Boeing developed MCAS for the 737 MAX after its larger, repositioned CFM LEAP-1B engines altered the aircraft’s pitch characteristics. The system was intended to preserve handling commonality with the 737 Next Generation and reduce simulator-training requirements, but it could command nose-down stabilizer trim using data from a single angle-of-attack sensor. On Lion Air Flight 610 in 2018 and Ethiopian Airlines Flight 302 in 2019, falsely high sensor readings repeatedly activated MCAS shortly after takeoff. Investigators determined that both aircraft became difficult to control; all 346 passengers and crew on the two flights died.
Boeing initially omitted MCAS from pilot manuals and training materials and first publicly identified the system to operators on November 10, 2018, twelve days after the Lion Air crash. On April 4, 2019, Boeing acknowledged that MCAS contributed to both accidents. The FAA-commissioned Joint Authorities Technical Review found that Boeing had not provided adequate technical information or thoroughly stress-tested the system, while the NTSB’s September 26, 2019 findings criticized Boeing’s assumptions about pilot responses and the FAA’s review process. In 2020, regulators approved changes requiring agreement between both angle-of-attack sensors, preventing repeated activation and allowing pilot override; the FAA required simulator training for MAX pilots by 2021.
