Two Boeing 737 Max 8 airplanes have crashed under similar circumstances in the past six months, one in October in Indonesia and the other in Ethiopia last week. These were new planes, and both had a control system installed that has been implicated in the Indonesian crash, and that might have played a role in the most recent disaster.
The system, known as the Maneuvering Characteristics Augmentation System (MCAS), has one very specific purpose. When flying in manual mode, the MCAS uses data from an “angle of attack” sensor to push the nose of the plane down if the plane’s orientation seems to be approaching the point when the plane would stall, which is a very dangerous condition. The software was designed to compensate for a new instability that resulted from some small physical-design modifications.
If the MCAS malfunctions, there is a procedure to cut the software out of the loop. But it requires throwing a separate switch, not merely pulling up on the plane’s control stick. If the switch isn’t flipped, the MCAS will keep nosing the plane down after five seconds. Back in November, as pilots and airline-industry observers mulled over the Indonesian crash, they fingered this “counterintuitive” system as part of the problem. Leeham, an aerospace news service, also noted that the novel behavior of the MCAS “was described nowhere” in the aircraft’s or pilot’s manual. This was a problem, Leeham wrote, because pilots had been told that an earlier version of the 737 and the Max 8 were the same, and could be flown interchangeably.