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737 Stuck Manual Trim Technique

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737 Stuck Manual Trim Technique

Old 30th May 2019, 23:49
  #181 (permalink)  
 
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[QUOTE]
Originally Posted by yoko1
I am also qualified on the 737, so I’m familiar with the system and procedures. Whoever said to use the cutout switches without regard to either a published procedure or before attempting to neutralize the trim was wrong - just plain wrong. Perhaps in the old days when pilots were expected to have a greater knowledge of the systems would such a divergence been acceptable. Not so much anymore.

I don’t really see the crew errors as being a case of the pilots personally being at fault, but rather continued evidence that airlines around the world (including, sadly, mine) continue to underinvest in their pilots, by some combination of accepting ridiculously low experience (i.e. the ET302 First Officer) and/or minimal training that is heavy on managing the automation and checking off boxes in the sim and light on managing the aircraft (to include proficiency in hand-flying) and preparation for unusual, out of the box situations.

As far as use of the trim cutout switches, we have to assume the ET crew was either following an established procedure or not. If they were attempting to accomplish the runaway stab procedure (the only one that I know of that uses the cutout switches), then they did it incorrectly leaving the aircraft in a grossly out of trim condition. If they were not following a published procedure and merely grasping at straws, then they were grasping at the wrong straw. The immediate remedy to the MCAS inputs was the yoke trim switch, not the trim cutout switch. Once the trim had been neutralized (or at least close), only then would it have been appropriate to disable all electric trim.
Totally agree on direction training is taking, a lot of 'check the tick box' and not a lot of 'what now" novel challenges.
This is likely not helped much by 'binary' simulator data that tend toward prepackaged scenarios and not as much freedom to go in and throw a random combinations of faults into the mix. This is my impression from comments here, I have no first hand knowledge on that.

On the stab trim runway when to hit the cutout switches may have changed along the way, the original 'most likely' trim runaway would be stuck switch/relay which would not be overridden by pilot trim input so focus was on stopping it quickly .
This from a prior post.

Quote:
Originally Posted by tdracer
Posted about this on one of the countless other MAX threads, but it probably bears repeating.
Not too long ago I was at special event at the Museum of Flight - not only was I seated at a table with a bunch of current and retired Alaska Air pilots, during the cocktail hour I ran into a flight test pilot friend who'd been involved in the MAX development (Alaska is an all 737 operator - classics and NGs). To a man, they all agreed that if the stab tirm started doing something you didn't understand or like, the very first thing they'd do is turn it off and trim it manually. Hence the reason Boeing didn't treat MCAS as a flight critical system. However these were all older, high time experienced pilots
That being said, they also all agreed that no sim training for MCAS (or any of the other MAX differences) was a huge miss...
My bolding in above.
One of the criticisms I see of the ET crew is that they did not first trim with electric trim before using cutouts as hinted at in the Lion Air triggered emergency AD.

It is not clear whether lack of manual trim after cutout was due to lack of familiarity with the flip out handles or aero loads or both.
Something had to be happening in the period while electric trim was disabled, we don't know what since the CVR transcript has not been released.
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Old 31st May 2019, 09:28
  #182 (permalink)  
 
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Didn't the ET crew leave it a long time before cutting off the stab trim switches.
So long in fact that the trim was so far out as to make manual trimming impossible.
This is not the procedure recomended. The equivalent of putting in rudder on a V1 cut when the aircraft has rolled on it's back.
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Old 31st May 2019, 10:16
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Originally Posted by reamer
Didn't the ET crew leave it a long time before cutting off the stab trim switches.
So long in fact that the trim was so far out as to make manual trimming impossible.
This is not the procedure recomended. The equivalent of putting in rudder on a V1 cut when the aircraft has rolled on it's back.
The trim was the following;
9 sec MCAS, followed by
3 sec crew up, followed by
6 sec MCAS, followed by
9 sec crew up, followed by
cutout.

So, the crew put in a total of 12 sec manual trim up, MCAS put in a total of 15 sec trim down. But MCAS trims at about 50% larger speed than manual crew trim, so the end result was significantly out of trim at the time of cut out.
But for all those that say the crew hit cutout without trimming at all, they did in fact trim continuously for 9 sec before hitting cut out. Sadly they didnt trim all the way to zero yoke force. But it is neither stated in any training material that they should do that, it is merely a suggestion that they can do manual trim before cut out. Which they in fact did, for 9 consecutive seconds up until the moment of cutout, which in my opinion is quite a lot of manual trim.

In my opinion the failure here is squarely in Boeings hands as Boeing never in any training material made clear how difficult it is to trim with the trim wheel at such out of trim conditions and never ever made clear how important it is to trim to about zero yoke force before hitting cutout.
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Old 31st May 2019, 11:30
  #184 (permalink)  
 
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I am with You Steinar.
But that would be part of the basic handling of any non-FBW aircraft, and certainly the 737.
Even a issue on a normal go around, some massive trim.
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Old 31st May 2019, 14:06
  #185 (permalink)  
 
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Originally Posted by reamer
Didn't the ET crew leave it a long time before cutting off the stab trim switches.
So long in fact that the trim was so far out as to make manual trimming impossible.
This is not the procedure recomended. The equivalent of putting in rudder on a V1 cut when the aircraft has rolled on it's back.
Depends what you consider a "long time" - from the details we have they appear to have diagnosed a stab trim problem and hit the cutouts within 40sec, which is a lot quicker than I recall in e.g. MentourPilot's demo videos. It's also a lot quicker than the first LionAir crew.

They were actually trimming _up_ when they hit the cutouts - so you are dead wrong there because if they'd left it _longer_ before hitting the cutouts they would have been _less_ out of trim, not more.

Why quit when you are winning? - fear of counter attack maybe. Maybe they thought they were near enough back in trim that they could cutout and do the rest manually and that was better than risking another MCAS. Maybe, just possibly, they'd even practised that in the sim - but the sim, as we now know (and they didn't), wasn't/isn't accurate on the trim wheel forces. We know that some ET pilots were trying to replicate the LionAir scenario in sim from the leaked email from one of them (which, assuming it is not fabricated, is scarily prophetic - "throw in a GPWS PULL UP and it would be a crash for sure").


All this is, of course, just speculation. I would put money on there being something the crew could have done to avoid the crash (because statistically with air accidents that is a winning bet), I am not qualified to say what they should have done, and have no idea what I would have done. However, when we have one crash and lots of people saying "should have done X (fast/first/etc.)" and than another crash after similar failure when they did "do X" followed by people saying "did X too quickly, should have done Y first etc." there is the smell of lots of missing "in hindsight"s in the air (and it doesn't matter whether the field in question is aviation or not). When I see some people saying of the same event "they did X too quick" while others say "they did X too slow", it makes me think this one may not be soluble even with 20/20 hindsight.
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Old 31st May 2019, 18:12
  #186 (permalink)  
 
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Originally Posted by MurphyWasRight
It is not clear whether lack of manual trim after cutout was due to lack of familiarity with the flip out handles or aero loads or both.
Something had to be happening in the period while electric trim was disabled, we don't know what since the CVR transcript has not been released.
To this respect, the information transmitted by the interim report is :
At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and FirstOfficer confirmed stab trim cut-out
So, the cutout switches have been switched off between 05:40:35 and 05:40:41,
Then later:
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.
This can only indicate that the crew was unsure if the cutout switches had disabled the electric trim switches or not.
The information about what was the exact function of these switches had not been transmitted to the pilots or assimilated by them.
So they WERE trying to trim up and they discovered that they had neither electric trim nor manual trim available.

There are 2 other points which I noticed in the report and found much informative.
The first point is about uncommanded stabiliser movements:
From 05:40:42 to 05:43:11 (about two and a half minutes), the stabilizer position gradually moved in the AND direction from 2.3 units to 2.1 units. During this time, aft force was applied to the control columns which remained aft of neutral position.
As this happens during the time the cutout switches were on the off position, I understand that this means that the stabiliser has moved under aerodynamic forces.

The second point is:
At 05:42:10, the Captain asked and the First-Officer requested radar control a vector to return and ATC approved.
At 05:42:30, ATC instructed ET-302 to turn right heading 260 degrees and the First-Officer acknowledged.
At 05:42:43, the selected heading was changed to 262 degrees.
This sequence starts shortly after the FO has reported that manual trim was not working.
The airplane had already started to bank right significantly (between 5° and 15°) at 05:41:30.
When ATC instructed heading 260°, the bank angle is increased gradually from about 10° to 20°.
It is between the 40 seconds that follow the ATC instruction that the crew decided to reset the cutout switches since, at 05:43:10, the DFDR records 2 short electric trim impulses that successfully (but very moderately) moved the stabiliser nose up.
At the time of the short pulses, the bank angle was between 20° and 25°.
When MCAS kicks in 5 seconds after the pulses at 05:43:20, the bank angle increases briefly to 30°.
I understand that the final loss of control happens at a point when the attention of the crew is divided between controlling pitch and another objective (here turning from heading 110° to heading 260°) and that the turn may have played a role in their decision to reset the cutout switches.

Keep in mind that, during all this sequence, the stick shaker is active and the EFS feedback force against aft command is multiplied by 4.

I am not disputing that this crew did not make the best possible decisions.
But apportioning blame is totally useless.
The important stuff for improving the system that led to this situation is understanding how and why these decisions were made.
Apologies for being lengthy.

Last edited by Luc Lion; 31st May 2019 at 18:38.
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Old 31st May 2019, 21:30
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Originally Posted by Luc Lion
The first point is about uncommanded stabiliser movements:

As this happens during the time the cutout switches were on the off position, I understand that this means that the stabiliser has moved under aerodynamic forces.
Could also be the result of attempt at using the manual trim wheel, as it was a relatively small movement. It is also possible that it was an anomaly in the data.


Keep in mind that, during all this sequence, the stick shaker is active and the EFS feedback force against aft command is multiplied by 4.
I think this may be an incorrect understanding. The four times reference comes from the maintenance manual, but our FCOM says the force is doubled. I asked our fleet guys who supposedly asked Boeing and the answer was that the pressure delivered to the elevator feel system is approximately four times (increase from 200 psi to 800 psi) but the force actual felt at the control column is approximately doubled to around 40-50 lbs.
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Old 1st Jun 2019, 00:38
  #188 (permalink)  
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Originally Posted by yoko1
Could also be the result of attempt at using the manual trim wheel, as it was a relatively small movement. It is also possible that it was an anomaly in the data.
DFDR/CVR data won't have an error in a variable like trim position that suddenly happens at that time. A loss of data, that happens, but the engineering values are fixed, so any sensed variable will give an outcome with a constant ac and dc correction/slope and bias. Occasionally the engineering value conversions are incorrect, and the data outcome would be incorrect for all outputs of that variable, that is not the case here. The conversions are post process values... The crew holding very high back pressure against a trim are unlikely to have manually trimmed further forward against their control input, that is not a credible event, so we are left with the possibility that the force on the stab is sufficient to cause creep, and given the design, and IMHO, that should give concern to anyone designing or certifying the plane of that type.
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Old 1st Jun 2019, 09:40
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Originally Posted by yoko1
I think this may be an incorrect understanding. The four times reference comes from the maintenance manual, but our FCOM says the force is doubled. I asked our fleet guys who supposedly asked Boeing and the answer was that the pressure delivered to the elevator feel system is approximately four times (increase from 200 psi to 800 psi) but the force actual felt at the control column is approximately doubled to around 40-50 lbs.
This is an interesting question.
1) It appears that the manufacturer's document are not clear about the aircraft actual control forces.
2) In any hydraulic actuator/cylinder, the force is proportional to the fluid pressure acting upon the piston part of the actuator.
For the control force to be only doubled when pressure is actually fourfold, a device must be provided that reduces the force from the actuator to the control column.
As the 737 is a mechanical airplane, such a device - if it exists - should also be a hydro-mechanical unit.

Now what would be the point of increasing hydraulic pressure just to reduce its effect on the control system ?

No doubt some Civil Aviation Agency - and maybe even the FAA - will ask that the actual control forces and torque on the trim wheels be measured before drawing any conclusion as to the plausible scenario of the accidents or the 737 return to service.

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Old 1st Jun 2019, 14:14
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Originally Posted by yoko1
Could also be the result of attempt at using the manual trim wheel, as it was a relatively small movement. It is also possible that it was an anomaly in the data.
Effectively, when a pilot discovers that he is unable to trim manually his airplane, he may be tempted to try in the opposite direction for checking if the system is mecanically blocked. I read a description of such an action in a PC-6 accident report.
However, in such a case, the trim movement is brief.

The report states and shows in the diagrams that the stabilizer position moved gradually in the AND direction over a period of 2 and half minutes.
This gradual movement can't be imparted as a manual action.
And, as indicated by fdr, an error in DFDR data calibration would produce a constant offset

My best guess is that the brakes on the trim clutch are not totally effective in an extreme out-of-trim situation at high speed.
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Old 1st Jun 2019, 17:37
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Originally Posted by Luc Lion
Effectively, when a pilot discovers that he is unable to trim manually his airplane, he may be tempted to try in the opposite direction for checking if the system is mecanically blocked. I read a description of such an action in a PC-6 accident report.
However, in such a case, the trim movement is brief.

The report states and shows in the diagrams that the stabilizer position moved gradually in the AND direction over a period of 2 and half minutes.
This gradual movement can't be imparted as a manual action.
And, as indicated by fdr, an error in DFDR data calibration would produce a constant offset

My best guess is that the brakes on the trim clutch are not totally effective in an extreme out-of-trim situation at high speed.
The FDR traces are coarse so it is hard to know for sure whether the changes were truly continuous or a series of small steps, the raw data should show which.
Other possibilities, of course could also be 'all of the above'

1: Each (ineffective) manual trim attempt briefly unlocked the brakes allowing an aero load induced movement.
I have no knowledge on the mechanical interaction between manual trim and the brakes so this may not be possible.

2: The pilots went back a bit before attempting to go forward, this can be seen in the mentour video demonstration almost like they were bouncing backwards.
The cables do act as a spring so hard to say if this was enough to actually move the stab.
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Old 4th Jun 2019, 11:40
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Originally Posted by Fly Aiprt
This is an interesting question.
1) It appears that the manufacturer's document are not clear about the aircraft actual control forces.
2) In any hydraulic actuator/cylinder, the force is proportional to the fluid pressure acting upon the piston part of the actuator.
For the control force to be only doubled when pressure is actually fourfold, a device must be provided that reduces the force from the actuator to the control column.
As the 737 is a mechanical airplane, such a device - if it exists - should also be a hydro-mechanical unit.

Now what would be the point of increasing hydraulic pressure just to reduce its effect on the control system ?

No doubt some Civil Aviation Agency - and maybe even the FAA - will ask that the actual control forces and torque on the trim wheels be measured before drawing any conclusion as to the plausible scenario of the accidents or the 737 return to service.
I agree that there is conflicting information in the tech manuals, and I've asked our fleet managers to seek a correction. That being said, it is entirely possible that the force reduction is simply a consequence of the mechanical linkages (i.e. levers, gear or pulley reduction, etc) built into the system and is thus transparent to the pilots. I have been told that the pull force required on the control column should never exceed 50 lbs. Hopefully the accident investigation boards will include this issue in their investigations.
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