Boeing Co. is trying to assuage 737 Max customers concerned about a little-known anti-stall feature that has emerged as a focus of investigators probing a crash in Indonesia last month that killed 189 people.

Southwest Airlines, the largest 737 Max operator, American Airlines and United Airlines are among the carriers globally pressing Boeing for details of the formerly obscure system, representatives of the airlines say. The aircraft manufacturer first disclosed the possible link to the Lion Air crash on Nov. 7 and has been working with the Federal Aviation Administration to figure out the appropriate remedies, from updating software to improving pilot training.

But as more becomes known about the feature, some U.S. pilots flying the Boeing 737 Max seem increasingly confident that they are suitably trained to disable the automated trim system. Even so, leaders of the three U.S. pilots unions continue to pressure the Chicago-based manufacturer and U.S. regulators for more details about its design—and why it was omitted from pilots’ flight operating manuals.

Before the Oct. 29 crash near Jakarta, Boeing hadn’t widely disclosed that the so-called Maneuvering Characteristics Augmentation System would, in limited circumstances, lower the jet’s nose without any input from pilots. The company in recent days has provided assurances that other crucial changes to the upgraded 737 weren’t similarly overshadowed.

“We’re not hearing of any additional systems that we were unaware of before as the information exchange goes on,” said Dennis Tajer, a 737 captain and spokesman for the Allied Pilots Association, which represents pilots at American Airlines.

A Boeing spokesman declined to comment on the company’s discussions with its customers.

A day after the largest U.S. pilot union, the Air Line Pilots Association, warned regulators of a “potential, significant aviation system safety deficiency,” the United Airlines chapter took a more pragmatic view. United pilots are already trained on how to disable the anti-stall feature Boeing built into the new 737 Max, according to a report Friday from Bob Sisk, a United Boeing 767 captain and chairman of the Central Air Safety Committee.

“Despite the omission of the MCAS description in the initial 737 Max differences training, United pilots are properly trained in handling an MCAS malfunction,” Sisk wrote. “As explained in the new bulletin, when working properly, the system helps us avoid stalls.”

If the system responds erroneously, pilots are drilled to shut it off and continue flying the airplane, he said. “We have been working collaboratively with the fleet on this issue and will work toward a fix with the manufacturer for this or any other system that contributes to an accident anywhere around the world.”

The MCAS safety system is designed to automatically push down the nose of the plane if it is in danger of losing lift on the wings, a condition known as an aerodynamic stall. It was added to the 737 Max because flight tests showed the aircraft was more prone to stalls in some conditions than previous models.

If a gauge known as an angle-of-attack vane shows the aircraft is pointed too high relative to the oncoming air, a flight computer automatically pushes down the nose. The MCAS system only works while pilots are manually flying the plane.

In the case of the Lion Air episode, erroneous angle-of-attack gauge signals essentially tricked the plane into thinking it was in danger and commanding a dive, according to Indonesia’s National Transportation Safety Committee.

There is an existing emergency procedure requiring pilots to flip two switches that can halt the MCAS commands. However, Boeing didn’t specifically notify airlines and pilots that the new system existed or provide guidance on how to handle a malfunction.

American Airlines, the world’s largest carrier, was among those in the dark. There is a single mention of the MCAS in the company’s two-volume flight manual for 737 pilots: in the abbreviations section, without any elaboration, Tajer said.

“We value our partnership with Boeing, but were unaware of some of the functionality of the Maneuvering Characteristics Augmentation System (MCAS) installed on the Max 8,” Ross Feinstein, an American spokesman, said by email, referring to the model number of the Lion Air jetliner that crashed. “We must ensure that our pilots are fully trained on procedures and understand key systems on the aircraft they fly.”

United spokesman Frank Benenati said Friday the airline is “in close contact” with Boeing and “at the present time, the current information we have from Boeing and the FAA does not indicate that additional training is necessary.”

Southwest Airlines Co. officials met with Boeing to discuss unspecified software issues involving the Max aircraft earlier this week, according to spokeswoman Brandy King.

The airline received detailed technical information on the MCAS system implicated in the Lion Air crash earlier, King said. It issued a note to pilots on Nov. 10 detailing how the system worked and how they should respond if it malfunctioned.