Investigators have been focused on whether the plane, Lion Air Flight 610, crashed because the system, which is designed to pull the plane out of a dangerous stall, activated based on inaccurate data transmitted or processed from sensors on the fuselage.

The pilot union for American Airlines, which also flies the new version of the 737, the Max 8, said Tuesday that the emergency system in question had not been included by Boeing in the standard operating manual. In addition, the flight check list — which contains information for manually overriding the emergency system — was incorrect, the union said.

The emergency system is intended to maneuver the plane out of a stall, when its nose is often angled too sharply upward. The system automatically pushes the nose down. If activated incorrectly, it could have sent the plane into its fatal dive.

“We’ve just been informed that there’s an entire new system on the Max called the MCAS,” said Capt. Dennis Tajer, spokesman for the Allied Pilots Association, the union for pilots at American Airlines, and a 737 pilot.

MCAS stands for maneuvering characteristics augmentation system. The previous system, and the one in the standard manual, goes by a different shorthand, EFS, for elevator feel shift.

Soerjanto Tjahjono, the head of Indonesia’s National Transportation Safety Committee, said Boeing’s manual did not adequately describe how this automatic anti-stall system worked and what to do if it was triggered incorrectly.

Pilots would only have a few seconds to respond to this situation, aviation experts said, especially if a plane was flying at low altitude, as was the case with Lion Air flight, which plummeted into the Java Sea on Oct. 29.

The New York Times

Hannah Beech, Muktita Suhartono and James Glanz © 2018 The New York Times