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The Maneuvering Characteristics Augmentation System (MCAS) is a feature on Boeing 737 MAX aircraft intended to prevent stalls in flaps-retracted, low-speed, nose-up flight. The MCAS uses airspeed and other sensor data to compute when a dangerous condition has developed and then trims the aircraft nose down.

History
Stall warning horns were invented in the 1930s. In the 1960s, a stick shaker was first installed to notify the pilot more forcefully than the horn, of an impending stall. With the introduction of fly-by-wire systems in airliners, it became possible to alter the feel of the control in the pilot's hands depending on flight regime, or even to override the pilot's commands when the flight computers sense a dangerous condition, including an incipient stall.

Airbus led the industry in adoption of systems that would not permit a pilot to put an airliner in a dangerous regime. Airbus introduced flight control laws with the A320 in 1988. In 1995, Boeing introduced a similar system in the 777 that could be overridden by the pilot.

Motivation for and operation of MCAS
Boeing 737 MAX aircraft have engines installed higher and further forward than previous 737 models. This caused an upward pitching moment. In order to pass Part 25 certification requirements, Boeing installed the MCAS, which automatically applies nose-down trim when the aircraft is in steep turns or in low-speed, flaps-retracted flight. When the angle-of-attack exceeds a limit that depends on airspeed and altitude, the system activates without notice to the pilot. The system is deactivated when a pilot trims the aircraft using a switch on the yoke. The system is sensitive to failure of angle-of-attack sensors mounted outside the aircraft. The FAA and Boeing made the AOA Disagree alert an optional feature for the 737 MAX, deciding it was not critical for safe operation.

Accidents in MCAS-equipped Boeing 737 MAX aircraft
Two aircraft equipped with MCAS have crashed: Lion Air Flight 610 and five months later Ethiopian Airlines Flight 302. Preliminary investigative results of Lion Air Flight 610 suggested MCAS may have received erroneous angle of attack data and caused loss-of-control. A fault in the MCAS flight control system is now seen as a potential cause of both accidents.

The entire 737 MAX fleet was grounded worldwide pending the results of the crash investigations.

Boeing reaction
Boeing cited an Indonesian finding that the Lion Air crash was a maintenance failure.

Boeing introduced a software upgrade that notifies pilots of a failure of an angle-of-attack sensor, a crucial component of the MCAS system. It will be deployed to aircraft operators "in the coming weeks," the company said on March 11, 2019.

Legal action
On March 11, one day after the Ethiopian crash, the Department of Justice convened a grand jury, subpoenaing at least one member of Boeing staff to answer questions about the development of the 737 Max 8.

On March 19, Secretary of Transportation Elaine Chao called for an audit of the aircraft's certification process, specific to determin whether Boeing had undue influence on the certification process ("regulatory capture").