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737 Stuck Manual Trim Technique

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737 Stuck Manual Trim Technique

Old 24th May 2019, 16:14
  #101 (permalink)  
 
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The natural instinct of a pilot facing an uncommanded descent is to apply as much aft control column movement as he/she can manage. But if high control column forces are applied in this situation, is there a risk that this would compromise the ability of applying nose-up trim using the manual trim system? Whereas going against instinct and slightly relaxing control column forces might permit the manual trim system to be used?

I once flew an airliner type which used aileron and spoiler for roll control. If either system jammed, it was possible to unlock the control column in the roll sense, so that whichever system hadn't jammed could still be used - aileron only from the left seat or spoiler only from the right seat. However, if the aircraft was rolling when the control jam was experienced, opposing it with the control column (the natural reaction) could prevent the 'unlock' system being used. It once happened to me during flight; fortunately the procedure was well-known and practised quite often in the simulator. Landing using spoiler only was quite difficult though, as the spoilers only operated when the control column was significantly displaced - giving a sort of 'sloppy neutral' feel.
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Old 24th May 2019, 17:49
  #102 (permalink)  
 
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Has it been explained why Boeing changed the cut-out switches to Pri and B/U? It seems like a lot of our problems would be solved if we could get the electric "manual" trim back without re-enabling MCAS, etc.
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Old 24th May 2019, 23:08
  #103 (permalink)  
 
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Originally Posted by ImbracableCrunk View Post
Has it been explained why Boeing changed the cut-out switches to Pri and B/U? It seems like a lot of our problems would be solved if we could get the electric "manual" trim back without re-enabling MCAS, etc.
In all current abnormal/emergency procedures affecting trim, you switch both off (don't ask me why), so changing them to primary and backup seemed like a good thing (not to me)....
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Old 24th May 2019, 23:45
  #104 (permalink)  
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This indicates that the horizontal stab tail areas have increased in latter variants of the 737, approx 29 up to 33 sq m.
What effect might this have on the ability to move the stab with the manual trim wheel.
My question on one of the other threads might fit well here. 47' 1" and 33 sq m is a vast flying surface to screw from the wrong side. It's so instinctively wrong that I assume there must be a good reason it's hinged at the rear.

It's not good enough to cry that it's been flying for so long it must be okay. There's Toronto for a start, and if anything spells how difficult it is to unload the surface, that flight spells it out.

MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures
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Old 25th May 2019, 19:05
  #105 (permalink)  
 
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Loose rivets,
‘I assume there must be a good reason it's hinged at the rear.’
A logical view, but the reason probably lost in the 707 records, and a situation where today’s 737 engineers wish it wasn’t so.

Re; tail area increase.
From an aerodynamic view, the increased area was required for the longer, heavier aircraft, more thrust, etc. The forces might be higher, but the overall balance of forces was maintained.
From an engineering view, if the balance was disturbed with significant trim displacement, then the individual higher forces on components could be a problem (hinge moments).

The classic 737 had a problem with trim runaway; it was difficult to trim the horizontal stab with opposing elevator applied. The force imbalance was resolved by unloading the combined ‘stab - elevator’ with a roller coaster manoeuvre, the trim wheel being moved when the elevator was inline with the tail - reduced elevator reaction. This was the basis of the original certification and the forces required to balance the smaller aircraft.

The larger aircraft have increased stab area, the forces are still balanced, but individually could be higher. The need for the trim runaway manoeuvre dipped below the training horizon, possible with a self satisfied ‘it hasn’t happened yet’ (but still the same risk).
But the risk might not be the same.

The trim runaway drill requires pilot recognition and intervention to electrically isolate the trim, and then recover normal trimmed flight using the trim wheel. If the forces involved are higher than the classic, then more nose up pitching moment might not be available due to revised aerodynamic, mechanical, or physical limits, i.e. unable to overcome the higher stab nose-down pitching moment, elevator hydraulics suffer jack stall, or pilot strength. If so then the trim runaway manoeuvre might not be practical.
The crew’s involvement could reduce this problem by holding the trim wheel and with the speed of isolating the trim before reaching the limit condition - not necessarily the stab limit, but the point where the elevator is ineffective. Thus the basis of certification did not change, but the assumptions within the trim runaway drill did. Apparently these were not overtly recognised, but recall the EASA query about the full range of trim in the NG, this also introduced speed; higher speeds higher forces.

The Max has different aerodynamics / thrust, etc, (need for MCAS), so again whilst there was a change in force, but the balance was maintained. This could imply that the trim runaway recovery limit had less margin in crew intervention - time for recognition and action - ineffective elevator for less trim deviation.

The lack of factual evidence from incidents in the NG does not identify any change from the classic runaway drill, but the simulator training tests do (video).
Regrettably, evidence from the Max accidents appears to confirm these difficulties, which could be greater than in the NG (note Max simulator inaccuracies), particularly the effect of speed.

If the suppositions above have value, then the trim runaway drill in the Max requires urgent crew intervention - recognition and isolation, and simultaneous control of speed, and possibly altitude constraint.
Thus the revisited certification question is if the crew participation is realistic for a trim runaway in the Max; a change from the NG.
With AoA input to MCAS clearly it was not, but MCAS will be modified eliminating that specific failure; but the trim runaway condition remains.

1: Yes the human will manage with yet more training, recognise and act quickly. (Currency, memory recall, surprise, experience)

or

2: The accidents identify the realistic expectation of crew intervention for trim runway - time to recognise, time to act, startle, range of piloting experience. (Irrespective of MCAS / AoA distractions)

A judgement; but who judges ?

Last edited by PEI_3721; 26th May 2019 at 12:14. Reason: typo
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Old 25th May 2019, 22:45
  #106 (permalink)  
 
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Old 25th May 2019, 23:06
  #107 (permalink)  
 
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Originally Posted by PEI_3721 View Post
Loose rivets,
‘I assume there must be a good reason it's hinged at the rear.’
A logical view, but the reason probably lost in the 707 records, and a situation where today’s 737 engineers wish it wasn’t so.

Re; tail area increase.
From an aerodynamic view, the increased area was required for the longer, heavier aircraft, more thrust, etc. The forces might be higher, but the overall balance of forces was maintained.
From an engineering view, if the balance was disturbed with significant trim displacement, then the individual higher forces on components could be a problem (hinge moments).

The classic 737 had a problem with trim runaway; it was difficult to trim the horizontal stab with opposing elevator applied. The force imbalance was resolved by unloading the combined ‘stab - elevator’ with a roller coaster manoeuvre, the trim wheel being moved when the elevator was inline with the tail - reduced elevator reaction. This was the basis of the original certification and the forces required to balance the smaller aircraft.

The larger aircraft have increased stab area, the forces are still balanced, but individually could be higher. The need for the trim runaway manoeuvre dipped below the training horizon, possible with a self satisfied ‘it hasn’t happened yet’ (but still the same risk).
But the risk might not be the same.

The trim runaway drill requires pilot recognition and intervention to electrically isolate the trim, and then recover normal trimmed flight using the trim wheel. If the forces involved are higher than the classic, then more nose up pitching moment might not be available due to revised aerodynamic, mechanical, or physical limits, i.e. unable to overcome the higher stab nose-down pitching moment, elevator hydraulics suffer jack stall, or pilot strength. If so then the trim runaway manoeuvre might not be practical.
The crew’s involvement could reduce this problem by holding the trim wheel and with the speed of isolating the trim before reaching the limit condition - not necessarily the stab limit, but the point where the elevator is ineffective. Thus the basis of certification did not change, but the assumptions within the trim runaway drill did. Apparently these were not overtly recognised, but recall the EASA query about the full range of trim in the NG, this also introduced speed; higher speeds higher forces.

The Max has different aerodynamics / thrust, etc, (need for MCAS), so again whilst there was a change in force, but the balance was maintained. This could imply that the trim runaway recovery limit had less margin in crew intervention - time for recognition and action - ineffective elevator for less trim deviation.

The lack of factual evidence from incidents in the NG does not identify any change from the classic runaway drill, but the simulator training tests do (video).
Regrettably, evidence from the Max accidents appears to confirm these difficulties, which could be greater than in the NG (note Max simulator inaccuracies), particularly the effect of speed.

If the suppositions above have value, then the trim runaway drill in the Max requires urgent crew intervention - recognition and isolation, and simultaneous control of speed, and possibly altitude constraint.
Thus the revisited certification question is if the crew participation is realistic for a trim runaway in the Max; a change from the NG.
With AoA input to MCAS clearly it was not, but MCAS will be modified eliminating that specific failure; but the trim runaway condition remains.

1: Yes the human will manage with yet more training, recognise and act quickly. (Currency, memory recall, surprise, experience)

or

2: The accidents identifies a realistic expectation of crew intervention for trim runway - time to recognise, time to act, startle, range of piloting experience. (Irrespective of MCAS / AoA distractions)

A judgement; but who judges ?
Remember the stab on the 707 was much larger than the 737 and the elevator was manually powered- I.e. not powered! 737 is hydraulic. Yet using the correct technique the 707 could be flown safely with runaway stab. Jammed stab. Any which way stab. But we all knew how to fly it and knew that stopping the runaway was paramount.
So the 737with it’s powered elevator and smaller stab, and the fact that you can trim back electrically BEFORE switching off electrical power means it shouldn’t really be that hard. Why the guys on the last two fatal accidents didn’t will be revealed but I’m betting on lack of training leading to lack of basic flying skills, system knowledge and airmanship
cheers
y
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Old 25th May 2019, 23:34
  #108 (permalink)  
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Professor Simon mentions 8,000 feet for the simulator crew to action the Roller Coaster recovery, but remind me, just how much height did the ET flight have before it started ingesting rocks? Remember, the jackscrew had gone a long way soon after the autopilot disconnect.

Is much known about that particular vested interest issue? Frankly, I became hostile to that quasi-politition after his first public display.

We've discussed before the ability of a simulator to truly represent such a set of scenarios.

And, the shock factor, along with general psychological factors. And of course, the bazillion other arguments. I'm still with the 'could have done better but huge mitigation' camp.
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Old 25th May 2019, 23:42
  #109 (permalink)  
 
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Originally Posted by Loose rivets View Post
Professor Simon mentions 8,000 feet for the simulator crew to action the Roller Coaster recovery, but remind me, just how much height did the ET flight have before it started ingesting rocks?
The more relevant question is how far could the pilot have driven the stab with his yoke switch before they cut out power to the trim motor? Get the stab in the right place first, and no yo-yo maneuver required. The first Lion Air 610 crew and the Captain of the second Lion Air 610 had no problem with opposing the MCAS input with trim. It was only the second Lion Air flight First Officer and the Ethiopian Captain who did not seem to understand that they could still trim with the yoke switch.
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Old 26th May 2019, 04:11
  #110 (permalink)  
 
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Originally Posted by Takwis View Post
Well, they both "blipped" it, and we still don't know why...was it functional? Was it jammed? Or had they forgotten how to trim, as so many have opined?
If you are referring to the Ethiopian flight, there are plenty of examples from the FDR output that shows a pilot trim input followed by stab movement - there just weren’t enough pilot inputs to counter the MCAS inputs. There are too interesting “blips” at the end. However, by this time the aircraft had exceeded Vmo and there is no guarantee that anything is going to work as it is supposed to once outside the certified envelope.

I think it is much more informative to look at both Lion Air flights. On the first flight, both pilots (there was a transfer of aircraft control) successfully countered the MCAS input with the yoke trim switch until they turned off the stab cutout switches. On the second Lion Air flight (the accident aircraft), the Captain flew for something like 7-8 minutes and maintained aircraft control. At some point he transferred control to the First Officer, and I would suggest that this is the “inflection point” we see in the FDR output where the pilot inputs no longer keep up with the MCAS input. The Captain was likely heads down in the QRH at this point and didn’t realize his First Officer was losing the battle with MCAS.

As I pointed out previously, the evidence strongly indicates that 3 of the 5 aviators who acted as the flying pilot with a malfunctioning MCAS were actually able to keep the plane flying. Unfortunately, two of them were not. Given more time to work the problem, I think it is reasonable to ask whether either of the accident crews would have eventually gotten to the same solution as the first Lion Air crew.
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Old 26th May 2019, 11:54
  #111 (permalink)  

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I asked before: If the crew on MAX is holding the yoke thumb rocker for trim, and the MCAS comes for the second push after the cooling off period, will it override the pilot's elec command? And we do not know.

Boeing tells us while MCAS is active the thumb switch will kill it, and I trust them. The question above is about a priority rule in a different scenario, another mode. - and we do not know, only assume.

The connection here is that we see a blip of ELEC TRIM switch on the traces, but we really have no way of knowing what was the commanded input of the crew on the rockers. Or do we? Having read those statements about AoA disagree not plugged in by mistake, and underperforming pilots being the only problem, I have an axe to grind now.

Come on, you self-certified the ND inputs to 1x 0,6 units and then went to build it with a recurring open loop of 2,5 units?! Thus the possibility of crew's inputs being voted out by the internal logic needs to be investigated. Given what we learned, is it really a lower probability of that logic being FUBAR, than a young, proud enthusiastic captain who flies a MAX 3 weeks after a deadly crash with the solution splashed all over the news and the internet not pressing the rocker switch??
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Old 26th May 2019, 12:02
  #112 (permalink)  

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For the record, I am on 737 driver's team with respect to the notion that our responsibility is to make sure our side of the street gets cleaned immaculately.

Yet for all we know, the Ethiopean captain may have been here with us in the LionAir thread, even contributing. Until the full truth comes out unpolished, their souls won't rest in peace in fear of such grief and tragedy recurring.
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Old 26th May 2019, 12:32
  #113 (permalink)  
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The more relevant question is how far could the pilot have driven the stab with his yoke switch before they . . .
Yes, but it's not the question I asked. The graph of the series of thumb switch input blips - plural - is one of the main puzzles, that and leaving the power so high, but for the moment, the height above terrain re Professor Simon's Youtube statement is, or was, the issue.
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Old 26th May 2019, 16:22
  #114 (permalink)  
 
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80% of accidents are due to pilot error. Always have been and, apparently, always will be. A mechanical defect might be the initial cause, but in most cases a competent pilot can still safely fly the airplane to a landing. Runaway trim is not limited to the 737 it can happen on any aircraft with electric trim and there are procedures written to correct this fault that any pilot who deserves to be appointed as PIC should know and be able to implement. With the degradation in pilot skills we see happening in every country now, due to the shortage of trained pilots, we should expect more of these types of accidents.
Blaming the aircraft does not help. The ability of a manufacturer to make changes to the design is restricted due to tort pressure/regulation inertia and unlikely to happen, even in this case. If a design is changed there will be multiple court cases saying "See? We told you the design was bad!"
The days when we could expect a normal crew to do a normal job are almost gone. The newbies are not taught as well, are not as experienced when taking the left seat and apparently are guilty of not knowing what they do not know so will continue to fall behind in competence, leading to an increase in pilot error accidents.
It is a different world we are entering. Get used to it.
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Old 26th May 2019, 16:46
  #115 (permalink)  
 
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boofhead,
You might wish to read the links below and consider alternative views of error,
‘… that ‘human error’ should not be used to explain adverse outcomes.
Instead we should try to understand why the same behaviour usually makes things go right and occasionally makes things go wrong.’


https://static1.squarespace.com/stat...ech-Report.pdf

http://www.iploca.com/platform/conte...afetyMyths.pdf
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Old 26th May 2019, 18:21
  #116 (permalink)  
 
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Originally Posted by FlightDetent View Post
I asked before: If the crew on MAX is holding the yoke thumb rocker for trim, and the MCAS comes for the second push after the cooling off period, will it override the pilot's elec command? And we do not know.

Boeing tells us while MCAS is active the thumb switch will kill it, and I trust them. The question above is about a priority rule in a different scenario, another mode. - and we do not know, only assume.

The connection here is that we see a blip of ELEC TRIM switch on the traces, but we really have no way of knowing what was the commanded input of the crew on the rockers. Or do we?
We have at our disposal three separate FDR outputs (2 Lion Air, 1 Ethiopian). In each and every one of these outputs, automatic trim inputs (STS, MCAS) and pilot trim inputs are shown on separate traces. On yet another trace, stab position is shown. This is how we can tell that 1) the pilot trim was working, and 2) that this input stopped and countered MCAS every single time it was used. So yes, we actually do know.

As far as whether the pilot trim was taken directly from the rocker switch on the yoke or somewhere else, I don't know, but I submit that it does not matter. The only way for pilots to input trim through the electric stab motor is through the yoke rocker switch. I know some other aircraft have different setups for this, but not the 737.

Again, what is clear from these outputs is that at least 3 of the 5 pilots who actively controlled the aircraft during a MCAS malfunction were able to hold their own against these unwanted stab inputs. Two did not resulting in subsequent loss of control.

The issue as to why two of these pilots (particularly the Ethiopian Captain) lacked the basic flying skills the other three possessed should certainly be a key area of investigation.
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Old 26th May 2019, 19:25
  #117 (permalink)  

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Thanks for clearing that for me. Those FDR traces, what is their data pick up sensor? I thing the yoke switch itself would be overkill, but if it is only a subroutine in the code ....
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Old 26th May 2019, 20:47
  #118 (permalink)  
 
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Originally Posted by safetypee View Post
boofhead,
You might wish to read the links below and consider alternative views of error,
‘… that ‘human error’ should not be used to explain adverse outcomes.
Instead we should try to understand why the same behaviour usually makes things go right and occasionally makes things go wrong.’


https://static1.squarespace.com/stat...ech-Report.pdf

http://www.iploca.com/platform/conte...afetyMyths.pdf
So it is not relevant why they failed to follow the correct procedures? It is not reasonable to expect a person occupying the left seat of an airliner with hundreds of people in the back relying on him to know what to do for a situation that is included in the emergency procedures checklist and that he has been trained on, to do it correctly?
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Old 26th May 2019, 22:16
  #119 (permalink)  
 
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yanrair, # 107
You appear to have overlooked the main point; perhaps the explanation of the difference between balance and absolute force lacked clarity (# 105 737 Stuck Manual Trim Technique).

All aircraft are in balance, irrespective of size or power controls (a simplistic analogous description of a beam balance *). If a system is disturbed or breaks, the ‘twang’ of the more heavily loaded (force not weight) tends to be more difficult to restore.

You similarly overlook the argument that as the aircraft variants have developed, the time available to recognise and act with trim runaway may have reduced. If so, there are situations where it is unreasonable to expect the crew to achieve the performance required for the severity of outcome. The safety margin is in knowing how and when the situation is sufficiently understood to act.

Aviation should not bet on anything, nor shrink behind terms such as basic skill, knowledge, or airmanship without explanation of what these entail and the context in which they are being used. The misuse of these or the assumptions in them - that every pilot will have the same understanding and ability to act, may well be at the root of the Boeing trim problem.

* https://dictionary.cambridge.org/dic...h/beam-balance NB aerodynamic notes.

P.S. instead of quoting lengthy posts in full, select the relevant section, or link the post as a reference under its # designation. How to do that: - knowledge, basic skill.



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Old 26th May 2019, 23:38
  #120 (permalink)  
 
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.
Originally Posted by boofhead View Post
80% of accidents are due to pilot error. Always have been and, apparently, always will be. A mechanical defect might be the initial cause, but in most cases a competent pilot can still safely fly the airplane to a landing. Runaway trim is not limited to the 737 it can happen on any aircraft with electric trim and there are procedures written to correct this fault that any pilot who deserves to be appointed as PIC should know and be able to implement. With the degradation in pilot skills we see happening in every country now, due to the shortage of trained pilots, we should expect more of these types of accidents.
Blaming the aircraft does not help. The ability of a manufacturer to make changes to the design is restricted due to tort pressure/regulation inertia and unlikely to happen, even in this case. If a design is changed there will be multiple court cases saying "See? We told you the design was bad!"
The days when we could expect a normal crew to do a normal job are almost gone. The newbies are not taught as well, are not as experienced when taking the left seat and apparently are guilty of not knowing what they do not know so will continue to fall behind in competence, leading to an increase in pilot error accidents.
It is a different world we are entering. Get used to it.
We can’t and mustn’t get used to more crashes
Train pilots properly. Two possible outcomes from LION AND ET final reports>>>
1 The MCAS inputs + airspeed issues were totally manageable (even if bad design) with well trained pilots. So cause of crash is poor pilots / training issues, with MCAS a contributor MCAS now fixed. Leaves only world wide pilot training issue - nightmare!
2 MCAS + UAS STICK SHAKERS ETC beyond any normal regular line pilot even from major word class airlines. MCAS Is now fixed. Won’t happen again..sigh of relief. Carry on.
I believe 1. to be the true story.
But.......who will write the reports? National authorities of the affected airlines.
Y
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