A Boeing bulletin to 737 Max operators warns that erroneous angle-of-attack (AOA) data trigger automatic nose-down inputs and emphasizes that pilots must follow specific flight crew operations manual procedures to keep the aircraft from descending uncommanded.
The flight crew operations manual (FCOM) bulletin stems from the probe into the Oct. 28 accident of a Lion Air 737-8, Boeing said. Data gleaned from the flight data recorder (FDR) confirmed that the crew was receiving erroneous (AOA) data, the Indonesian National Transportation Safety Committee, which is leading the investigation, has confirmed.
"An erroneous AOA can cause the pitch trim system to trim the stabilizer nose down," Boeing explained in the bulletin. "The nose-down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but can restart after the electric stabilizer trim switches are released. The [bulletin] directs flight crews to existing procedures to address this condition."
The bulletin warns that erroneous AOA data can lead to "repetitive cycles of nose-down input" unless the system is deactivated using the STAB TRIM CUTOUT switches and manual trim inputs.
Boeing's bulletin instructs operators to insert the checklist reminders into their 737 Max flight crew operations manuals. FAA just mandated the action via airworthiness directive 2018-23-51.
While the accident investigation is in its early stages, Boeing's bulletin suggests that documented procedures that could have kept the Lion Air aircraft aloft may not have been followed. The cockpit voice recorder, which would likely shed light on what the pilots believed they were experiencing and the steps they took to troubleshoot the issue, has not been recovered.
The aircraft went down into the Java Sea about 13 min. after departing Jakarta, killing all 189 onboard.