Following the October 29th Lion Air plane crash in Indonesia which killed all aboard, Boeing has issued a safety warning for erroneous readings on the sensors of its 737 Max aircraft. The Operations Manual Bulletin directs all airlines operating the Boeing 737 Max to follow “existing flight crew procedures to address circumstances where there is erroneous input from an AOA sensor.”

AOA, or Angle of Attack, refers to the angle at which the oncoming flow of wind hits the airplane or its wings. Though the exact cause of the crash is unclear at the moment, investigators suspect that a software glitch or misinterpretation by pilots may have contributed to the accident, reports The Wall Street Journal.

According to the “black box” data found in the Boeing 737 Max 8 aircraft, the airspeed indicator malfunctioned on its last four flights. The Lion Air aircraft, which had only been in use for a few months, is one of Boeing’s newest commercial aircrafts. The Boeing 737 Max 8 is currently in use by two US airlines, Southwest and American Airlines, though no issues have been reported by those airlines yet.

Following Boeing’s statement, the FAA has issued an airworthiness directive for the 737 Max which mandates safety protocols for US carriers, according to Bloomberg. The emergency order requires airlines to follow Boeing’s instructions if encountering the 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.”