Go Back  PPRuNe Forums > Flight Deck Forums > Rumours & News
Reload this Page >

MAXís Return Delayed by FAA Reevaluation of 737 Safety Procedures

Rumours & News Reporting Points that may affect our jobs or lives as professional pilots. Also, items that may be of interest to professional pilots.

MAXís Return Delayed by FAA Reevaluation of 737 Safety Procedures

Old 30th Sep 2019, 14:20
  #2741 (permalink)  
 
Join Date: Jul 2002
Location: Ireland
Posts: 596
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by Tomaski
but somewhere along the way this knowledge fell out of general circulation and with it an appreciation for the seriousness of runaway trim and the need for flight crews to be regularly exposed to this malfunction and the proper steps to counter it.
Agreed, and I suspect that runaway trim will feature more in future training and sim scenarios. However, now that the Pandoraís box of trim wheel operation has been opened, it will have to be addressed. As has been mentioned a number of times on this thread the solution must be one which allows all pilots to effectively use manual trim, not simply the average pilot.

And as you pointed out, this affects all iterations of the NG model as well as the MAX.
Speed of Sound is offline  
Old 30th Sep 2019, 14:34
  #2742 (permalink)  
 
Join Date: Sep 2019
Location: leftcoast
Posts: 2
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by OldnGrounded
Yes, the diameter of the manual trim wheels changed and, yes, it was a long time ago. It's possible that there is some side loading of the jackscrew, but probably not enough to be a major factor, considering the angle it forms with the stab lever arm and the fact that the ball nut is in a gimbal ring. Yes, pilots are no longer taught the porpoise maneuver, but it has no chance of saving the day unless there's sufficient altitude AGL.

Almost certainly, the real problem with manual trimming in the sorts of situations we are considering is just that the H-stab is huge and, at some combinations of attitude, (out-of-) trim positions and airspeed, the aero loads make it either unlikely or impossible to crank those wheels enough to recover.
I did find the post- I wuz wrong- it referred to the NG NOT the MAX

They also put a damper in there as well because there was also a new trim motor.

Quote:
Regarding the trim wheels: When the NG was being introduced, I happened to be the Lead Engineer in charge of them and a whole lot of other stuff. There were some issues. The new display system created a pinch point between the dash and the wheel. We had to make the wheel smaller. And the new trim motor resulted in the wheel, which is directly connected to the stabilizer by a long cable, springing back when electric trim was used. It was an undamped mass on the end of a spring. We had to add a damper.
Result: Depending on the flight conditions, the force to manually trim can be extremely high. We set up a test rig and a very fit female pilot could barely move it.
Grebe is offline  
Old 30th Sep 2019, 14:51
  #2743 (permalink)  
 
Join Date: Jul 2003
Location: An Island Province
Posts: 1,257
Likes: 0
Received 1 Like on 1 Post
To use an analogy, there was a time when the hazards of low-level windshear and microburst phenomenon were not well-appreciated. Now we practice windshear recoveries all the time because prompt recognition and response can be critical to a successful outcome. Why do we not do the same with Runaway Stab Trim? It is a "memory item" after all.’

We might not be able to avoid wind shear / microburst, although we should make every effort to do so (wind shear avoidance training?).
Runaway trim in the 737 Max is currently not avoidable, and in some circumstances unrecoverable, but could be with a modified trim system.

Pilot training requirements are not meant to compensate for non-acceptable design on the compliance and safety standpoint.

https://www.easa.europa.eu/newsroom-...-clarification
alf5071h is offline  
Old 30th Sep 2019, 15:51
  #2744 (permalink)  
 
Join Date: Nov 2007
Location: dublin
Posts: 2
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by OldnGrounded
346 people, including two aircrews led by experienced PICs are dead because, in the real-world, the combination of indications, alarms and HAL-commanded flight control movements actually was confusing -- deadly confusing. An enormous number of ATPs (including real, live Sky Gods like Sully), knowledgeable engineers and others who have paid extremely careful attention to the accident flights and the information that has since come to light have come to the conclusion that the same fate could easily befall any pilots facing the same circumstances. And the world's CAAs have kept the MAX grounded for many months because they have determined that it would be unsafe to let it fly without significant changes to the MCAS system (among other things, not all know to us in detail).

And, despite all of that, we still have people blaming the pilots.
Can I ask a question? How did the jump seat pilot on the flight the day before the fatal Lionair crash know what to do?
it seems to me that the situation was indeed survivable since, well, it landed safely and the captain (and this is not how it should be done) hardly mentioned it in the tech.log.
I think there is a clue here
cheers
y
yanrair is offline  
Old 30th Sep 2019, 15:52
  #2745 (permalink)  
 
Join Date: Jun 2019
Location: VA
Posts: 210
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by alf5071h

We might not be able to avoid wind shear / microburst, although we should make every effort to do so (wind shear avoidance training?).
Runaway trim in the 737 Max is currently not avoidable, and in some circumstances unrecoverable, but could be with a modified trim system.

ďPilot training requirements are not meant to compensate for non-acceptable design on the compliance and safety standpoint.Ē

I suspect what we have a here is a conflict between theory and reality. Yes, it would be wonderful if aircraft were so reliable that prompt pilot recognition and action were not required in response to an aircraft malfunction. That maybe true someday, but not likely anytime in the near future.

The exact methodology will vary from airline to airline (and from aircraft to aircraft), but at my airline on the 737NG we use a Quick Reference Card (QRC) for critical actions (sometimes referred to as "memory items"). This is in addition to the critical maneuver responses (windshear, GPWS, engine-failure, upset & stall recovery, TCAS response) that must be done promptly without reference to a checklist. This QRC currently has thirteen different procedures, each representing one or more possible aircraft malfunctions, only one of which is Runaway Stabilizer. These procedures have been put in place not so much because of the likelihood of any particular failure, but because of the hazards associated with a delayed response. To the extent that these procedures exist, it is an acknowledgement by both the manufacturer and the certificate authority that there is some non-zero risk of a malfunction, and that prompt action is necessary. Please keep in mind that right now I'm only talking about the 737NG.

From my vantage point (and again, circumstances might vary across airlines), one of the key issues is that the necessary training follow-through is lacking. Of the thirteen QRC procedures I referenced above, at least five of them have not been presented to me in the sim since my initial checkout some years ago. One of those malfunctions is Runaway Stabilizer. Do you see the problem here?

As has been previously stated, the key to avoiding the manual trim problem in the 737 is prompt recognition and response. The way to speed up that recognition and response is through regular exposure and practice. I'm all for a redesign of the system, but in the meantime someone needs to take a hard look at whether the the actual training supports the assumptions built into the procedures.
Tomaski is offline  
Old 30th Sep 2019, 16:02
  #2746 (permalink)  
Thread Starter
 
Join Date: Apr 2015
Location: Under the radar, over the rainbow
Posts: 851
Received 26 Likes on 15 Posts
Originally Posted by yanrair
Can I ask a question? How did the jump seat pilot on the flight the day before the fatal Lionair crash know what to do?
Perhaps s/he checked the QRH. Perhaps watching the spinning trim wheels from the jump seat triggered some insight into the situation. As you know, we don't really know with any clarity.

. . . it seems to me that the situation was indeed survivable since, well, it landed safely . . .
So? The fact that one of three crews known to have faced the situation didn't end up smashed to pieces, with all of their passengers, hardly suggests that the situation is adequately "survivable" to justify continuing to permit the aircraft to fly -- and there isn't a CAA on Planet Earth that believes it does.
OldnGrounded is offline  
Old 30th Sep 2019, 16:30
  #2747 (permalink)  
 
Join Date: Nov 2010
Age: 56
Posts: 981
Received 15 Likes on 8 Posts
Originally Posted by tiddles52
lets not forget ET was going out of a hot and high airport at MTOW. Conditions not experienced much in the USA, so any talk of cut thrust, nose down and manage it are very US-centric. The "high" airports in the US don't tend to be hot!

I can think of at least 5 cities with 200K+ people with an elevation over 5000', that will have 35deg C+ temperature over the summer.
hans brinker is offline  
Old 30th Sep 2019, 16:32
  #2748 (permalink)  
 
Join Date: Jul 2002
Location: Ireland
Posts: 596
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by yanrair

it seems to me that the situation was indeed survivable since, well, it landed safely
So did QF32 but Rolls Royce still investigated the reason for the incident and changed out the offending oil pipe.
Speed of Sound is offline  
Old 30th Sep 2019, 17:11
  #2749 (permalink)  
 
Join Date: Nov 2007
Location: dublin
Posts: 2
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by alf5071h
ĎTo use an analogy, there was a time when the hazards of low-level windshear and microburst phenomenon were not well-appreciated. Now we practice windshear recoveries all the time because prompt recognition and response can be critical to a successful outcome. Why do we not do the same with Runaway Stab Trim? It is a "memory item" after all.í

We might not be able to avoid wind shear / microburst, although we should make every effort to do so (wind shear avoidance training?).
Runaway trim in the 737 Max is currently not avoidable, and in some circumstances unrecoverable, but could be with a modified trim system.

ďPilot training requirements are not meant to compensate for non-acceptable design on the compliance and safety standpoint.Ē

https://www.easa.europa.eu/newsroom-...-clarification
Alf
i donít think runaway stab is avoidable on any type? Certainly not on any 737 or even 707 for that matter.
Any electrical input can trigger the motor and if that input, false or otherwise , doesnít stop, the stab keeps running.Manual elec. trim, autopilot trim, Mach trim, STS trim or MCAS trim all have the potential to initiate a runaway. Itís not just a Max issue although MCAS is on max only.
Unrecoverable? Donít think so. Even on the Lionair the previous day it was indeed recovered using the correct procedure,
albeit with the aid of a third party jump seat pilot who clearly DID know what to do. I hope he has illuminated the investigation with his private knowledge.
Is that a fair comment? If not Iím genuinely keen to know why? We seem to have forgotten the one that got away.

And a modified MCAS will reduce any chance of that alpha vane induced failure mode occurring again, but will not remove any other cause.
So I would rather fly with a crew that can handle a runaway stab whether continuous, intermittent but repetitive, any undesired and uncommanded movement of the stab that is not normal and which is starting to upset the aircraft basic trim state. Itís the trim moving in such a way as to cause the plane to deviate from its desired path -uncommanded -which tells you itís misbehaving. At least it was when Pontius was a pilot. And nothing has changed aerodynamics since then.
Cheers
y
yanrair is offline  
Old 30th Sep 2019, 17:16
  #2750 (permalink)  
 
Join Date: Nov 2007
Location: dublin
Posts: 2
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by Speed of Sound


So did QF32 but Rolls Royce still investigated the reason for the incident and changed out the offending oil pipe.
Hi Speed,
not sure what your point is here? Boeing are changing the offending MCAS following an investigation. My point is that the first Lionair landed safely despite a major design fault through the knowledge of a pilot. So did QF32.
Apologies if Iím missing something obvious.
Cheers
y
yanrair is offline  
Old 30th Sep 2019, 17:25
  #2751 (permalink)  
 
Join Date: Nov 2007
Location: dublin
Posts: 2
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by OldnGrounded
Perhaps s/he checked the QRH. Perhaps watching the spinning trim wheels from the jump seat triggered some insight into the situation. As you know, we don't really know with any clarity.



So? The fact that one of three crews known to have faced the situation didn't end up smashed to pieces, with all of their passengers, hardly suggests that the situation is adequately "survivable" to justify continuing to permit the aircraft to fly -- and there isn't a CAA on Planet Earth that believes it does.
Donít think it was insight. Think he had been taught what to do. Nobody could have known what to do unless they had been trained that way. And having travelled for 35 years in jump seats with several major UK/IRL airlines while commuting I can tell you that you donít intervene in what the guys up front are doing unless
1 you think you are about to die
2 you know the correct procedure and for some reason they donít.
i
It takes extraordinary confidence and courage to say to the operating crew ď this is what you need to doĒ
But then thatís just my opinion of course.
Cheers
Y
yanrair is offline  
Old 30th Sep 2019, 18:06
  #2752 (permalink)  
 
Join Date: Jul 2002
Location: Ireland
Posts: 596
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by yanrair

My point is that the first Lionair landed safely despite a major design fault through the knowledge of a pilot. So did QF32.
QF32 landed safely due to some excellent fault analysis, incredible workflow management, cool heads, and a lot of luck

An aircraft should be capable of being operated safely by any crew trained to minimum certified standards, not just exceptional crews.
Speed of Sound is offline  
Old 30th Sep 2019, 18:21
  #2753 (permalink)  
 
Join Date: Jan 2008
Location: Reading, UK
Posts: 15,984
Received 306 Likes on 158 Posts
Originally Posted by yanrair
My point is that the first Lionair landed safely despite a major design fault through the knowledge of a pilot. So did QF32.
Apologies if I’m missing something obvious.

Thereby demonstrating that it's only on a bad day that all the holes line up ... ?
DaveReidUK is offline  
Old 30th Sep 2019, 18:24
  #2754 (permalink)  

Plastic PPRuNer
 
Join Date: Sep 2000
Location: Cape Town
Posts: 1,898
Received 0 Likes on 0 Posts
"Almost certainly, the real problem with manual trimming in the sorts of situations we are considering is just that the H-stab is huge and, at some combinations of attitude, (out-of-) trim positions and airspeed, the aero loads make it either unlikely or impossible to crank those wheels enough to recover."

Then why are they still there? As a panacea? Decoration? Tradition probably....

Mac
Mac the Knife is offline  
Old 30th Sep 2019, 18:40
  #2755 (permalink)  
 
Join Date: Sep 2019
Location: leftcoast
Posts: 2
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by Mac the Knife
"Almost certainly, the real problem with manual trimming in the sorts of situations we are considering is just that the H-stab is huge and, at some combinations of attitude, (out-of-) trim positions and airspeed, the aero loads make it either unlikely or impossible to crank those wheels enough to recover."

Then why are they still there? As a panacea? Decoration? Tradition probably....

Mac
Interesting to note -AFIK the 767 never had similar mechanical trim wheels but did/does have a few cable controls for spoilers etc.
The 767 with an early glass cockpit and redundant sensors-some truly redundant computers and AFIK had electric trim ( but no mechanical backup ? ). Now we add a tanker version ( KC-46) and a MCAS system which uses multiple sensors.

The 767 in all variations has apparently had few HS related problems.

Those who do not learn from history - do not learn from history..

MY guess is that IF they removed the trim wheel ( by ANY method ) as compared to NG, it would require new certification, cost bucu $$$$$, and then raise more questions as to why no RAT, no APU, et

Or in simple terms - faster and cheaper was absolute priority above ALL else.Those grunts who did not agree were encouraged to spend time with family..

Last edited by Grebe; 30th Sep 2019 at 18:49. Reason: comprehension corrections to sentence structure
Grebe is offline  
Old 30th Sep 2019, 19:04
  #2756 (permalink)  
 
Join Date: Feb 2016
Location: S.E.Asia
Posts: 1,961
Received 11 Likes on 5 Posts


If you look at this youtube video below it is all down to slick remarketing and telling the world this aircraft is safe. (all done in 30 seconds!)


Which of course facts have proven it is not. I predicted some time ago the Max will not be certified until 2020. Trying to sell it to airlines and passengers might be a whole new ball game.




Originally Posted by LowObservable
Meanwhile, here is the latest Boeing contribution to the discussion...

https://www.youtube.com/watch?v=2REqWEUPcMk&app=desktop

... complete with the standard piano track that is overused in movies to indicate that someone has The Real Feels.
Mike Flynn is offline  
Old 30th Sep 2019, 19:20
  #2757 (permalink)  
 
Join Date: May 2008
Location: denmark
Posts: 10
Likes: 0
Received 3 Likes on 1 Post
Originally Posted by yanrair
Alf
I donít think runaway stab is avoidable on any type? Certainly not on any 737 or even 707 for that matter.
Any electrical input can trigger the motor and if that input, false or otherwise , doesnít stop, the stab keeps running.Manual elec. trim, autopilot trim, Mach trim, STS trim or MCAS trim all have

This is easy, well known, and is regularly done in the automation industry.. And on real fly by wire aircraft like the Falcon, except that the Falcon had a flawed design/risk analysis :-(
One method is to use an AC motor powered from DC via. an 3 phase DC/AC converter. The DC/AC converter need to perform an accurate PWM modulation to generate an rotating magnetic field. Anny fault like a shorted transistor would generate no field or a DC field blocking the rotation. (and burn a fuse)
The system need at least two redundant control paths with diagnostics checking for agreement between the two channels, this information can be communicated either via. communication protocols, or via. discrete signals.
The two control channels typically disconnect the High and Low side gate drivers, and signal to the DSP/MCU that it should not generate an PWM field.
https://en.wikipedia.org/wiki/Safe_Torque_Off
https://www.sdsdrives.com/downloads/...ick_guides.pdf
Falcon Runaway Trim

The biggest risk is if the designers fail to properly analyze all failure modes..
If we compare the B737 MAX with the Falcon, then both aircraft had incorrect trim movement.
In the case of the Falcon it was a matter of fixing and re-qualifying some HW and SW since the architecture of the system was sound.
In the case if the B737 MAX it is much more difficult.
HighWind is offline  
Old 30th Sep 2019, 20:32
  #2758 (permalink)  
 
Join Date: Dec 2014
Location: Somerset
Posts: 40
Received 0 Likes on 0 Posts
Originally Posted by MemberBerry
From what I understand the system is indeed designed so that holding the trim wheels while the electric motor is adjusting the trim will disengage the electric motor, via a clutch mechanism. But, if you don't touch the trim wheels, that clutch should remain engaged and electric trim should work fine, in both directions, no matter how out of trim the stab is.

Also, no matter how low friction that ball nut is, it seems what's preventing the use of the trim wheels when severely out of trim is somehow friction related. For example in Mentour's video it was getting harder and harder to trim AND the more they trimmed AND. At some point, at around 2.5 units from full AND, it became impossible to trim more AND. Assuming that was not a quirk of the simulator, this indicates that manual trimming in both directions is affected when the stabilizer is out of trim, both AND and ANU changes become harder.
I note your reply on the clutch thanks. My logic for my description of the clutch /trim wheel / stab motor went like this:-

If holding the trim wheel against rotation from the stab motor stops HS movement then that is the torque at which the clutch disengages - this will be the maximum torque the stab motor can exert.

I think there is agreement that in a severe out of trim condition the trim wheels can't be used to move the HS even when a very large force is applied to the handles. Given that this force must be greater than that above (to just hold the trim wheel) doesn't that mean that the torque being applied (which is not enough to turn the jackscrew) is then greater than that at which the clutch disengages. If this is the case then the stab motor will cause its clutch to disengage if it tries to drive the jackscrew. What do you think?

As I conceded previously all these arguments depend on actual values of the forces involved which we do not know - to some extent they may also depend on the design and operation of the clutch ( breakout torque, slipping torque, reset torque etc).
Cheers
Europa01 is offline  
Old 30th Sep 2019, 20:48
  #2759 (permalink)  
 
Join Date: Sep 2019
Location: leftcoast
Posts: 2
Likes: 0
Received 0 Likes on 0 Posts
Also, no matter how low friction that ball nut is, it seems what's preventing the use of the trim wheels when severely out of trim is somehow friction related. For example in Mentour's video it was getting harder and harder to trim AND the more they trimmed AND. At some point, at around 2.5 units from full AND, it became impossible to trim more AND. Assuming that was not a quirk of the simulator, this indicates that manual trimming in both directions is affected when the stabilizer is out of trim, both AND and ANU changes become harder.
I think (member berry ) meant to say --Mentour's video it was getting harder **** and harder to trim ANU *** since the 'problem' to correct was severe AND made so by simulating ( MCAS ) which trimmed AND at a faster rate than yoke switch trims ANU. And of course maybe the same problem with the opposite ANU runaway ?
Grebe is offline  
Old 30th Sep 2019, 21:14
  #2760 (permalink)  
 
Join Date: Jan 2008
Location: Reading, UK
Posts: 15,984
Received 306 Likes on 158 Posts
Originally Posted by MemberBerry
Also, no matter how low friction that ball nut is, it seems what's preventing the use of the trim wheels when severely out of trim is somehow friction related.
And not the aerodynamic load on the stab ?

Hmmm.

DaveReidUK is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.