By now, you have heard much about the Lion Air crash on October 29, 2018. But let’s recap. Right after Lion Air Flight 610 crashed after requesting to return to the airport, reports began to be floated about the maintenance of the aircraft. Edward Sirait, the president of the airline, told reporters that the plane had also encountered a technical problem on its previous flight that was subsequently resolved by maintenance technicians. “This aircraft last flew from Denpasar [Bali] to Cengkareng [Jakarta airport] and it was released to fly,” he said. “There was a report about a technical problem but we have worked on the technical issue based on the procedure from the aircraft manufacturer.”
The previous flight, JT43, also encountered “unreliable airspeed indications.” One passenger said the plane dropped suddenly several times during that flight. Another passenger recounted that the aircraft returned to the gate prior to departing. The aircraft was reportedly experiencing unusual engine sounds. Flight JT43 made a distress call and requested to return to the airport as well. The pilot later said the aircraft was flying normally and would not need to return after all.
Shortly after Flight 610 crashed, the Indonesian transport minister suspended Lion Air’s technical director and three other officers due to speculation over the airworthiness of the aircraft. The focus was firmly on maintenance.
Once the flight data recorder was found and data downloaded, it was determined that its engines were running, indicating no explosion or inflight break up in the air, shifting focus to operational issues.
Further research by Indonesia’s National Transportation Safety Committee into the previous flights of that aircraft revealed it had airspeed indicator problems during its final four flights. Inaccurate airspeed readings continued for three days after analyzing the contents of a flight data recorder that was recovered. Each time the aircraft was cleared for takeoff. Again, the focus turned to maintenance.
Then, based on FDR data, it was announced that the anti-stall system engaged after takeoff from Jakarta with the nose being pushed down repeatedly. Quickly after that, Boeing issued a safety warning about flight-control software that might cause pilots problems and could lead to a steep descent of the 737 MAX 8. The FAA added their own directive quickly after. The investigation then started to focus on a software issue that could lead to misinterpretation in the cockpit. The automatic anti-stall system appeared to engage due to erroneous readings from a sensor, the angle of attack indicator, that may have triggered it. The angle of attack sensor measures the position of the wings relative to the flow of air and provides information about a potential stall. In that situation, the 737 MAX has a system that automatically pushes the nose down to prevent a stall.
Maintenance workers, they said, had failed to repair the sensor despite pilots’ reports from a flight the previous day that suggest the anti-stall system engaged improperly on that flight too. The ball was now lobbed back into the maintenance side of the court. The pilot flying the jet the day before the crash, shut down the plane’s anti-stall system, and continued on.
Meanwhile, two pilot unions (and later a third) put forth that the risks of the new safety feature, the Maneuvering Characteristics Augmentation System (MCAS) system, were not sufficiently explained in their manuals or training. Boeing executives quickly met with the pilot unions at American and Southwest Airlines to allay their concerns about the system.
The initial Indonesian National Transportation Safety Committee report said the aircraft’s “flight computers received false AOA readings that may have triggered MCAS—which would have pushed the nose of the aircraft down and continued to do so as long as the system was active.” MCAS is not part of previous 737 designs, the Boeing memo released shortly after the accident said. The system also was not covered in MAX flight crew operations manual (FCOM) or difference training for 737NG pilots. In that memo from Boeing, it says an erroneous AOA could trigger automatic nose-down pitch-trim. The Boeing memo referred operators to a procedure for runaway pitch trim that includes switching the system off as the proper response.
Maintenance logs show the angle-of-attack sensor was replaced October 27. After penultimate flight, where the pilots flew the plane manually and continued on, maintenance workers reportedly “flushed the left pitot air data module and the static air data module, and cleaned the electrical connector plug for a computer that transmits elevator feel to the control column. The angle-of-attack sensor was not reported to have been examined,” according to a Forbes report. But the issue continued to occur. “At this stage, we cannot determine if [the actions were] correct or not,” Nurcahyo Utomo, head of Indonesia’s national transport safety committee, said at a news conference.
It is never a good idea to second guess an investigation in progress. But some clear problems have already been uncovered. Many things must go wrong to result in an event of this magnitude. Mistakes were made. However, if the system had been fully explained to the crew, if they had been trained on its implications and recovery in a malfunctioning scenario, and if the maintenance crews had been able to troubleshoot the angle of attack sensor problem the first time the discrepancy was written up, this tragedy could have been avoided.