A grounded Lion Air Boeing Co. 737 Max 8. Photographer: Dimas Ardian/Bloomberg (Bloomberg)

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.

1. What system is under scrutiny?

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.

2. Why did Boeing add the MCAS to the plane?

Boeing designed the 737 Max to deliver a 14 percent fuel-savings to compete with the A320neo from the company’s European rival, Airbus SE. The use of new, bigger engines required Boeing’s designers to mount the units farther forward on the wings in order to give them proper ground clearance when taking off or landing. That changed the flying characteristics in a way that the engineers thought required the additional system to prevent stalls. Boeing, which promoted the plane as similar enough to past 737 models that pilots wouldn’t need extra simulator training, only a short computer course on the plane’s differences, said the new feature wasn’t significant enough to be highlighted in the plane’s manuals or pilot training.

3. What did the MCAS do in the Lion crash?

The system was activated by a reading from a single faulty angle-of-attack sensor that had been improperly repaired after incidents on earlier flights. That caused the plane to dive repeatedly during the harrowing, 11-minute flight as the crew desperately tried to regain control. Boeing says there is a simple procedure for shutting off the MCAS in such situations, but the Lion Air pilots apparently didn’t follow it. When the same malfunction occurred on the airplane’s penultimate flight, an off-duty pilot riding in the cockpit saved the day by telling the crew to cut power to the motor in the system that was driving the nose down, according to two people familiar with Indonesia’s investigation. The procedure is part of a checklist all pilots are required to memorize.

4. Why is the MCAS under suspicion in the crash in Ethiopia?

Satellite tracking of the plane’s flight path showed similarities with that of the doomed Lion Air plane. Ethiopia’s transport minister has said that flight-data recorders showed “clear similarities” between the two crashes. And a piece of equipment found in the wreckage of the Ethiopian Airlines flight indicates the jet was configured to dive, based on a preliminary review, according to a person familiar with the investigation. The item, called a jackscrew, is used to raise or lower the angle of the horizontal part of the plane’s tail. To avert a stall, MCAS swivels the stabilizer to force the tail up and the nose down.

5. What has Boeing said?

After the Lion Air crash, Boeing issued a bulletin reminding pilots how to disengage the motor that moves the jackscrew and horizontal stabilizer if the plane is trying to dive when it shouldn’t. The Chicago-based planemaker is working on its modifications to the system’s software in coordination with the U.S. Federal Aviation Administration. In the meantime, aviation authorities and airlines around the world have grounded the global fleet of 737 Max planes.

6. How have U.S. officials responded?

The Transportation Department’s inspector general is conducting a review of how the 737 Max was certified to fly, and a grand jury under the U.S. Justice Department is also seeking records in a possible criminal probe. The FAA has said it plans to mandate the Boeing software changes in the MCAS which will make it less likely to activate when there is no emergency. The agency and Boeing said they are also going to require additional training and references to it in flight manuals.

To contact the reporter on this story: Alan Levin in Washington at alevin24@bloomberg.net

To contact the editors responsible for this story: Jon Morgan at jmorgan97@bloomberg.net, Lisa Beyer

©2019 Bloomberg L.P.