A preliminary report released Wednesday by Indonesia’s National Transportation Safety Committee (KNKT) on the October 29 fatal crash of a Lion Air Boeing 737 Max confirms that a series of instrument failures occurred during the airplane’s previous four flights, including the one before the accident flight in which the pilots managed to override automatic nose-down inputs that appear to have led to the crash the following day. The report also details how the pilots of the ill-fated flight struggled to overcome the commands triggered by the airplane’s maneuvering characteristics augmentation system (MCAS) starting two minutes after takeoff and repeatedly until the crash 11 minutes later.
The 78-page document includes readouts from the digital flight data recorder (DFDR), which recorded a difference between left and right angle of attack (AOA) of about 20 degrees and continued until the end of the recording. During rotation the left control column stick shaker activated and continued for most of the flight, the report said. Investigators have determined that faulty AOA sensor inputs led the flight control system to force the airplane’s nose downward, resulting in a steep dive from which the pilots could not recover. Despite an account of the aircraft’s last known moments, the report does not list a definitive cause for the crash.
Pilots flying the same airplane a day earlier encountered the same phenomenon while traveling from Denpasar to Jakarta, but they managed to gain control by shutting down the MCAS and flying manually to their destination. Before that flight, a maintenance engineer informed the captain that crews replaced and tested the AOA sensor. After takeoff, the aircraft’s stick shaker activated and at 400 feet the indicated airspeed (IAS) disagree warning showed on the primary flight display. The pilots also experienced difficulty with trim input three times, which prompted the captain to shut down the automatic trim system—a procedure Boeing pointed out appears in the runaway stabilizer non-normal checklist in the 737 Max flight crew operations manual.
The report revealed that the captain performed three non-normal checklists and none contained the instruction “Plan to land at the nearest suitable airport.” The aircraft landed in Jakarta safely.
In response to KNKT’s findings, Boeing issued its own statement on Wednesday and said the report not only failed to include installation records of the AOA sensor replaced before the October 28 flight but also failed to indicate whether the sensor was new or refurbished.
“Although the report states that the pilot was satisfied by the information relayed by the engineer that the AOA sensor had been replaced and tested, on the subsequent flight the pilots again experienced problems with erroneous airspeed data, and also experienced automatic nose-down trim,” Boeing said.
The manufacturer pointed out that the pilot ran the runaway stabilizer non-normal check “but [the report] does not state he communicated that fact in the maintenance documentation following that flight.”
Boeing also said the report failed to indicate whether or not the pilots of flight JT610 performed the runaway stabilizer procedure or cut out the stabilizer trim switches as did the pilots of the Denpasar-Jakarta flight on October 28.