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question about 737NG stab trim malfuction

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question about 737NG stab trim malfuction

Old 5th Nov 2010, 14:52
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Question question about 737NG stab trim malfuction

on the 737NG QRH , we have 3 non-normal checklist about the stab trim, they are: stabizer out of trim; runaway stabilizer; Stabilizer Trim Inoperative.
my question is about the latter two.when having a stab trim inop, we need to manually trim the aircraft,
the flap setting for the approach is flap15 according to the NNC.
but when having a runaway stabilizer, it`s a memorized items,if the senario goes like this:
Control column. . . . . . . . . . . . . . . . . Hold firmly
Autopilot (if engaged) . . . . . . . . . . . . .Disengage
the runaway continues:
STAB TRIM CUTOUT
switches (both) . . . . . . . . . . . . . . . . CUTOUT
Stabilizer. . . . . . . . . . . . . . . . . . .
Trim manually
when doing the approach ,the QRH does not provide any clue for flap 15 or any other setting ,so defaultly we use the flap 30.
my question is these two stam trim problems both need manual trim, why the flpa setting is different?
thanx~~~
citizensun is offline  
Old 5th Nov 2010, 15:03
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Complete guess since no B737 specific knowledge.

For INOP, the likely position of the stab when failed can be estimated - it's likely stuck at the cruise condition. Therefore, Boeing can make an assessment of the likely change in trim requirement between the assumed failure case and the landing case. Sounds like they are directing the use of a lower flap setting (15 instead of 30) to minimize the likely out-of-trim between the assume failure condition and the landing condition.

For runaway, there's no way of really knowing where the trim eventually stops - depends on direction and speed of crew reaction. They may therefore feel unable to specify a specific landing flap - I am a little surprised they don't provide any guidance, but they may feel it's "airmanship" and not for them to be dogmatic about it.

For example, if the stab runs away in the cruise condition in the nose down direction, then I'd expect a crew to land with the least flaps possible for the airfield, since that will minimize the trim offset - similar considerations as for the INOP case.

But for a nose-up runaway, it may be that the stab stops at somewhere near the flaps 30 trim position anyway - in which case, using the flap setting that is closest to being in trim would be a good choice in most cases.
Mad (Flt) Scientist is offline  
Old 5th Nov 2010, 15:29
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thank you for guess ,
in the 737 when having a stab trim problem ,no matter it is runaway or inop, the pilot both can use the manual trim wheel to trim the aircraft as needed. maybe the answer is not as you mentioned。
but thank you anyway Mad (Flt) Scientist~
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Old 5th Nov 2010, 16:46
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Does the manual trim wheel move the stab, same as for the auto/powered system, or does it move something else (like rezeroing the feel system on the elevators)?

I can't conceive of how, following a stab runaway - which could be a failure right at the surface control - an alternate control scheme can regain control. If the drive has failed, it's failed. I can conceive of having a secondary control if it's say a trim switch failure, and you use an alternative command path.

If the stab is mechanically jammed, it's jammed. No alternate, secondary or manual control will move it.


IF it is the case of recentering the elevator feel, you can't recover aerodynamic trim authority - you're limited by elevator deflection and how much stab that can counter. So the flap config would be significant.
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Old 5th Nov 2010, 17:33
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when doing the approach ,the QRH does not provide any clue for flap 15 or any other setting ,so defaultly we use the flap 30.
my question is these two stam trim problems both need manual trim, why the flpa setting is different?
thanx~~~
Because "Stab trim Inop" means the stab is jammed and cannot be adjusted to an in-trim condition for the approach, while a "Stab Trim Runaway" has a moveable stab which can be trimmed manually. Flap 30 requires a much bigger trim correction than flap 15, so the lesser flap setting is used if you can't trim in order to keep the aircraft flyable.
Whippersnapper is offline  
Old 5th Nov 2010, 17:42
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There are cables to the stabilizer and two electrical trims, one for manual and one for the autopilot (roughly from memory without looking in any books or checklists).

If the stabilizer is stuck, it's stuck and won't move. Therefore, most likely, the restriction on the flaps, since you will only have the elevator to counter the nose down.

The cable to the stab does not run away. Only a malfunction in one of the electrical trims will cause a runaway. The manual trim wheel can still move the stabilizer, once the electrical motors are cutout with the cutout switches, and hence compensate for flap selection.
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Old 5th Nov 2010, 20:02
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Thanks cosmo, that makes sense to me. INOP can mean a motor failure (in which case you can trim manually) or a physically jam (in which case you can't) and to cover the latter they are recommending a lower landing flap. Whereas its assumed that runaway can be halted by the cutoff and then trim is definitely available, hence no flap restriction.

Personally, I'd have been tempted to state that the flaps 15 applies only if manual trim is found not to work, so allowing the full landing flap to be more available. But maybe most of the failures are jams - I'm having a hard time believing that though. Or maybe just a case of Keep It Simple?

Always interesting to se how other people have handled failures - often you find differences in philosophy, and sometimes something you can learn from - or steal shamelessly, if you prefer!
Mad (Flt) Scientist is offline  
Old 5th Nov 2010, 21:01
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This question was asked on 20th October 2010

(Look about page 5 of this forum)

737 NN LDG Config
Hi,
why performing the "Runaway Stabilizer NNC" the QRH ask for a "normal" Flaps and Vref for landing, while performing the "Stabilizer Trim Inoperative NNC" the QRH asks for a Flaps 15 LDG ?

In both situations both the cut-out trim switches (autopilot & Main elec) are OFf, so the flight conditions should be the same......

thanks in advance


------------------------------------------------------

Our Ops Dept asked this very question of Boeing (2008) and the reply was as follows;

"In the RUNAWAY STABILIZER non normal condition, manual trim is always readily available, there is no need to limit the landing flap to flaps 15.

The STABILIZER TRIM INOPERATIVE checklist may be addressing a condition in which the crew must continuously break out the disconnect clutch or in which the stabilizer is physically jammed; it is therefore more appropriate to limit the flight crew to flaps 15 to avoid large trim requirements."

Hope this helps
ad-astra is offline  
Old 6th Nov 2010, 13:21
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thank you ad-astra
and thank all~

but another question comes:what is that disconnect clutch exit for?
is that redundant?
citizensun is offline  

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