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Old 4th Apr 2019, 16:04
  #3115 (permalink)  
Join Date: Apr 2010
Location: Europe
Posts: 21
Originally Posted by 737mgm View Post
Fair point, I should have written crews were overwhelmed instead of too easily overwhelmed. Nonetheless, despite all the stress and confusion that was surely occurring, the "only thing" that was happening is that the airplane was trimming nose down. According to Boeing this can be overriden with the electric trim switch and I have not seen any proof yet that this isn't the case. As I already wrote in an earlier post it is just very puzzling to me why a pilot would let his/her aircraft get to an extreme nose down state if it is possible to counter this with electric trimming. We obviously need more information to be able to understand what truly happened.
"Simulation has demonstrated that the thumb switch trim does not have enough authority to completely trim the aircraft longitudinally in certain corners of the flight envelope, e.g. gear up/flaps up, aft center of gravity, near Vmo/Mmo corner, and gear down/flaps up, at speeds above 230 kts.
In those cases, longitudinal trim is achieved by using the manual stabilizer trim wheel to position the stabilizer. The trim wheel can be used to trim the airplane throughout the entire flight envelope.
In addition, the autopilot has the authority to trim the airplane in these conditions.
The reference regulation and policy do not specify the method of trim, nor do they state that when multiple pilot trim control paths exist that they must each independently be able to trim the airplane
throughout the flight envelope."
Reference: Explanatory Note to TCDS IM.A.120 - Boeing 737 Issue 10 page 15 [Ref 1]. My reading of this, and the preceding paragraph in the referenced document, plus an equivalent safety issue on the 747-8 / -8F [Ref 2], is that the aisle stand trim switches can be use throughout the flight envelope, but the yoke trim switches cannot.

Ref 1 - http://www.easa.europa.eu/sites/defa...20ISS%2010.pdf
Ref 2 - http://www.easa.europa.eu/sites/defa...ESF%20B-13.pdf

Last edited by Semreh; 4th Apr 2019 at 16:43.
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