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 possibly in another one flown by Ethiopian Air 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. If a so-called angle-of-attack sensor on the outside of the airplane reports that its nose is aimed too high, the MCAS is programmed to automatically lower it, allowing the plane to regain speed and lift.

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 training, 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.

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 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 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?

U.S. federal authorities are exploring a criminal investigation of how the 737 Max was certified to fly passengers, according to people familiar with the probe. Prompted by information obtained after the Lion Air crash, the investigation is being conducted in part by the Transportation Department’s Inspector General’s office, which conducts both audits and criminal investigations in conjunction with the Justice Department. Boeing declined to comment about the investigation.

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

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