PPRuNe Forums - View Single Post - question about 737NG stab trim malfuction
Old 5th Nov 2010, 15:03
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Mad (Flt) Scientist
 
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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.
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