Maneuvering Characteristics Augmentation System (MCAS)

The Maneuvering Characteristics Augmentation System (MCAS) is a flight control law developed by Boeing which became notorious for its role on the 737 MAX as a hidden feature that caused two fatal accidents, killing 346 people on board.

MCAS was intended on the 737 MAX to mimic pitching behavior of the previous generation of the series, the Boeing 737 NG, correcting an elevated angle of attack (AoA) situation by electronically adjusting the horizontal stabilizer and trim tab to push the nose down. Unlike on the Boeing 767 Tanker where MCAS was first used, pilot inputs on the control column does not disable the system on the MAX. A description of MCAS did not exist in the flight manuals during certification, leaving pilots unaware of the system when the airplane entered service in 2017.

Twelve days after the crash of Lion Air Flight 610, Boeing publicly revealed MCAS in a message to airline operators, noting that the system operates "without pilot input." Pilots were advised to perform a recovery procedure to reverse the nose down commands caused by MCAS in the event of a malfunction. The procedure could not prevent the crash of Ethiopian Airlines Flight 302, which occurred while a fix to MCAS was under development at the time. The aircraft was grounded worldwide in 2019 as a result of both crashes.

Boeing admitted that the MCAS played a role in both accidents, where it acted on false indications from exterior sensors. Boeing said that MCAS's functions were distinctly not an anti-stall system. The reports emerged that Boeing knowingly withheld, for at least a year before the Lion Air crash, that a key sensor warning system did not function as advertised. Repairs to the aircraft included an overhauled computer architecture, accurate simulation of forces potentially resisting manual trim, and other electromechanical defects.

In 2020, an FAA Airworthiness Directive approved design changes for each MAX aircraft, requiring input from two AoA sensors for MCAS activation, elimination of the system's ability to repeatedly activate, and allowing pilots to override the system if necessary. For each aircraft, the FAA will issue an airworthiness certificate, without delegation to Boeing, upon completion of an AoA sensor system test and a validation test flight. The FAA began requiring all MAX pilots to undergo MCAS-related training in flight simulators by 2021 after two years of grounding.