B737 stabilizer trim brake failure
Thread Starter
Joined: Aug 2011
Posts: 43
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From: Tokyo (ENRI)
B737 stabilizer trim brake failure
Hi All,
I have a technical question about the B737(NG) stabilizer trim system.
Assuming the two brakes on the stabilizer trim actuator assembly fail (to engage), now the stabilizer is free to backdrive the jackscrew and change its position based on aerodynamic loads, i.e., the stabilizer can float freely between the stops. Is it in such a situation possible to simply grab the trim wheel and hold it in order to prevent a stabilizer trim runaway? Or are the forces way too high to do so (breaking thumbs).
Also, if those brakes fail, is it an option to leave the AP on and let it control the stabilizer by means of the electric stabilizer actuator system? I guess this would lead to a continuous trim input from the AP, which may be prevented by the system.
Thank you
I have a technical question about the B737(NG) stabilizer trim system.
Assuming the two brakes on the stabilizer trim actuator assembly fail (to engage), now the stabilizer is free to backdrive the jackscrew and change its position based on aerodynamic loads, i.e., the stabilizer can float freely between the stops. Is it in such a situation possible to simply grab the trim wheel and hold it in order to prevent a stabilizer trim runaway? Or are the forces way too high to do so (breaking thumbs).
Also, if those brakes fail, is it an option to leave the AP on and let it control the stabilizer by means of the electric stabilizer actuator system? I guess this would lead to a continuous trim input from the AP, which may be prevented by the system.
Thank you
Joined: Oct 2006
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From: berkshire
stab runaway
if similar to B707, then on BA B707s there was a hand operated friction brake on the left that put a pad in contact with the rim of the hand trim wheels to slow down and jam the trim in position. also F/E could grab hold of the spinning wheels.
Joined: Oct 2008
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From: Above & Beyond
B737 Runaway stab trim memory items
1 Control column. . . . . . . . . . . . . . . . . Hold firmly
2 Autopilot (if engaged) . . . . . . . . . . . . .Disengage
Do not re-engage the autopilot.
Control airplane pitch attitude manually with
control column and main electric trim as
needed.
3 If the runaway stops:
■ ■ ■ ■
4 If the runaway continues:
STAB TRIM CUTOUT
switches (both) . . . . . . . . . . . . . . . . CUTOUT
If the runaway continues:
Stabilizer
trim wheel . . . . . . . . . . Grasp and hold
- - - - - - - - - - - - - - - - - - - - - - -
5 Stabilizer . . . . . . . . . . . . . . . . . . . Trim manually
6 Anticipate trim requirements.
2 Autopilot (if engaged) . . . . . . . . . . . . .Disengage
Do not re-engage the autopilot.
Control airplane pitch attitude manually with
control column and main electric trim as
needed.
3 If the runaway stops:
■ ■ ■ ■
4 If the runaway continues:
STAB TRIM CUTOUT
switches (both) . . . . . . . . . . . . . . . . CUTOUT
If the runaway continues:
Stabilizer
trim wheel . . . . . . . . . . Grasp and hold
- - - - - - - - - - - - - - - - - - - - - - -
5 Stabilizer . . . . . . . . . . . . . . . . . . . Trim manually
6 Anticipate trim requirements.
Thread Starter
Joined: Aug 2011
Posts: 43
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From: Tokyo (ENRI)
Thanks!
I am also wondering which position the stabilizer will go to in case you let go. In general the stabilizer provides a nose up moment (lift vector down) to counteract the pitch-down tendency of the whole aircraft due to the total lift working on a point aft of the ac CG.
But, I would say that it depends on the position of the center of pressure (CoP_stab) on the stabilizer with respect to its hinge point where it will move to. Assuming that the CoP_stab is aft of the hinge point, the stabilizer will move up (if you let go of it), reducing the negative lift force generated, causing the aircraft to pitch down. That in turn may be causing an increasing angle of attack on the stabilizer, and since its an inverted foil, this will increase negative lift again, causing a nose down moment? So it will find some equilibrium, or oscillate? Or will it end up at one of the mechanical stops?
I think this is a rather dynamic problem depending on many variables but maybe someone can shed light on this.
F. S.
I am also wondering which position the stabilizer will go to in case you let go. In general the stabilizer provides a nose up moment (lift vector down) to counteract the pitch-down tendency of the whole aircraft due to the total lift working on a point aft of the ac CG.
But, I would say that it depends on the position of the center of pressure (CoP_stab) on the stabilizer with respect to its hinge point where it will move to. Assuming that the CoP_stab is aft of the hinge point, the stabilizer will move up (if you let go of it), reducing the negative lift force generated, causing the aircraft to pitch down. That in turn may be causing an increasing angle of attack on the stabilizer, and since its an inverted foil, this will increase negative lift again, causing a nose down moment? So it will find some equilibrium, or oscillate? Or will it end up at one of the mechanical stops?
I think this is a rather dynamic problem depending on many variables but maybe someone can shed light on this.
F. S.
Joined: Nov 2000
Posts: 562
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From: Grobelling through the murk to the sunshine above.
Assuming the two brakes on the stabilizer trim actuator assembly fail (to engage), now the stabilizer is free to backdrive the jackscrew and change its position based on aerodynamic loads
(Edited after closer study of the AMM following correction below)
Last edited by Pub User; 22nd January 2012 at 21:23.
Thread Starter
Joined: Aug 2011
Posts: 43
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From: Tokyo (ENRI)
Have to disagree on this one. The Boeing AMM states clearly that the Stabilizer trim assembly has two similar ratchet type brakes. These brakes disengage in case of an AP trim request or Trim switch (on the yoke).
There is also a clutch in the trim assembly which allows a manual override of the trim, always. The priority of control for the stabilizer position is:
* Manual
* Electric
* Autopilot
Furthermore, the jackscrew is designed such that it actually does allow backdriving. From Wiki: "Depending upon their lead angle, ball screws can be back-driven due to their low internal friction..."
F.S.
There is also a clutch in the trim assembly which allows a manual override of the trim, always. The priority of control for the stabilizer position is:
* Manual
* Electric
* Autopilot
Furthermore, the jackscrew is designed such that it actually does allow backdriving. From Wiki: "Depending upon their lead angle, ball screws can be back-driven due to their low internal friction..."
F.S.
Joined: Sep 2008
Aviation Qualifications: ATPL
Posts: 3,430
Likes: 1,192
From: 41S174E
There are two brakes. They are built into the motor/clutch unit at the bottom of the only jackscrew.
Not really because when trim starts running away you don't know if it's being driven by the A/P circuit , the main circuit, or by aerodynamic loads.
You will disengage the a/p quick smart in the hope of stopping the runaway and once you've done that you won't re engage it. So I don't think that would ever be a viable option.
Also, if those brakes fail, is it an option to leave the AP on and let it control the stabilizer by means of the electric stabilizer actuator system?
You will disengage the a/p quick smart in the hope of stopping the runaway and once you've done that you won't re engage it. So I don't think that would ever be a viable option.
Joined: Oct 2019
Aviation Qualifications: ATPL
Posts: 171
Likes: 72
From: north of Harlow and south of Cambridge
A strange coincidence…
It is now Dec 25, 2024. For some strange reason the stabilizer brake system was in my head when I woke up at 03:00 this morning. I stopped flying the 737s Dec 2019, but I still fly in my sleep most every night.
upon obtaining the first coffee, I did a google search “B737 stabilizer brake system,” as one does these days, and found this old thread.
this is one of the several components of the 737 that i always wanted to know more about. Especially after the Alaska Airlines MD80 had the stabilizer malfunction that caused them to lose control. (screw thread come out of the jackscrew?) which may have had a completely different type system. But I always had, in the back of my head, the same questions that the OP brought up. Knowing how the hardware actually operates can only give clarity if any malfunction happens.
I always found it a bit uncomfortable that it was so difficult to get access to the real nuts and bolts of the machine I flew for so many years. Over the course of the 1990 to 2019 Boeing progressively put less and less hard systems information into their manuals. Perhaps they thought pilots were unable to understand. Perhaps most did not want to know. I did. And I did bug the Boeing reps for sections of the maintenance manuals, but soon that was frowned upon and the rep was told not to share that information with me.
After working on lawn mowers as a child and automobiles during the 60s and 70s, and having a habit of taking most things apart to see how they worked, often unable to put them back together into working condition, I wanted to know how the 737-100,200,300,400,500&800s worked underneath the covers.
this philosophy might have gotten out of control and bit their asses when it came to the MCAS issue. In this case, not only a lack of nuts and bolts information, but an entire passive denial of the system’s existence.
my coffee is almost finished and I will go back to being a has been 737 driver in the fog of the past.
upon obtaining the first coffee, I did a google search “B737 stabilizer brake system,” as one does these days, and found this old thread.
this is one of the several components of the 737 that i always wanted to know more about. Especially after the Alaska Airlines MD80 had the stabilizer malfunction that caused them to lose control. (screw thread come out of the jackscrew?) which may have had a completely different type system. But I always had, in the back of my head, the same questions that the OP brought up. Knowing how the hardware actually operates can only give clarity if any malfunction happens.
I always found it a bit uncomfortable that it was so difficult to get access to the real nuts and bolts of the machine I flew for so many years. Over the course of the 1990 to 2019 Boeing progressively put less and less hard systems information into their manuals. Perhaps they thought pilots were unable to understand. Perhaps most did not want to know. I did. And I did bug the Boeing reps for sections of the maintenance manuals, but soon that was frowned upon and the rep was told not to share that information with me.
After working on lawn mowers as a child and automobiles during the 60s and 70s, and having a habit of taking most things apart to see how they worked, often unable to put them back together into working condition, I wanted to know how the 737-100,200,300,400,500&800s worked underneath the covers.
this philosophy might have gotten out of control and bit their asses when it came to the MCAS issue. In this case, not only a lack of nuts and bolts information, but an entire passive denial of the system’s existence.
my coffee is almost finished and I will go back to being a has been 737 driver in the fog of the past.
