Boeing Company today published an update to the flight crew operations manual for its 737 Max 8, warning of a possible fault in the aircraft’s angle of attack system that could cause the aircraft to violently pitch nose down, according to the FAA. The AOA measures the angular difference between the direction the aircraft is moving and the pitch of the aircraft’s wing. The agency followed the ops manual update with an emergency airworthiness directive against the aircraft, warning operators the pitch-over threat exists even when the aircraft is being hand flown by pilots. The agency said operators have less than 30 days to comply with the AD.
A 737 Max 8 being operated last week as Lion Air flight 610 slammed into the waters of the Java Sea shortly after takeoff from Jakarta, killing all 189 people aboard. Investigators have been cautious however not to claim the AOA issue addressed by the AD is the only cause of the accident. It is also unclear whether today’s AOA warning alters initial concerns last week over the Lion Air aircraft’s airspeed indications.
Concerns are focused on both the angle of attack indicator itself and the software that transmits the AOA’s information to the aircraft. Of concern is whether incorrect data from the AOA, fed to other aircraft systems, may have caused the aircraft to pitch over on its own or whether the cockpit crew may have misinterpreted potentially erroneous flight instrument indications.
The Air Current said the AOA issue is dangerous while the aircraft is being flown manually. “The erroneous AOA input can pitch the aircraft’s stabilizing trim down for up to 10 seconds at a time.” An earlier Wall Street Journal story said Lion Air 610’s crew did revert to manual flying of the aircraft in order to maintain control.
Boeing procedures normally include emergency instructions to pilots in the event of a faulty stabilizer trim system, that may have in this case, have convinced the aircraft its wing was about to stop flying. Whether the Lion Air crew had time to attempt any emergency remedies to regain control of their aircraft is unknown. More than 200 Boeing 737 Max 8 aircraft have been delivered to date.