Maneuvering Characteristics Augmentation System (MCAS) is a flight control law created for the Boeing 737 MAX-8 and -9. Boeing chose this software solution to mitigate a pitch problem stemming from the new, larger engines. MCAS activates when the flap is up and improves aircraft handling characteristics by decreasing pitch-up tendency at higher angles of attack. It moves a control surface at the horizontal stabilizer to push the nose down, boosting longitudinal stability characteristics during high-angle-of-attack flight. The result is a handling improvement that avoids the need for pilot re-training while proving to potentially be used as a stall identification or protection system.
What is MCAS in aircraft?
The Maneuvering Characteristics Augmentation System (MCAS) is a flight stabilizing feature developed by Boeing, and it prevents stalls in specific flight phases and aircraft configurations, but it became notorious for its contribution in two fatal accidents of the 737 MAX in 2018 and 2019.
MCAS is Maneuvering Characteristics Augmentation System, a flight control law built into the Boeing 737 MAX's flight control computer. It is a longitudinal stability enhancement developed by Boeing to help the aircraft emulate the handling characteristics of the earlier Boeing 737 Next Generation without requiring pilot retraining.
The system was introduced to manage aircraft handling at high pitch angles and to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high angle of attack. New engines are attached to the wings higher and further forward than in previous models like the 737-800, creating a higher propensity for the plane to pitch up under certain conditions. MCAS decreases this pitch-up tendency at higher angles of attack by providing a nose-down command to oppose the pitch-up.
MCAS operates autonomously, using airplane input data like airspeed, angle of attack, angle of bank, flaps position, and load factor. When the aircraft's angle of attack exceeds a threshold calculated from airspeed and altitude, MCAS activates provided the autopilot is disengaged. It then moves the horizontal stabilizer to push the airplane's nose down, boosting pitch characteristics during flaps-up flight at airspeeds approaching stall and during steep turns with high load factors.
Although Boeing described MCAS to pilots as a system whose sole function is to trim the stabilizer nose down, the International Civil Aviation Authorities team review (JATR) commissioned by the FAA stated that MCAS is a stall identification or protection system. Its design aims to give a steady increase in stick force as the stall is approached, as required by regulation, and to meet FAA certification requirements for smooth handling characteristics in all flight conditions.
Is MCAS still used on airplanes?
Yes, MCAS is still used on airplanes. The 737 MAX still has MCAS, but it has been redesigned. After the redesign, MCAS operates only in manual, flaps-up flight, compares data from two angle-of-attack sensors instead of one, and is limited to a single activation per event. If the sensors disagree, the system is automatically inhibited. These changes restore full authority to the flight crew through the control column and satisfy the requirements set by the FAA, Transport Canada and EASA, all of which concluded that the airplane can be safely operated with or without MCAS. Boeing therefore states that the updated MCAS will not contribute to a future accident, as happened in 2018 and 2019 with the 737 MAX, while acknowledging that the name and basic trim function remain the only elements the MAX system now shares with the KC-46 Pegasus, whose earlier two-sensor architecture served as the starting point for the redesign.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
