Preliminary investigations into the loss of a Lion Air Boeing 737 Max 8 on 29 October show that the aircraft encountered speed and altitude issues on the four flights prior to the deadly crash.
These issues, recorded on the aircraft’s flight maintenance log, indicated that there were problems with the left primary flight display, concerning indication of the aircraft’s speed and altitude.
The pilot of the flight directly before JT610 noted “indicated air speed and altitude disagree” and “feel differential pressure” in the aircraft's maintenance log. He also informed an engineer about the issues. Prior to that flight, an angle-of-attack sensor had been replaced.
The report released by Indonesia’s National Transportation Safety Committee (NTSC) states that the engineer went on to flush the left pitot air data module as well as the static air data module to correct the speed and altitude error. The electrical connector plug of the elevator feel computer was also cleaned to rectify the feel differential pressure. Tests were then performed on the ground, with the engineer satisfied that the issued had been resolved before dispatching the jet.
Data retrieved from the aircraft’s flight data recorder shows that on the fateful 29 October flight, the pilots reported a “flight control problem” and had to ask air traffic control for the aircraft’s altitude and speed.
Before the aircraft crashed, the flight crew also requested that the controller block altitude 3,000ft above and below it for traffic avoidance, since the altitude of the aircraft could not be determined, as the instruments had different readings.
Data from the recorder shows that the aircraft automatically pitched nose-down more than two dozen times during the 11-minute flight, with the pilots pulling the nose up each time. The automatic nose down trim stopped only when the flaps were extended, but when they were retracted it activated again. The cycle continued before the pilots eventually lost control of the aircraft.
The report adds that there was also a 20° difference between the left and right angle-of-attack sensors throughout the flight. The left control column stick shaker was also activated and continued for most of the flight.
The aircraft’s cockpit voice recorder has not yet been recovered.
The aircraft’s Maneuvering Characteristics Augmentation System (MCAS) is an area of focus for the investigation. At issue is also how erroneous angle-of-attack sensor information could have forced the aircraft's nose down.
MCAS is a new feature added to the 737 Max to enhance pitch characteristics while the aircraft is being manually flown at a high angle of attack with the flaps up. The system commands stabiliser movement, without pilot input, to pitch the aircraft nose down.
A good part of the report focused on the flight directly before the crash. Pilots on that Denpasar-Jakarta flight determined that the left primary flight display was faulty, after checks following an air speed disagreement warning. Soon after take off, after the flaps were retracted, the aircraft also automatically pitched nose down. The pilot however moved to run off MCAS and continued the flight manually and without further incident to Jakarta.