How a Boeing Safety Feature Became a Suspect in Crashes

Planes have been flying with the aid of computers for so long that the term “autopilot” has become part of the language, usually meaning a situation of calm predictability. Yet an automated computer feature is suspected by investigators to be a factor in the crash of a Boeing 737 Max operated by the Indonesian carrier Lion Air — and in another flown by Ethiopian Airlines five months later.

The Maneuvering Characteristics Augmentation System, or MCAS, is a set of sensors and software that Boeing installed in the 737 Max to help pilots respond to a situation in which the wings are losing lift because the plane is climbing too steeply. Insufficient lift can produce an aerodynamic stall, which can cause a plane to plummet. The MCAS was designed so that if the so-called angle-of-attack sensor on the left side of the exterior of the airplane reports that its nose is aimed too high, the system automatically lowers it, allowing the plane to regain speed and lift. Boeing is planning to alter the system to consider feeds from both the left and right sensors in case one is malfunctioning.