Pilots who fly Boeing’s 737 MAX in the U.S. say the airline manufacturer didn’t tell them about features of a new flight-control system that reportedly are part of the investigation into last month’s deadly crash in Indonesia.

The pilots say they were not trained in new features of an anti-stall system in the plane that differ from previous models of the 737.

The automated system is designed to help pilots avoid raising the plane’s nose too high, which can cause the aircraft to stall. It automatically pushes the nose of the plane down.

But if that nose-down command is triggered by faulty sensor readings — as suspected in the Lion Air crash — pilots can struggle to control the plane and it can go into a dive and perhaps crash.