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MN-FLT-OH-201 B-18 Page 17 of 22
Flight Crew Operations Manual Bulletin No. GOT-18 , Dated March 1, 2021 (continued)
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.
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