Maneuvering Characteristics Augmentation System

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The Maneuvering Characteristics Augmentation System (MCAS) is a flight stabilizing feature developed by Boeing that became notorious for its role in two fatal accidents of the 737 MAX in 2018 and 2019, which killed all 346 passengers and crew among both flights.

Because the CFM International LEAP engine used on the 737 MAX was larger and mounted further forward from the wing and higher off the ground than on previous generations of the 737, Boeing discovered that the aircraft had a tendency to push the nose up when operating in a specific portion of the flight envelope (flaps up, high angle of attack, manual flight). MCAS was intended to mimic the flight behavior of the previous Boeing 737 Next Generation. The company indicated that this change eliminated the need for pilots to have simulator training on the new aircraft.^{[citation needed]}

After the fatal crash of Lion Air Flight 610 in 2018, Boeing and the Federal Aviation Administration (FAA) referred pilots to a revised trim runaway checklist that must be performed in case of a malfunction. Boeing then received many requests for more information and revealed the existence of MCAS in another message, and that it could intervene without pilot input.^[1][2] According to Boeing, MCAS was implemented to compensate for an excessive angle of attack by adjusting the horizontal stabilizer before the aircraft would potentially stall. Boeing denied that MCAS was an anti-stall system, and stressed that it was intended to improve the handling of the aircraft while operating in a specific portion of the flight envelope. Following the crash of Ethiopian Airlines Flight 302 in 2019, Ethiopian authorities stated that the procedure did not enable the crew to prevent the accident, however further investigation revealed that the pilots did not follow the procedure properly.^[3] The Civil Aviation Administration of China then ordered the grounding of all 737 MAX planes in China, which led to more groundings across the globe.

Boeing admitted MCAS played a role in both accidents, when it acted on false data from a single angle of attack (AoA) sensor. In 2020, the FAA, Transport Canada, and European Union Aviation Safety Agency (EASA) evaluated flight test results with MCAS disabled, and suggested that the MAX might not have needed MCAS to conform to certification standards.^[4] Later that year, an FAA Airworthiness Directive^[5] approved design changes for each MAX aircraft, which would prevent MCAS activation unless both AoA sensors register similar readings, eliminate MCAS's ability to repeatedly activate, and allow pilots to override the system if necessary. The FAA began requiring all MAX pilots to undergo MCAS-related training in flight simulators by 2021.