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Old 5th Apr 2019, 20:11
  #3366 (permalink)  
astonmartin
 
Join Date: Jan 2008
Location: Netherlands
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Originally Posted by Just This Once...
I'm still struggling with the though that a grossly mis-trimmed stab on an NG cannot be recovered by either electric trim or by the strength of 1 or even 2 pilots cranking the wheels when the aircraft is at certain parts of the flight envelope.

As covered by others above, it seems bizarre to rely on having the airspace and time available to perform an acceleration or deceleration to a trimmable speed and/or perform a manoeuvre in the opposite sense to what you are trying to achieve in order to put the stab in a safe position.

Hopefully Boeing can enlighten us as to which parts of the performance envelop allow for PF solo cranking, PM dedicated cranking, both pilots cranking or just plain impossible to crank the wheel due to aerodynamic load. The NG is still flying and I am sure the AOCs would like to know, even if the FAA is asleep at the wheel at a benign 1G.
During testflights with the B737 PG and NG I have used the stabilizer trim wheel many times. Also with high speeds, it is possible to crank the wheel. But with more elevator input (ANU), it becomes increasingly difficult to trim the stabilizer (ANU). This is because the (ANU) elevators push down on the trailing edge of the stabilizer.

Furthermore, the phenomenon of ‘elevator blowback’ may have played a role at the last, high speed phase of the flight. If that was the case, the elevators were blown back towards neutral, despite the combined hydraulic and muscle forces.
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