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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 5th Apr 2019, 16:49
  #3321 (permalink)  
 
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Originally Posted by ecto1
I think there is a reasonable consensus (lest's say 50% chances) that vane shaft broke, keeping AOA vane attached or not, and subsequent alarms and checklists (both useful and useless) threw the crew out of balance. They didn't manage airspeed and lost ability to trim. They reengaged electric trim, was not strong enough either, forgot to CUTOUT, mcas trimmed down, EOF.

I think one of the biggest problems here is the compounding of stupidities. Each stupidity on its own is very much survivable, but all of them is a huge mess.

0. Of course MCAS MUST NOT BE operative with AOA disagree. Minimal software mod.

1. one AOA clearly fails, why not use a switch to transfer everything to the other (manually or automatically). It's a 3 way switch (AOA input L/NORM/R). In the event of stick shaker on, AOA disagree, check if any AOA is stupid (75 is quite stupid), switch to the other side, no more alarms in the cabin, crisis over in 10s tops. minimal wiring loom mod.

Even if you don't do it:

2. We have now perfect data about the influence of AOA over airspeed. 30 knots tops over the full AOA range and airspeed. Probably 15 knots 0 to 15 degrees 0 to 300 knts, probably less than 5 knots in the really tricky areas (slow). Upon AOA disagree, both airspeeds should use a default AOA value (4 deg maybe) instead of throwing UAS. and offer a reading with a possible +-7 knot deviation. But keep autothrottle and autopilot, maybe a caution message (airspeed calculation inacurate, stay 20 knots away from limits). Not a really disturbing unreliable airspeed, just because of a few knots. Minimal software mod.

3. Same with altitude. (altitude calculation inaccurate, stay 1000 feet clear from limits). Minimal software mod.

So that the only remaining alarm would be a stick shaker plus AOA disagree, and you still have autopilots. Much, much easier to handle. But if this is still enough for you to have the aircraft out of trim and miss speed management,

4. If speed goes over 280, message: reduce speed to regain trim ability). Minimal software mod.

My point is: most probably ANY of those mods would have saved the day, and all of them are pretty evident.

To me the problem is simply a huge lack of effort at design level to 1) Imagine 2) prepare for failures.


Chances are that the very same secuence of events, without MCAS final strike, has happened more than once before in other 737 variants (AOA fails, unreliable air data, stick shaker, big confusion, lack of proper aviation and or navigation and possibly all the way to overspeed and uneffective trim). Only without MCAS the aircraft would have been more or less in trim and therefore not nosediving and making the news.
Completely agree, great post, I have been thinkig along the same lines since the first crash, this one only reinforces that.

0) I think Boeing is working on that belatedly.

1). I flew the F50, and you could silence all/most alarms with a dedicated switch on one panel. I have read several accident reports that made me wonder if the crew would have done better without distracting and conflicting (stick shaker and over-speed simultaneously on Aeroperu flight PL603) error messages. Having a switch to deselect a system that gives an erroneous indication is an established concept in certifying aircraft (, and I think it should be a mandatory re-fit to all B737). Having one stick shaker go of on the failed AOA side with no other way to switch it off than going through the CB-panel isn't good enough.

2) + 3) Seems sensible, there might be areas in the envelope where this wouldn't work, so I can't positively endorse without more info.

4) Yes, UAS/AOA failure, max speed should be kept lower, so advise the pilots to lower their speed by reducing power is good, however, just lowering the limit so the clacker goes of brings me back to item 1)


Definitely Boeing needs to:
-Limit MCAS authority, and preferably by adding aerodynamic fixes like tail strakes.
-Disable MCAS in the case of AOA disagree.
-Install the AOA indicator and AOA-disagree indicator, preferably better than the current way.
-Install an AOA left/norm/right switch that limits error messages/stall warnings for those cases where it is clear to the crew which one is wrong.
-Bring back the option to disable auto trim (AP, STS ,MCAS, LAM), without losing the thumb-trim.
-Improve the NNC, to differentiate between MCAS and "regular" runaway trim.
-Verify it is possible to trim the aircraft back to in-trim from a runaway trim, aircraft full AND situation at low altitude, and see if there is any speed restrictions required for this, for all B737.
Obviously al of this should be paid by Boeing.

Maybe not all are required, but at this point I think Boeing better go the extra mile to make sure this doesn't happen again.

Also I do believe there should be more required training for pilots and more push in the SOPs to manually handle the aircraft, so people will not try to switch on the AP to resolve a problem requiring manual flight.

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Old 5th Apr 2019, 17:00
  #3322 (permalink)  
 
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The inference that manual trim was not available may not be correct, in fact the report refers to "manual electric trim" meaning the control column trim switches, this is in fact the "main electric trim", as distinct from manual trim. It is possible that the FO misunderstood the Captain's request for manual trim and attempted to trim by using the control column trim switches which not surprisingly did not work due to the operation of the cut out.

I think it is possible that the FO may have tried to use the control column switches at the Captain's request rather than winding the trim wheel due to the stress of the moment, even though he had only just selected the cut out. The FO's statement at 05:41:54 that "it is not working" would appear to me that the control column trim switch was not working, whereas if the manual wheel was jammed I'm sure he would have said something along the lines of "I can't move it" or "it is jammed".

I base this on my experience on the NG, where I have heard pilots and instructors both in simulator training and normal operations referring to trimming "manually" meaning manual operation of the electric trim switches, I've never seen anyone actually manually rotating the manual trim wheel in the aircraft during flight.
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Old 5th Apr 2019, 17:09
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Originally Posted by fizz57
Disagree. May address the issues that caused the latest accident, but contradicts the entire concept of the 737's instrumentation system, and will invalidate the safety calculations on which certification is based, as well as possibly opening the doors to other failure modes.

The 737 instrumentation is a dual-redundant system, comprising two completely separate systems such that in the event of a single failure one completely operational system is still available. With the aid of appropriate disagreement monitors, standby instruments and checklists, pilots are trained to detect and diagnose such failures and take appropriate action.

This system was state-of-the-art at the time the 737 was launched and is still in use in smaller, non-FBW airliners. It is totally adequate for its stated task, that is to provide information to a well-trained human crew. It should never have been allowed to provide inputs to a system that will automatically drive flight control surfaces.
Respectfully, I don't agree with you. The concept of the 737 has been changed dramatically from the original 93' span/94' length, 110K Lbs to the current 117' span/118' length, 195K Lbs. Adding STS, LAM, and now MCAS also contradicts the original B737 instrumentation system. The B737 overhead panel should have been thrown out when the designed the NG, and definitely with the MAX (FYI, DC9 and B717 are the same type rating). Having a switch to deselect a faulty indicator does not open the door to other failure modes. Not having an AOA disagree light as standard is just bad/cheap design (and Boeing agrees, all their MAXes will be retrofitted, and I believe all their more recent designs use 3 AOAs). Not informing crews about MCAS was the wrong thing to do, and not having a checklist for it letting crews re-trim the aircraft to neutral before using the cut-out for MCAS failure (as opposed to regular runaway trim) IMHO was a big mistake, just like certifying MCAS working on one AOA at a time was a big mistake.
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Old 5th Apr 2019, 17:13
  #3324 (permalink)  
 
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Originally Posted by threemiles
The peak is the FAA "Continued Airworthiness Notification to the International Community" dated March 12, a document type that was non existant before this date, is not based on ICAO rules and had just one obvious purpose. CMA.

CANIC was actually an outcome from the Comair 5191 crash. One of the NTSB findings related to poor inter-agency communication between various national and international certification authorities which delayed dissemination of training material that could have prevented the crash. This may be the first time that a CANIC was issued, however it is not a document type that was invented in the past few months.

"Recognizing the need for improved authority-to-authority communication, on September 26, 2006, the FAA released Order 8040.5 describing the policy and procedures for developing Federal Aviation Administration (FAA) airworthiness directives (AD) on imported products where the State of Design Authority has issued mandatory continuing airworthiness information (MCAI). The order introduced a process for evaluating all MCAI and determining whether individual MCAI meet certain criteria that would permit a quicker issuance of an FAA AD.Further, the FAA has implemented a process for Continued Airworthiness Notification to the International Community (CANIC) that provides notification to other international regulatory authorities when the FAA issues an airworthiness directive. This is the reciprocal action to that identified above in FAA Order 8040.5. Further, the FAA has instituted a process of regular continued operational safety program reviews with all manufacturers who hold, or have applied for, FAA type certificates on their aircraft. This review program allows a more open and consistent avenue of communication between the FAA, counterpart regulatory authorities, and manufacturers, should airworthiness issues arise."


https://lessonslearned.faa.gov/ll_ma...24&LLTypeID=10

Last edited by slacktide; 5th Apr 2019 at 17:14. Reason: formatting
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Old 5th Apr 2019, 17:15
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Originally Posted by AfricanSkies
Maybe in a 737, But it's wasn't in other jets, like the Fokkers. The trim wheel is just geared ..again question the 737 design..are so many clack clack clack turns necessary?
Late reply, but:
It is needed because of the force required to move the THS at high speed due to the hinge being at the back end. This causes the airflow to incraese the required trim force, had the THS been hinged at the front and and the jackscrew at the trailing edge of the THS the force required to trim would have decreased with an increase in speed. I am not a designer so I have no idea if you could even have built it that way, but I believe aerodynamically speaking this is the reason so many revolutions are required.
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Old 5th Apr 2019, 17:21
  #3326 (permalink)  
 
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Originally Posted by RatherBeFlying
Electric Trim Availability?

It's been pointed out that the range allowed for electric trim is less than can be obtained from the trim wheel (airload permitting).

Does that mean electric trim can not be used to return trim outside its range back to within its range?
No, the manual says explicitly that electric trim can always be used to bring it back into the normal range, but not to move it outside that range:

from 737NG FCOM 9.20.8 Stabilizer Trim:
In the event the stabilizer is trimmed to the end of the electrical trim limits, additional trim is available through the use of the manual trim wheels. If manual trim is used to position the stabilizer beyond the electrical trim limits, the stabilizer trim switches may be used to return the stabilizer to electrical trim limits.
Bernd
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Old 5th Apr 2019, 17:22
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Originally Posted by MemberBerry
Obviously hitting the mechanical limit disables trimming. It's a hard limit. But there are also electric switches preventing you from reaching the full nose down (and up) position using electric trim even before you hit the mechanical limit. Boeing regards this additional limitation as a safety feature. And indeed, in case of electric trim runaway, it can limit the amount of trim the runaway could apply, so it can improve safety in some situations.

Later edit:

You can read in more detail about that in the latest Satcom Guru article: https://www.satcom.guru/2019/04/stab...and-range.html

In that article there is a table describing the available ranges for the various types of electric trim, compared to manual trim with the trim wheels.

For example, for the 737-800, the full trim range available with the trim wheels is -0.20 to 16.9 units. For manual electric trim with the flaps retracted the range available is 3.95 units to 14.9 units, so far away from the mechanical stops.

That's what the EASA document was talking about. You can't access the full range of mechanical trim positions using manual electric trim. With the flaps retracted on the 737-800 you would only have access to a range of 10.95 units of trim out of the total range of 17.1 trim units available with the trim wheels.

Unfortunately MCAS doesn't use the same limits as manual electric trim, so it can bring the trim closer to the full nose down limit. In the Ethiopian flight it manages to bring it to 0.4 units of trim at some point.
Question, when it says manual electric trim is unavailable outside of the range you mentioned, does that mean in both directions? I understand you can't use electric manual trim to go further ANU than 14.5 and not further than 3.95 AND, but if you find yourself with the flaps up and full AND trim can you use the thumb switch to correct, or do you have to crank the handle till reaching 3.95 AND trim beforhe the thumb switch works? (all this based on me thinking the low number being AND, and the high number ANU)

Asked and answered while I was typing:

Originally Posted by RatherBeFlying
At initial stick shaker, the aircraft can be expected to be in trim or not far from it. Better to head off MCAS at the pass than let it compound an already difficult situation to one where recovery is uncertain.

Electric Trim Availability?

It's been pointed out that the range allowed for electric trim is less than can be obtained from the trim wheel (airload permitting).

Does that mean electric trim can not be used to return trim outside its range back to within its range?
Originally Posted by bsieker
No, the manual says explicitly that electric trim can always be used to bring it back into the normal range, but not to move it outside that range:

from 737NG FCOM 9.20.8 Stabilizer TRIM:
Quote:
In the event the stabilizer is trimmed to the end of the electrical trim limits, additional trim is available through the use of the manual trim wheels. If manual trim is used to position the stabilizer beyond the electrical trim limits, the stabilizer trim switches may be used to return the stabilizer to electrical trim limits.
Bernd

Last edited by hans brinker; 5th Apr 2019 at 17:30. Reason: adding answer
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Old 5th Apr 2019, 17:31
  #3328 (permalink)  
 
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Main Electric trim:
Flaps retracted: 3.95 - 14.5 units
Flaps extended: 0.05 - 14.5 units
AP trim 0.05 - 14.5 units
Manual trim: -0.2 - 16.9 units

0 degrees is 4 ANU

Interesting the green band on the NG is 1.5 to 6.5, while on the MAX it is 2.5 to 8.5

I saw a report where thrust was at 94% to impact?
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Old 5th Apr 2019, 17:36
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Originally Posted by RatherBeFlying
At initial stick shaker, the aircraft can be expected to be in trim or not far from it. Better to head off MCAS at the pass than let it compound an already difficult situation to one where recovery is uncertain.

Electric Trim Availability?

It's been pointed out that the range allowed for electric trim is less than can be obtained from the trim wheel (airload permitting).

Does that mean electric trim can not be used to return trim outside its range back to within its range?
Unbelievable ! But that would explain why the electric trim ANU stopped twice at about the same position (2.3/2.4 units).
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Old 5th Apr 2019, 17:36
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QUOTE]Hans Brinker wrote:
Definitely Boeing needs to:
-Limit MCAS authority, and preferably by adding aerodynamic fixes like tail strakes.
-Disable MCAS in the case of AOA disagree.
-Install the AOA indicator and AOA-disagree indicator, preferably better than the current way.
-Install an AOA left/norm/right switch that limits error messages/stall warnings for those cases where it is clear to the crew which one is wrong.
-Bring back the option to disable auto trim (AP, STS ,MCAS, LAM), without losing the thumb-trim.
-Improve the NNC, to differentiate between MCAS and "regular" runaway trim.
-Verify it is possible to trim the aircraft back to in-trim from a runaway trim, aircraft full AND situation at low altitude, and see if there is any speed restrictions required for this, for all B737.
Obviously all of this should be paid by Boeing.[/QUOTE] What stands out most to me in reading this entire thread is that, while MCAS appears to have been activated in error on 4 occasions in some 500,00 total cycles of the MAX fleet, it appears to have activated correctly precisely zero times. This suggests to me

1) that the certification requirement that MCAS was intended to meet was excessive and not really required for safe operation of the aircraft; and

2) that, had MCAS been designed to deactivate itself upon AoA disagree and done nothing else (including notify the flight crew), neither these two accidents would have happened nor would have any other related to the certification requirement.

Based on the numbers to date, the statistical probability of an AoA disagree sufficient to deactivate a well-designed MCAS happening on the same flight on which the flight crew managed to get near to an accelerated stall is vanishingly small.

Regarding much of the MAX design being grandfathered under the certification process, it's worth noting that the NG series (which was also grandfathered) has essentially the same accident/fatality rate as the A320 series, which suggested that grandfathering the NG had no effect on operational safety.
​​​​​​​
Hans Brinker also wrote:

​​​​​​​1). I flew the F50, and you could silence all/most alarms with a dedicated switch on one panel. I have read several accident reports that made me wonder if the crew would have done better without distracting and conflicting (stick shaker and over-speed simultaneously on Aeroperu flight PL603) error messages. Having a switch to deselect a system that gives an erroneous indication is an established concept in certifying aircraft (, and I think it should be a mandatory re-fit to all B737). Having one stick shaker go of on the failed AOA side with no other way to switch it off than going through the CB-panel isn't good enough.
Perhaps the user interface aspect of a cockpit where multiple alarms are going on and off deserves more attention than it has received to date. In particular, the issue of the stick shaker creating such a racket seems to be a real issue. It's far easier to be overwhelmed by aural cues than by visual ones.
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Old 5th Apr 2019, 17:39
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Originally Posted by deltafox44
Unbelievable ! But that would explain why the electric trim ANU stopped twice at about the same position (2.3/2.4 units).
Apparently always possible to electric trim back to middle even when outside of normal electric manual trim range.

Originally Posted by bsieker
No, the manual says explicitly that electric trim can always be used to bring it back into the normal range, but not to move it outside that range:

from 737NG FCOM 9.20.8 Stabilizer Trim:
In the event the stabilizer is trimmed to the end of the electrical trim limits, additional trim is available through the use of the manual trim wheels. If manual trim is used to position the stabilizer beyond the electrical trim limits, the stabilizer trim switches may be used to return the stabilizer to electrical trim limits.
Bernd
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Old 5th Apr 2019, 17:41
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Originally Posted by hec7or
The inference that manual trim was not available may not be correct, in fact the report refers to "manual electric trim" meaning the control column trim switches, this is in fact the "main electric trim", as distinct from manual trim.
I got the same impression reading the preliminary report.

From the report, emphasis mine:

Between liftoff and 1000 ft above ground level (AGL), the pitch trim position moved between 4.9 and 5.9 units in response to manual electric trim inputs. At 1000 ft AGL, the pitch trim position was at 5.6 units.
I'd like to know exactly what terms were used in this exchange:

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. The Captain told him to try. At 05:41:54, the First-Officer replied that it is not working.
I still can't figure out why nobody pulled back the power with continuous overspeed warnings on both sides and obviously too much thrust for attempted level flight.
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Old 5th Apr 2019, 17:55
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Originally Posted by Rob21
Do you know how many hand inputs from the pilot will take to complete one turn? Three, four? Supposing the answer is three, the pilot will have to make 44 hand movements to change one degree. If the pilot is able to execute one turn in 4 seconds (fast), a change of only one degree will take 59 seconds, or one degree per minute.
Not enough time to recover from a dive. If my math is correct, the only way to counteract MCAS input is with electric trim. But if you need to cut off trim motors to "kill" MCAS, how the pilot will manually turn the trim wheel to achieve enough amplitude to cancel MCAS inputs?
The key, as stated by Ranrair, (which see above) is to react immediately, before the trim gets beyond the point where the only functioning control is the mechanical trim wheel & cable drum.

The problem is, as has been stated by many now, MCAS actions mask what is essentially a runway-by-increments stabilizer, and there was no training or FCOM information for pilots to assess the behaviour of the system for themselves and when faced with heavier control column and UAS + #1 stall warning right after liftoff, tried to diagnose the problem without the necessary tools and experience to do so, costing valuable response time.
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Old 5th Apr 2019, 18:01
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Simulation of airflow around B737 body

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Old 5th Apr 2019, 18:03
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Originally Posted by Airbubba
I still can't figure out why nobody pulled back the power with continuous overspeed warnings on both sides and obviously too much thrust for attempted level flight.
Any reduction in power would have created a significant AND moment. The longitudinal acceleration was positive throughout- right up until the final MCAS AND. (0.1g at end equivilant to 0-60mph/0-100kmh in 27seconds.) The actual acceleration provided by thrust would need to be calculated using both vertical (above 1g) and longitudinal (above 0g) acceleration data.

That thrust moment of the engines coupled with the aerodynamics of the engine cowling was in fact assisting them with pitch (and would do so with diminishing effect up until VH) The fact that it also masked the true trim when they initially cut it out is possibly the reason why they didn't attempt manual trim wheel as soon as cut outs were flicked.
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Old 5th Apr 2019, 18:03
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Originally Posted by deltafox44
Unbelievable ! But that would explain why the electric trim ANU stopped twice at about the same position (2.3/2.4 units).
Electric trim limit on the NG (may vary slightly by series) is apparently 12.5 units ANU, can't think the Max will be much different.
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Old 5th Apr 2019, 18:06
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Originally Posted by Airbubba
I still can't figure out why nobody pulled back the power with continuous overspeed warnings on both sides and obviously too much thrust for attempted level flight.
Crew/ Confusion/ Overload

When multiple warnings go off at the same time with conflicting informations the crew gets in trouble.

Stick shaker = STALL warning, and at low altitude that is something to be scared off so you try to get the nose DOWN and power UP. And at the same time one gets HIGH speed indications.

Priority ONE, if the crew get the time to troubleshoot before crashing, is to figure out what warnings are true and what warnings are false.

Then add that the LEFT AOA and the RIGHT AOA give different info, add an anti STALL system, (MCAS) that starts playing with your feet and trim position, and crew overload exceeds available time to find out what is true and false.

Add the human factor too. Every beginning pilots get : Lesson 1, first minutes => Speed is life.

Each and every pilot, from beginning PPL upwards is more scared of a low altitude stall then off an overspeed. => Safety first => So the Power stays UP.
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Old 5th Apr 2019, 18:09
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Originally Posted by deltafox44
Unbelievable ! But that would explain why the electric trim ANU stopped twice at about the same position (2.3/2.4 units).
I think actually it may be related to this:

Originally Posted by Smythe
Interesting the green band on the NG is 1.5 to 6.5, while on the MAX it is 2.5 to 8.5
Maybe they only applied as much electric trim as needed to bring it close to the green band, that starts at 2.5 on the MAX. The stick shaker suggesting they are close to a stall might have made them reluctant to use more.

I don't think there was anything preventing them to apply more nose up trim if they wanted to, until they used the cutout switches.
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Old 5th Apr 2019, 18:13
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Originally Posted by positiverate20
Any reduction in power would have created a significant AND moment. The longitudinal acceleration was positive throughout- right up until the final MCAS AND. (0.1g at end equivilant to 0-60mph/0-100kmh in 27seconds.) The actual acceleration provided by thrust would need to be calculated using both vertical (above 1g) and longitudinal (above 0g) acceleration data.

That thrust moment of the engines coupled with the aerodynamics of the engine cowling was in fact assisting them with pitch (and would do so up until VH) The fact that it also masked the true trim when they initially cut it out is possibly the reason why they didn't attempt manual trim wheel as soon as cut outs were flicked.
Can any B737 pilot tell me how significant the AND is from reducing the thrust from T/O power (in this case I believe it was at 94% N1), to a power setting giving 210kts F5 and 250 kts Fup at 10.000' ?
I fly brand A, so I have no idea, but an answer in units or seconds of thumb trim input would be helpful. The cowling wasn't helping, as I am sure most of the cowling was below the CG at the AOA they were flying,( it only provides an ANU moment at very high AOA)

Also, I have a hard time believing they left the power at full on purpose, as I read they set a lower altitude and speed in the window, presumably expecting the AT to reduce power.
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Old 5th Apr 2019, 18:30
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I give it a burst of trim every 10 knots during accelerating. I also have to counter STS trimming initially nose-up to maintain speed.
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