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B737 stabilizer trim brake failure

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B737 stabilizer trim brake failure

Old 16th Jan 2012, 10:38
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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
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Old 17th Jan 2012, 13:00
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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.
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Old 17th Jan 2012, 13:21
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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.
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Old 18th Jan 2012, 10:05
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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.
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Old 18th Jan 2012, 13:17
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Snoop

Don't think its aft of the hinge (CoP) point because that is inline with the rear spar.
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Old 18th Jan 2012, 14:09
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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
There is no way aerodynamic loads could backdrive the jackscrew, because of the geometry of the screwthread.

(Edited after closer study of the AMM following correction below)

Last edited by Pub User; 22nd Jan 2012 at 22:23.
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Old 18th Jan 2012, 14:53
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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.
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Old 18th Jan 2012, 19:54
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There are two brakes. They are built into the motor/clutch unit at the bottom of the only jackscrew.

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?
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.
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