A preliminary report on the Oct. 29 crash of Lion Air flight 610 (JT610) confirms the accident aircraft, a Boeing 737 MAX 8, was not airworthy on at least its last two flights, spotlighting gaps in the airline’s maintenance practices and safety culture.

Much of the investigation focus has been on how the JT610 flight crew responded to flight-control issues during an 11-min. flight that ended when the Boeing 737 MAX 8 dove into the Java Sea, killing all 189 onboard. But early findings by Indonesia National Transportation Safety Committee (NTSC) confirm that mechanics with Lion Air subsidiary Bantam Aero Technic, tasked with correcting problems on the aircraft, failed to do so. As a result, both the JT610 pilots and the crew that flew the aircraft’s previous flight—an Oct. 28 leg from Denpasar to Jakarta—were assigned aircraft that never should have flown. 

Pilots reported a series of technical faults with the three-month-old aircraft, PK-LQP, during four flights over three days before JT610’s accident flight. Among them: angle-of-attack (AOA), altitude and airspeed sensor disagreements between the pilot's and first officer’s instruments. Maintenance was done each night and ground tests determined the aircraft was airworthy.

The pilots on the Oct. 28 flight detected issues as soon as the aircraft rotated. Their left AOA sensor was reporting a figure 20 deg. higher than the right sensor. This prompted an automatic stall-protection system to adjust the aircraft’s stabilizer and push the aircraft’s nose down. The pilots responded by pulling back on the stick, but the auto-trim system—apparently driven by the single, incorrect AOA reading—kept pushing the nose down. 

The crew ran through several checklists, including one for Runaway Stabilizer that included a step to toggle switches that disconnect the automatic system. With the aircraft back under control, the crew opted to keep flying, even with a major warning sign still present.

“The flight from Denpasar to Jakarta experienced stick-shaker activation during the takeoff rotation and remained active throughout the flight,” the report said. “This condition is considered [an] un-airworthy condition and the flight [should not have continued].” 

The pilots told investigators they kept flying because none of the checklists they referenced recommended diverting to the nearest airport. Lion Air’s operations manual and Indonesia’s aviation regulations require the pilot-in-command to “discontinue the flight when un-airworthy mechanical, electrical or structural conditions occur.”

Following the flight’s arrival in Jakarta, the pilot filed an electronic report summarizing the issues, listing “airspeed unreliable” and altitude (ALT) disagreement. He also wrote that the aircraft’s speed-trim system (STS) was “running in the wrong direction.” The report does not say the pilot told mechanics he went through the “Runaway Stabilizer” checklist.

Maintenance conducted following the flight included flushing the left-side pitot and static air data sensors to address the airspeed and altitude disagreements, the report said. A ground test showed the equipment was serviceable. The report does not reference any work or tests on AOA sensors.

The next day, JT610’s crew experienced the same issues as the Oct. 28 crew from the flight’s beginning. AOA sensors disagreed, with the left one reporting a higher value than the right. The automatic nose-down stall-protection system activated, and the pilot countered by pulling back on his control column. This back-and-forth cycle continued until the end of the flight.

The automatic nose-down inputs were apparently commanded by the Maneuvering Characteristics Augmentation System (MCAS) flight-control law added to the MAX family to help it handle like 737NGs, but the report does not reference MCAS specifically. MCAS’s triggers include data from either the left or right AOA input, with the source alternating after each flight. Following a power down/power up of the aircraft, the system starts with left-side AOA input.

The NTSC report, a factual update at the probe’s one-month mark that includes little analysis and no conclusions, does not explain why the crew lost control of the aircraft. It also does not say what, if any, troubleshooting steps the pilots took. Searchers are still attempting to recover the cockpit voice recorder.

While the probe is far from complete, Lion Air has implemented a series of changes to address possible issues uncovered by investigators. The airline has instructed pilots to provide as much details as “they deem necessary to provide a full comprehensive description of [any] technical defect to the engineering team.” It also is revamping how BAT line-maintenance teams troubleshoot reported problems and verify that issues have been corrected. The airline also wants its maintenance teams to be more mindful of repetitive problems that could point to larger issues with either hardware or the work being done on it.

Sean Broderick, sean.broderick@aviationweek.com