Little Understood About MAX Technical Failures

Boeing 737 MAX aircraft

The MCAS relies on data from one of the MAX’s two AOA vanes, with the source changing after each flight. When the MCAS senses the aircraft’s nose is too high, it applies automatic nose-down horizontal stabilizer trim.

Flight-data recorder data from JT610 clearly shows the AOA sensor disagreement, but the Indonesia National Transportation Safety Committee (NTSC) preliminary report, released in November, offers little insight on why the AOA values did not agree. After the accident, regulators queried operators and scoured databases looking for reports of MAX flight-control anomalies. None described MCAS-related malfunctions.

The NTSC preliminary report revealed that the three-month-old Lion Air 737-8, PK-LQP, had a series of technical faults during four flights over three days before JT610’s accident flight. Among them: AOA, altitude and airspeed sensor disagreements between the pilot’s and first officer’s instruments. One of the aircraft’s AOA sensors was swapped after the aircraft’s last flight on Oct. 27, but problems persisted the next day, on a flight from Denpasar to Jakarta. The MCAS activated, and the pilot countered with trim inputs via a column-mounted switch. Pilots will also get new training and updated flight manuals.

Collected and summarized from the source below by Minh Pham