinnett says: “Mechanically, on the NG there is a column cutoff switch that stops any automatic trim when the column is back to a certain spot. On the MAX, we still needed automatic trim when you got to that spot. MCAS differs from speed trim at elevated alpha because it bypasses that switch by design. To do that, it activates based on AOA rather than based on speed—which is what speed trim does. Speed trim is a function of airspeed, and MCAS is a function of angle-of-attack and Mach number, but it only triggers off AOA.”

In the initial briefing sessions for pilots on March 27, “we didn’t talk specifically about either of the accidents, but we ran through MCAS scenarios,” says Sinnett. “From the accidents, we now know how MCAS can behave when there is an erroneously high AOA input, so we walked through scenarios where that could occur. We demonstrated those in the simulator.”

In these sessions, pilots and regulators were able to interact via intercom and a big screen with Boeing pilots in the 737 MAX engineering cab. Following the sessions, “we went back to the classroom and said, ‘Here are the things that concern us most when we look at the scenario of the two accidents we just experienced,’” Sinnett says. “Upon reflection on what has occurred, it appeared the system could present a high-workload environment—and that’s not our intention. So we looked at changing the design to compare values from multiple AOA indicators to essentially eliminate the unintended trigger condition that causes MCAS to activate."

Sinnett says pilots appear satisfied that the three main layers of protection now added to the MCAS will prevent any potential repeat of the circumstances involved in the Lion Air and Ethiopian Airlines accidents. “We answered a lot of questions during the discussion, and then we went back into the simulator and demonstrated a number of different scenarios to run against these changes,” Sinnett says. “And the most compelling thing is that the AOA failure case turns into a run-of-the-mill AOA failure case like you might have on any other airplane. We didn’t get any negative feedback. It was all very positive, and any of the pilots who got into the simulator and saw the before and after, it was like, ‘Yes, OK, this is now a nonissue.’”

The first main layer of protection provided by the update is a cross-channel bus between the aircraft’s two FCCs, which now allows data from the two AOA sensors, or alpha vanes, to be shared and compared. AOA data continues to be fed from left and right vanes into their respective air data inertial reference units before being passed to the flight computers. However, the AOA data in both computers is now continuously compared. The change is made by software only and requires no hardware modification.

“In a situation where there is erroneous AOA information, it will not lead to activation of MCAS,” says Sinnett. He underlines that the entire speed-trim system, including the MCAS, will be inhibited for the remainder of the flight if data from the two vanes varies by more than 5.5 deg. If an AOA disagreement of more than 10 deg. occurs between the sensors for more than 10 sec., it will be flagged to the crew on the primary flight display.

The second layer of protection is a change to the logic in the MCAS algorithm that provides “a fundamental robust check to ensure that before it ever activates a second time, pilots really want it to activate,” says Sinnett.

The change would have protected the system from continuing to activate in the case of the Lion Air accident, in which the left AOA vane was stuck in the 20-deg.-nose-high position. In that circumstance, the logic rechecked if the MCAS was required and, registering the apparent nose-high position from the errant vane, commanded more nose-down trim. “Now it sees you’re in the same spot, it says you’ve got a stuck vane and says, ‘I’m not going to activate again,’” Sinnett notes.

“That’s assuming it will activate in the first place, which it won’t because one AOA vane with a high value won’t activate,” he adds. “When you do defense in depth, you have to artificially fail one layer to make sure you adequately design and test the next layer—so that’s what we had to do.”

Explaining the background to the third layer of protection, Sinnett says: “We also made sure if the second layer of protection failed somehow in some weird way and allowed MCAS to activate multiple times, the system now ensures the sum total is command-limited.” The result is that the pilots always have maneuvering authority remaining with the control column. “Pilots will always have the ability to override—although they had that before in other ways, like with the trim switch, for example. But with the software update, the column itself will always provide at least 1.2g of maneuvering capability. So you don’t just have the ability to hold the nose level, you can still pitch up and climb.”
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