The Boeing 737 MAX MCAS Explained

Aviation Week & Space Technology
Sean Broderick Guy Norris Graham Warwick
Wed, 2019-03-20 04:00

The Maneuvering Characteristics Augmentation System (MCAS) is a flight control law managed by the flight control computer (FCC) and introduced on the 737 MAX to help it handle like a 737 Next Generation (NG), particularly at slow speeds and high angles of attack (AOA).

1 | Leap Engines and Pitch-up Moment

The MAX’s larger CFM Leap 1 engines create more lift at high AOA and give the aircraft a greater pitch-up moment than the CFM56-7-equipped NG. The MCAS was added as a certification requirement to minimize the handling difference between the MAX and NG

2 | MCAS Activation

The system activates when the aircraft approaches threshold AOA, or stickshaker activation, for the aircraft’s configuration and flight profile. The MAX flight-control law changes from speed trim to the MCAS because the MCAS reacts more quickly to AOA changes.

3 | Angle of Attack Vanes

The MCAS’s primary data sources are the MAX ’s two AOA sensing vanes, one on either side of the nose. Boeing designed the MCAS to receive input from only one of the sensors during each flight. The left and right sensors alternate between flights, feeding AOA data to the FCC and the MCAS.

4 | Stabilizer Deflection

When threshold AOA is reached, the MCAS commands 0.27 deg. of aircraft nose-down stabilizer deflection per second for 9.3 sec.—a total of 2.5 units of trim. When the FCC reads the AOA as back to below threshold, the MCAS is reset, and the aircraft’s trim returns to the pre-MCAS configuration. Inaccurate AOA data will trigger the MCAS every 5 sec. until the data is corrected or the system is disabled.

5 | Disabling the System

Pilots can interrupt the MCAS in two ways: via the yoke-mounted electric trim switches, or using the STAB TRIM CUTOUT switches on the center console. The trim switches interrupt the MCAS for 5 sec. and establish a new stabilizer trim reference point. Toggling both cutout switches de-powers the MCAS and the speed-trim system.