MN-FLT-OH-201
B-18 Page 15 of 22
Flight Crew Operations Manual Bulletin No. GOT-18 , Dated March 1, 2021 (continued)
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 a 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.
The Speed Trim function contains a High AOA mode that is used to provide
increased stabilizer commands at elevated AOA. As AOA increases toward stick
shaker, the Speed Trim function transitions to the High AOA mode to command
additional nose down stabilizer to increase control column forces. AOA is used
only to transition into and out of the High AOA mode and does not determine the
amount of stabilizer command.
The Speed Trim function controls the stabilizer trim motor at a high rate with
flaps extended and low rate with flaps retracted.
March 1, 2021
