737 MAX Flight Crew Operations Manual
Flight Controls -
System Description
MN-FLT-OH-201
9.20.11
The MCAS function contains logic that limits the amount of nose down stabilizer
trim movement to preserve elevator authority for pitch control. When MCAS
activates, the command limit logic computes a nose down stabilizer trim limit. If
stabilizer trim reaches the computed limit, 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. Manual stabilizer trim, main
electric stabilizer trim and autopilot trim are not affected by the command limit
logic. The command limit logic inhibits the Speed Trim System when the
computed nose down stabilizer trim limit is reached:
• within five minutes following MCAS activation, or
• before the autopilot has been engaged for one continuous minute
following MCAS activation.
The command limit logic is reset after five minutes or after the autopilot has been
engaged for one minute, whichever occurs first.
Conditions for operation are listed below:
• AOA above threshold
• Up to Mach 0.84
• 10 seconds after takeoff
• Flaps up
• Autopilot not engaged
March 1, 2021
