United Airlines is warning pilots to be vigilant to ensure they can regain control of the Boeing 737 Max 9 if its automated systems unexpectedly make the aircraft dive, the likely scenario on October 29 when a Lion Air airplane crashed after takeoff in Indonesia, according to a new internal bulletin reviewed by Skift.

In the Lion Air crash, which killed all 189 aboard, automated systems likely erroneously believed the aircraft was in a stall and needed to dive to increase airspeed, investigators have said. In reality, investigators suspect a mechanism, called the angle of attack sensor, failed, and the airplane was not in a stall. But the aircraft descended nonetheless, and pilots were unable to recover.

“United Airlines is issuing this pilot bulletin to address this potential nose-down trim issue, which could cause the flight crew to have difficulty controlling the airplane, and lead to

excessive nose-down attitude, significant altitude loss, and possible impact with terrain,” United said in its bulletin to pilots.

United is one of three U.S. airlines that fly the Max, Boeing’s newest model. Union officials representing pilots at the other two carriers — Southwest Airlines and American Airlines — have said that this possible scenario was not in their pilot manuals.

Boeing and the Federal Aviation Administration have asked airlines, including United, to better prepare crews to understand the aircraft’s automated system.

A United spokesman said Tuesday night: “We have received the bulletin from Boeing and directives from FAA and are complying.”

United’s began flying the Boeing 737 Max 9, an advanced version of the decades-old aircraft, in June. Earlier models do not have this sophisticated system, officials have said.

United’s notice to pilots underscored that they should be aware of possible issues, noting “if a single erroneously high angle of attack sensor input is received by the flight control system, there is a potential for repeated nose-down trim commands to the horizontal stabilizer.” The bulletin advised to pilots to react if the plane incorrectly identifies a stall, asking them to disengage autopilot and auto throttle, among other things.

Ross Feinstein, a spokesman for American, said his airline, which flies the smaller 737 Max 8, has given similar advice to its pilots. Like its pilots union, American was unaware of some of the advanced functionality of the new Max aircraft, Feinstein said.

“We value our partnership with Boeing, but were unaware of some of the functionality of the Maneuvering Characteristics Augmentation System installed on the MAX 8,” Feinstein said.

Feinstein said American updated its pilot manuals on Friday. But he added that American has not had any problems with its angle of attack sensors.

“The work with the FAA and Boeing is on-going, and we will continue to keep pilots informed of any updates,” he said.