737 MAX Flight Crew Operations Manual
Flight Controls -
System Description
MN-FLT-OH-201
9.20.9
The trim authority for main electric and autopilot trim is less than that for manual
trim.
• Main Electric Trim
•flaps extended 0.05 to 14.5 units
•flaps retracted 3.95 to 14.5 units
• Autopilot Trim 0.05 to 14.5 units
• Manual Trim -0.20 to 16.9 units.
The green band range of the STAB TRIM indicator shows the takeoff trim range.
An intermittent horn sounds if takeoff is attempted with the stabilizer trim outside
the takeoff trim range.
Speed Trim System
The Speed Trim System (STS) provides speed and pitch stability augmentation.
Speed stability augmentation is provided by the Speed Trim function in the Speed
Trim System. Pitch stability augmentation is provided by the MCAS function in
the Speed Trim System. The Speed Trim System only operates while the autopilot
is not engaged. If the SPEED TRIM FAIL light is illuminated, Speed Trim System
(Speed Trim function and MCAS function) operation is inhibited. Main electric
stabilizer trim switches can be used to override Speed Trim System inputs.
If the left and right AOAs disagree by 5.5 degrees or more with the flaps retracted,
the SPEED TRIM FAIL light illuminates and the Speed Trim System (Speed Trim
function and MCAS function) is inhibited for the remainder of the flight.
Note:
The Speed Trim System (Speed Trim function and MCAS function) is
inhibited with either the STAB TRIM PRI cutout switch or the STAB
TRIM B/U cutout switch in the CUTOUT position.
Speed Trim Function
Speed trim is a speed stability augmentation function designed to improve flight
characteristics during operations with low gross weight, aft center of gravity and
high thrust when the autopilot is not engaged. The purpose is to return the airplane
to a trimmed speed by commanding the stabilizer in a direction opposite the speed
change. The Speed Trim function monitors inputs of stabilizer position, thrust
lever position, flap position, airspeed and vertical speed to determine the amount
of stabilizer trim and rate at which to control the stabilizer trim motor. As the
airplane speed increases or decreases from the trimmed speed, the stabilizer is
commanded in the direction needed to return the airplane to the trimmed speed.
This increases control column forces to encourage return to the trimmed speed. As
the airplane returns to the trimmed speed, the commanded stabilizer movement is
removed.
March 1, 2021