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Old 11th Mar 2019, 11:52
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Centaurus
 
Join Date: Jun 2000
Location: Australia
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Boeing advice on "aerodynamically relieving airloads" using manual stabilizer trim

Reference Boeing 737 Classics and NG FCTM under the chapter Non-Normal Operations/Flight Controls and sub heading Manual Stabilizer trim.

Edited for brevity one paragraph states: "Excessive air loads on the stabilizer may require effort by both pilots to correct mis-trim. In extreme cases it may be necessary to aerodynamically relieve the air loads to allow manual trimming. Accelerate or decelerate towards the in-trim speed while attempting to trim manually."

What control movements are needed to "aerodynamically" relieve the airload? This is not amplified in the FCTM and from experience I believe few pilots know what Boeing mean by "aerodynamically relieve the airload".
In fact I wouldn't be surprised in the two recent Boeing 737 MAXI accidents and the apparent inability for their crews to recover from unusual attitudes (nose down) could be traced in part to absence of knowledge on how to aerodynamically relieve airloads if using manual stabilizer trim.

My understanding of the meaning of "aerodynamically relieving" is best illustrated as follows: An aircraft suffers a severe nose down runaway stabilizer trim.causing the aircraft to initially dive and rapidly lose height. Any delay caused by surprise factor further compromises flight path control. Both pilots haul back hard on the elevators while attempting back trim using manual stabilizer trim. Due to increasingly heavy aerodynamic airloads against the stabilizer the effort to manually rotate the trim wheels in this condition is considerably higher than normal.

To relieve these airloads so that manual stabilizer trim can be used to wind off the forward position of the stabilizer which has caused the problem in the first place, it may be necessary for the crew to first attempt to raise the nose well above the horizon. With the nose high, the control column is immediately released from all back pressure. This action momentarily "aerodynamically relieves the airloads that in turn allows rapid unimpeded manual operation of the stabilizer trim control; to return the stabilizer to mid-range and thus permit more effective elevator effectiveness.

This "yo-yo" technique may be the only effective way of overcoming the difficulty of using the manual stabilizer trim during an attempted recovery from a high speed dive where electrical operation of the stabilizer is unavailable. To my knowledge this technique is not covered during simulator training for manual stabilizer trim operation

Last edited by Centaurus; 11th Mar 2019 at 12:03.
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