Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Boeing advice on "aerodynamically relieving airloads" using manual stabilizer trim

Tech Log The very best in practical technical discussion on the web

Boeing advice on "aerodynamically relieving airloads" using manual stabilizer trim

Old 8th Apr 2019, 02:29
  #41 (permalink)  
 
Join Date: Jun 2002
Location: CYYC (Calgary)
Posts: 4,802
737 Driver,

Here's the link you referred to:

https://leehamnews.com/2019/04/05/bj...irst-analysis/
India Four Two is offline  
Old 8th Apr 2019, 11:03
  #42 (permalink)  
 
Join Date: Jan 2008
Location: uk
Posts: 752
Originally Posted by Derfred View Post
Great point PJ.

In addition to awareness of the number of manual turns required per unit of trim, I noted with interest in one of Mentour’s YouTube videos the difference in ability between him and the RHS pilot when it came to rotating the wheel.

He did it easily and rapidly while simultaneously pulling control column back pressure. But the RHS pilot had considerable difficulty.

There is obviously “technique” involved, which may come with practice.

Manual trimming is not regularly practiced in my airline... I think I’ve tried it once in the type rating.
As an engineer I would note that unless two hands are used (which was tried in the now deleted video) the LHS pilot is going to be using their right hand while the RHS pilot is using their left - if both pilots are right handed, then LHS will find it easier.

However, in the Mentour video I think you are referring to, there is probably another factor at work:
- when they cut the switches they are out of trim, so it's hard to turn due to airload
- it becomes easier when both pilots join in
- as they get back in trim you can see it moves more easily
- when Mentour is trimming on his own it is after they have got back in trim and in fact he says "just minor adjustments"
- this is easily done because they are back in trim (or nearly)

But like I say, that's an engineering take on it, pilot opinions may vary.
infrequentflyer789 is offline  
Old 8th Apr 2019, 11:20
  #43 (permalink)  
 
Join Date: Jun 2006
Location: Brisbane
Posts: 891
789, thanks. I don’t have a link to the video I was recalling - I’ll have to search for it again. You might be right.

The aspect I was considering was that Mentour is obviously a Sim Trainer who may have had the opportunity for extensive practice at manual trimming in the Sim, whilst the rest of us don’t.

I’ll try to find the video again.

EDIT: Found it. This is the video I was looking at:

It’s positively irritating for pilots as these videos are obviously made for non-pilots.

So FFW to 19:00 minutes

It’s obviously not set up for a highly elevator loaded stab, it’s just a runaway stab simulation in which the stab is not really even running away for long.

I just thought the F/O vs CPT technique interesting. But you are right, when the Capt joins in, they are doing it together.

Cheers

Last edited by Derfred; 8th Apr 2019 at 11:36.
Derfred is offline  
Old 8th Apr 2019, 19:08
  #44 (permalink)  
 
Join Date: Feb 2007
Location: Edinburgh
Age: 60
Posts: 36
I’m not sure if it helps your very interesting discussions, but I had to be on an old 737-400 as well as a Max today. The trim wheel on the 400 was 10 inches in diameter, the Max was 9 inches. I tried the Max, and yes, it took approx 15 turns of the wheel per unit of trim.
As I said, hope it helps. Cheers.
clarkieboy is online now  
Old 9th Apr 2019, 02:17
  #45 (permalink)  
 
Join Date: Oct 2006
Location: N/a
Posts: 51
The B737stab trim system has two speed modes for the autopilot and main electric trim, I.e. high rate trimming with flap extended and low rate trimming with flap up.

I am wondering if the MCAS was programmed to always input high rate trimming? Hence if you are flying a MAX with flap up and the MCAS puts in a high rate AND burst for 10sec, then the pilots available ANU counter response on the main electric trim will only be at the low rate.
Artisan is offline  
Old 9th Apr 2019, 06:59
  #46 (permalink)  
 
Join Date: Jan 2008
Location: Reading, UK
Posts: 10,507
Originally Posted by Artisan View Post
The B737stab trim system has two speed modes for the autopilot and main electric trim, I.e. high rate trimming with flap extended and low rate trimming with flap up.

I am wondering if the MCAS was programmed to always input high rate trimming? Hence if you are flying a MAX with flap up and the MCAS puts in a high rate AND burst for 10sec, then the pilots available ANU counter response on the main electric trim will only be at the low rate.
Looking at the FDR trace, the longest application of ANU trim (about 9-10 seconds) appears to have moved the stab about two units. That's almost, but not quite, the rate that MCAS had been moving it in the opposite direction.
DaveReidUK is offline  
Old 9th Apr 2019, 19:18
  #47 (permalink)  
 
Join Date: Mar 2014
Location: Toronto
Age: 64
Posts: 37
From (only) a private pilot. I find prose descriptions of technical systems hard to follow, and prefer diagrams, schematics and flowcharts. I still do not fully understand the pitch control on 737 NGs and MAXs. The following diagram helped me, but likely contains errors and misunderstandings. Comments would be appreciated.

1. Does the autopilot operating in "normal" flight mode (excluding its STS and Mach Trim trimming) move the elevators or the stabilizer?

2. Does flipping the CUTOUT switch terminate the autopilot's control of the elevators or just its control of the stabilizaer jackscrew?

3. I have read about a "hidden" switch somewhere on the back of the center console that did something on the NG, but has been removed on the MAX. What is it?

4. I have read that the First Officer's trim switch is actually a pair of switches. If true, do the separate switches do different things?

5. I have read that the MAX has an automatic Elevator Jam Landing Assist system which manipulates the spoilers to assist in pitch control. The EJLA looks after things when the flaps are deployed; the MCAS looks after things when they're not. Hmmm. I wonder if there is an unintended software interaction?
Attached Files
File Type: pdf
737 pitch control.pdf (42.9 KB, 33 views)
YYZjim is offline  
Old 10th Apr 2019, 08:07
  #48 (permalink)  
 
Join Date: Dec 2015
Location: Cape Town, ZA
Age: 57
Posts: 346
Originally Posted by YYZjim View Post
From (only) a private pilot. I find prose descriptions of technical systems hard to follow, and prefer diagrams, schematics and flowcharts. I still do not fully understand the pitch control on 737 NGs and MAXs. The following diagram helped me, but likely contains errors and misunderstandings. Comments would be appreciated.

1. Does the autopilot operating in "normal" flight mode (excluding its STS and Mach Trim trimming) move the elevators or the stabilizer?

2. Does flipping the CUTOUT switch terminate the autopilot's control of the elevators or just its control of the stabilizaer jackscrew?

3. I have read about a "hidden" switch somewhere on the back of the center console that did something on the NG, but has been removed on the MAX. What is it?

4. I have read that the First Officer's trim switch is actually a pair of switches. If true, do the separate switches do different things?

5. I have read that the MAX has an automatic Elevator Jam Landing Assist system which manipulates the spoilers to assist in pitch control. The EJLA looks after things when the flaps are deployed; the MCAS looks after things when they're not. Hmmm. I wonder if there is an unintended software interaction?
We need a FAQ for this topic. I am not a pilot, but these are what I have read.
1. The autopilot can move any control it likes, within limits.
2. The trim cutoff switch only controls the horizontal stabiliser, not any other flight controls.
3. There is an underfloor cutoff switch at the bottom of the control column. It is still there, but its function changed on the MAX.
4. Each pilot's electric trim consists of two thumb-switches mounted close together. For each pilot, both have to be pressed for any action to occur.
5. No answer.
GordonR_Cape is offline  
Old 11th Apr 2019, 06:43
  #49 (permalink)  
 
Join Date: Dec 2013
Location: Weltschmerz-By-The-Sea
Posts: 651
This system description is from the 737NG FCOM.

It explains the control column (under-floor) cutout switches which have been modified in the MAX to prevent them from disabling MCAS stab trim movement. The “hidden switch” on the centre console (actually called the aisle stand) is in plain view. Its a black cover-guarded switch called the STAB TRIM OVERRIDE that will override the underfloor cutout switch. It is there to restore normal trim operation if one of those switches has failed. I don’t know if it is still fitted to the Max, but that would be moot in any event.


Stabilizer Trim​
Stabilizer trim switches on each control wheel actuate the electric trim motor ​
through the main electric stabilizer trim​ circuit when the airplane is​ flown ​
manually. With the autopilot engaged, stabilizer trim is accomplished through the ​
autopilot stabilizer trim​ circuit. The main electric and autopilot stabilizer trim ​
have two speed​ modes:​ high speed with flaps extended​ and low speed with flaps ​
retracted. If the autopilot is​ engaged, actuating either pair of stabilizer trim ​
switches automatically​ disengages the autopilot. The stabilizer​ trim​ wheels rotate ​
whenever electric stabilizer​ trim​ is​ actuated.​
The STAB TRIM MAIN​ ELECT cutout​ switch and the STAB TRIM ​
AUTOPILOT cutout switch, located on the control stand, are provided to allow ​
the autopilot or​ main​ electric trim​ inputs​ to be disconnected from the stabilizer ​
trim motor.​ ​
Control column actuated stabilizer trim cutout switches stop operation of the main ​
electric and autopilot trim when the control column movement opposes trim ​
direction. When the STAB TRIM override switch is​ positioned to​ OVERRIDE, ​
electric trim can be used regardless of control column position.​
Manual stabilizer control is accomplished through cables which allow the pilot to ​
position​ the stabilizer​ by rotating​ the stabilizer​ trim wheels. The stabilizer​ is held​ ​
in position by two​ independent​ brake systems. Manual​ rotation​ of the trim​ wheels ​
can be used to override autopilot or main​ electric trim. The effort required to ​
manually rotate the stabilizer trim wheels may be higher under certain flight ​
conditions. Grasping the stabilizer trim wheel will stop stabilizer motion
Australopithecus is online now  
Old 13th May 2019, 09:31
  #50 (permalink)  
 
Join Date: Oct 2007
Location: Here and there
Posts: 321
[COLOR=left=#000000]Grasping the stabilizer trim wheel will stop stabilizer motion.
I believe this action is only if the stab trim wheel keeps on "coasting" - that is turning slowly under aerodynamic forces if power is removed. Try stopping an electrical runaway stab trim with bare hands and you risk injury. [/COLOR]

It seems to me that regardless of the two accidents involving EMCAS the Boeing QRH procedure to counter a runaway stabilizer trim has not changed significantly from back in the days where the 737-200 began operating.
The key is prompt recognition that "something" has caused an un-commanded stab trim operation - whether up or down. The pilot action should be to immediately disconnect electrical power to the stab trim before it moves any further. That is by selecting both stab trim cutout switches to Cutout. Providing there is no delay the stab trim will normally be stopped from moving any further. In turn, operation of the manual trim should be controllable without significant effort.

It is only if the stab trim is allowed to run to its limits (in either direction) because of slow pilot reaction, will the situation become serious. Even then, use of the roller coaster method to relieve aerodynamic forces, is an emergency fall-back technique and assumes the crew are aware of this technique as an essential part of their training. Clearly excessive airspeed needs to be avoided if the roller coaster method is to be successful in regaining manual control of the stabilizer trim position.
The point being made is IMHO Boeing are entitled to assume the crew are technically competent when faced with the instructions contained in the QRH. We know from reading accident reports this is not always true..
It is this writers experience that very few pilots are aware, for example, that rolling the aircraft towards the nearest horizon may be necessary in order to get the nose to drop in event of an extreme nose up wings level unusual attitude. This should be part of training for the PPL and is elementary airmanship. Certainly it should be taught during unusual attitude recoveries as part of instrument flying. It is even described in airline FCTM of some types - including the Boeing series.
The ever increasing accent towards more automation has left basic airmanship in its wake.

Last edited by Judd; 13th May 2019 at 13:23.
Judd is offline  
Old 13th May 2019, 12:15
  #51 (permalink)  
 
Join Date: Nov 1999
Location: UK
Posts: 1,133
How can it be that the THS trim motor can be stalled by elevator input? Wouldn’t you make the motor strong enough to overcome the aerodynamic forces caused by full opposite elevators?

We were not told about this, or the yo-yo recovery, and I did a 737 type rating last year.

Uplinker is offline  
Old 13th May 2019, 12:22
  #52 (permalink)  
 
Join Date: Mar 2013
Location: Doha
Posts: 23
There is an article in Aviation Week dated 10/5 titled "Ethiopian MAX Crash Simulator Scenario Stuns Pilots" which talks about testing this situation in a simulator and the problems encountered.
HowardB is online now  
Old 13th May 2019, 12:52
  #53 (permalink)  
 
Join Date: Dec 2002
Location: UK
Posts: 1,832
HowardB,

Link -
https://aviationweek.com/commercial-...b3175d28a7b54a

Highlights for the non registered (free).

‘A simulator session flown by a U.S.-based Boeing 737 MAX crew that mimicked a key portion of the Flight 302 accident sequence suggests that the crew faced a near-impossible task of getting their 737 MAX back under control, and underscores the importance of pilots understanding severe runaway trim recovery procedures.’

‘What the U.S. crew found -. Keeping the aircraft level required significant aft-column pressure by the captain, and aerodynamic forces prevented the first officer from moving the trim wheel a full turn.
They resorted to a little-known procedure to regain control.’ (YoYo Roller Coaster)

The excessive descent rates during the first two steps meant the crew got as low as 2,000 ft. during the recovery.

The article continues discussing aspects of the recent accidents, worthy of inclusion in previous (closed) threads; failing that - register with AvWk for info.

Concluding:-
‘The simulator session underscored the importance of reacting quickly to uncommanded stabilizer movements and avoiding a severe out-of-trim condition, one of the pilots involved said. “I donʼt think the situation would be survivable at 350 kt. and below 5,000 ft,” this pilot noted.’

“This is the sort of simulator experience airline crews need to gain an understanding of how runaway trim can make the aircraft very difficult to control, and how important it is to rehearse use of manual trim inputs,”

safetypee is offline  
Old 14th May 2019, 11:46
  #54 (permalink)  
 
Join Date: Jan 2008
Location: Reading, UK
Posts: 10,507
Originally Posted by Judd View Post
I believe this action is only if the stab trim wheel keeps on "coasting" - that is turning slowly under aerodynamic forces if power is removed.
My understanding is that the trim wheel moves if, and only if, the motor/screwjack/stab are moving. If those are moving, it won't be due to aerodynamic forces.

Try stopping an electrical runaway stab trim with bare hands and you risk injury.
Previous posts suggest that it's doable provided that you grab it firmly enough. Same principle as grabbing a spinning bicycle wheel - do it half-heartedly and your skin will suffer. Bear in mind that you're not actually trying to overcome the trim motor, as that won't attempt to drive the stab if resistance is detected at the cable drum.



DaveReidUK is offline  
Old 18th May 2019, 23:05
  #55 (permalink)  
 
Join Date: Mar 2019
Location: On the Ground
Posts: 24
Originally Posted by maxximizer View Post


Foot works better.


I find it hard to "Grasp and Hold" with my foot.

Previous posts suggest that it's doable provided that you grab it firmly enough. Same principle as grabbing a spinning bicycle wheel - do it half-heartedly and your skin will suffer. Bear in mind that you're not actually trying to overcome the trim motor, as that won't attempt to drive the stab if resistance is detected at the cable drum.
To Grab (or "Grasp") implies encircling something. The edge of the wheel is less than a quarter inch thick. You can grasp it, but if your thumb gets a tiny bit under that edge, when the handle housing comes around (1/2 a second or so), you're going to really wish you hadn't. More like grabbing the tire and getting your thumb down between the spokes. Except with less give.
Takwis is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.