After the stab trim is set before take off, will it move by itself during the take off roll? If yes, why? Where can I find the reference?


1 Answer 1


The stab trim should not move by itself during the takeoff roll on the 737 NG.

The stabilizer can in principle be moved by the following things:

  • the stab trim switches on the control wheels: these are available during the takeoff roll, but do not operate automatically
  • the autopilot trim: the autopilot is turned off during the takeoff roll, so no trim inputs are made here
  • stabilizer trim wheels: these are mechanically linked and allow manual adjustment of the stabilizer, but no automatic operation
  • the Speed Trim System (STS): the STS starts to be active 10s after takeoff, so not during the takeoff roll

The details are described in the Flight Crew Operating Manual (FCOM):


The horizontal stabilizer is positioned by a single electric trim motor controlled through either the stab trim switches on the control wheel or autopilot trim. The stabilizer may also be positioned by manually rotating the stabilizer trim wheel.

Stabilizer Trim

Stabilizer trim switches on each control wheel actuate the electric trim motor through the main electric stabilizer trim circuit when the airplane is flown manually. With the autopilot engaged, stabilizer trim is accomplished through the autopilot stabilizer trim circuit. [...]

Speed Trim System

The speed trim system (STS) is a speed stability augmentation system designed to improve flight characteristics during operations with a low gross weight, aft center of gravity and high thrust when the autopilot is not engaged. [...]
Conditions for speed trim operation are listed below:

  • Airspeed between 100 KIAS and Mach 0.5
  • 10 seconds after takeoff
  • 5 seconds following release of trim switches
  • Autopilot not engaged
  • Sensing of trim requirement

(Boeing 737NG FCOMv2 - 9.20.8 - Flight Controls, System Description)



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