MCAS, or Maneuvering Characteristics Augmentation System, is a flight control law implemented on the 737 MAX to improve aircraft handling characteristics and decrease pitch-up tendency at elevated angles of attack.
MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance longitudinal stability characteristics with flaps up and at elevated Angles of Attack (AoA). The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control Computer (FCC) using input data from sensors and other airplane systems.
The MCAS function becomes active when the AoA exceeds a threshold based on airspeed and altitude. After AoA falls below the hysteresis threshold (0.5 degrees below the activation angle), MCAS commands nose up stabiliser to return the aircraft to the trim state that existed before the MCAS activation.
Context - The LEAP-1B engine
When Boeing set out to develop the 737 MAX, engineers had to find a way to fit a much larger and more fuel efficient engine under the wing. MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation.
The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous 737NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up.