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737 MAX Flight Crew Operations Manual
Flight Controls -
System Description
9.20.10 MN-FLT-OH-201
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.
The Speed Trim function operates most frequently during takeoff, climb and
go-around. Conditions for operation are listed below:
Maneuvering Characteristics Augmentation System (MCAS) Function
MCAS is a pitch stability augmentation function designed to operate at elevated
angles of attack (AOAs). The purpose is to increase control column forces by
commanding the stabilizer in the nose down direction at elevated AOAs.
The MCAS function becomes active when the airplane exceeds a threshold AOA.
Stabilizer trim inputs in the nose down direction are added incrementally if AOA
continues to increase. Stabilizer trim commands are based on AOA and Mach
number. As AOA is reduced below threshold AOA, the commanded stabilizer
trim is removed. If AOA is again increased above threshold AOA, MCAS
becomes active and again commands stabilizer trim inputs in the nose down
direction. The MCAS function controls the stabilizer trim motor at the flaps
extended Speed Trim function rate.
The FCC uses a corrected AOA value to activate MCAS. This logic uses both
AOA vanes and filters out small differences between the vanes and provides a
single corrected value to the MCAS function.
• Up to Mach 0.68
• 10 seconds after takeoff
• 5 seconds following release
of main electric stabilizer
trim switches
• Autopilot not engaged
• Sensing of trim requirement
March 1, 2021