MN-FLT-OH-201
B-18 Page 13 of 22
Flight Crew Operations Manual Bulletin No. GOT-18 , Dated March 1, 2021 (continued)
Flight Controls Chapter 9 - System Description Section 20
Stabilizer Trim
Main electric 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. The main electric and autopilot
stabilizer trim have two speed modes: high speed with flaps extended and low
speed with flaps retracted. In addition, with flaps extended, the autopilot trim
stops stabilizer nose up trim commands when airspeed is three knots or more into
the minimum maneuver speed (amber) bar. If the autopilot is engaged, actuating
either pair of main electric stabilizer trim switches automatically disengages the
autopilot. The stabilizer trim wheels rotate whenever main electric stabilizer trim
is actuated.
The STAB TRIM PRI cutout switch and the STAB TRIM B/U cutout switch are
located on the control stand. If either switch is positioned to CUTOUT, the
autopilot, main electric and Speed Trim System (Speed Trim function and
MCAS function) trim inputs are disconnected from the stabilizer trim motor.
Control column actuated stabilizer trim cutout switches interrupts operation of
main electric stabilizer trim, autopilot trim and the Speed Trim function when the
control column movement opposes trim direction. Aft control column movement
does not interrupt Maneuvering Characteristics Augmentation System (MCAS)
nose down trim commands. Forward control column movement interrupts
MCAS nose up trim commands.
In addition to the control column actuated stabilizer trim cutout switches, the
FCC contains a software column cutout function which adds a layer of
redundancy to mitigate a latent failure of the column cutout switches. The control
column cutout function interrupts operation of autopilot trim and the Speed Trim
function when the control column movement opposes trim direction. Aft control
column movement does not interrupt MCAS nose down trim commands.
Forward control column movement interrupts MCAS nose up trim commands.
The software column cutout function cuts out automatic stabilizer commands
beyond the cutout position of the control column actuated stabilizer trim cutout
switches. Main electric stabilizer trim commands are not inhibited.
When the STAB TRIM override switch is positioned to OVRD, main electric
stabilizer trim can be used regardless of control column position.
March 1, 2021
