The Maneuvering Characteristics Augmentation System (MCAS) is a flight control system designed by Boeing to enhance the handling characteristics of the Boeing 737 MAX aircraft. It is intended to prevent the aircraft from entering an aerodynamic stall, which is a condition where the aircraft loses lift and becomes uncontrollable.

The MCAS system works by automatically adjusting the aircraft's horizontal stabilizer, which is a movable surface located at the rear of the aircraft. The horizontal stabilizer controls the aircraft's pitch, or up-and-down movement.

The MCAS system uses data from the aircraft's sensors to determine when the aircraft is in danger of stalling. If the system detects that the aircraft is approaching a stall, it will automatically activate and adjust the horizontal stabilizer to push the aircraft's nose down, thereby preventing the stall.

The MCAS system was designed to be a backup system, and was not intended to be the primary means of controlling the aircraft. However, it has been implicated in two fatal crashes of Boeing 737 MAX 8 aircraft, the Ethiopian Airlines Flight 302 crash in March 2019 and the Lion Air Flight 610 crash in October 2018.

Following these crashes, Boeing has made significant changes to the MCAS system, including adding additional safeguards and making it easier for pilots to override the system. The MCAS system has also been renamed to the Maneuvering Characteristics Improvement System (MCIS).