The cockpit of a new Boeing 737-800
Credit: Anton Novoderezhkin/TASS via Getty Images

According to pilots flying Boeing’s 737 MAX for both American Airlines and Southwest Airlines, they were not made aware of a crucial change in an automatic system that has been linked to the fatal Lion Air crash last month.

In October, a Lion Air flight crashed into the sea just off the coast of Indonesia, killing all 189 people on board. Now, investigators are focusing their efforts on figuring out if the plane crashed because of an update to a safety system that was designed to pull the plane out of a dangerous stall, The New York Times reported. Investigators believe the system may have been triggered on inaccurate data transmitted or processed from sensors on the fuselage, causing the plane to nosedive into the water.

According to The Times, Boeing has been busy selling the new 737s to different airlines and showcasing it as a plane that needed little to no additional pilot training, which is an attractive financial incentive for airlines.

But, according to the pilots union for American Airlines, the system upgrade wasn’t included in Boeing’s standard operating manual.

“We don’t like that we weren’t notified,” Jon Weaks, president of the Southwest Airlines Pilots Association, told TIME. Dennis Tajer, a 737 captain and spokesman for the Allied Pilots Association at American Airlines Group Inc., additionally noted that his union members were equally concerned about the omitted information.

“This is not about silos and layers of bureaucracy, this is about knowing your airplane,” Tajer said. “We will always be eager and aggressive in gaining any knowledge of new aircraft.”

And, because of this lack of communication, pilots are now left wondering if Boeing left anything else out.

“The companies and the pilots should have been informed,” Weaks said. “It makes us question, ‘Is that everything, guys?’ I would hope there are no more surprises out there.”

However, according to a Boeing spokesperson, the company is doing everything it can to ensure pilots are well prepared to fly the planes.

“We are taking every measure to fully understand all aspects of this incident, working closely with the investigating team and all regulatory authorities involved,” the company shared in a statement with TIME. “Safety remains our top priority and is a core value for everyone at Boeing.”

But, according to Roger Cox, a retired investigator with the U.S. National Transportation Safety Board and a former airline pilot who spoke to TIME, crews have every right to be angry at Boeing.

“I would be pretty pissed” about the missing information, he said. “This is important systems information that pilots should know about.”