Indonesia’s National Transportation Safety Committee (NTSC) presented its preliminary findings Wednesday, almost one month after the Boeing 737 MAX 8 aircraft crashed at high speed into the water minutes after takeoff, killing all 189 people on board.
According to a two-page summary document, issues were reported by pilots on the plane’s penultimate flight from Denpasar to Jakarta, who noted the instruments were showing inaccurate readouts from the angle-of-attack (AoA) sensors.
AoA sensors send information to the plane’s computers about the angle of the plane’s nose relative to the oncoming air to help determine whether the plane is about to stall.
The report said that the plane was “automatically trimming” on the previous flight — that is, the computer was adjusting the aircraft’s attitude — so the pilots switched to manual trim and, as their safety checklists didn’t recommend an emergency landing, they continued to Jakarta.
Further maintenance on the AoA sensor was carried out in Jakarta prior to Flight 610’s takeoff the next morning. After the flight took off, the instruments recorded a substantial discrepancy in the aircraft’s angle — as much as 20 degrees.
Within 90 seconds of takeoff, the co-pilot asked air traffic control to confirm air speed and altitude. Thirty seconds after that he reported that they had experienced a “flight control problem,” the report added.
After the aircraft’s flaps retracted following takeoff, the automatic trim problem noted on the previous night’s flight returned, until the flight data recorder stopped recording when the plane crashed.
As part of the continued investigation, the faulty AoA sensor will undergo further testing, the NTSC said. It plans to finish its report within 12 months.
Searching for the voice cockpit recorder
Responding to the report, Boeing said it was “deeply saddened” by the loss of the Lion Air flight — but maintained the 737 MAX 8 “is as safe as any airplane that has ever flown the skies.”
Most of the information from the preliminary report was obtained from the flight data recorder (FDR) that was pulled from the bottom of the sea on November 1.
However, investigators are still looking for the cockpit flight recorder, which is believed to be buried in thick mud on the ocean floor. That data recorder, if found, will reveal more about the pilots’ final efforts to control the flight and how they differed from the pilots on the previous flight.
“We need to know what was the pilot discussion during the flight. What was the problem that may heard on the CVR. So why the action difference, this is the thing we need to find. At the moment I don’t have the answer,” said Captain Nurcahyo Utomo, NTSC’s head of aviation.
In the course of the investigation, officials have pointed to issues with the plane’s maneuvering characteristics augmentation system (MCAS).
The feature is new to Boeing’s MAX planes and automatically activates to lower the nose to prevent the plane from stalling, based on information sent from its external sensors, including the AoA sensors.
“We don’t receive any information from Boeing or from (the) regulator about that additional training for our pilots,” Zwingli Silalahi, Lion Air’s operational director told CNN on November 14.
The report said Flight 610 reported a “flight control problem” in the minutes after taking off from the Indonesian capital en route to the city of Pangkal Pinang, on the island of Bangka.
Differences were recorded between the left and right AoAs, which continued until the end of the information captured by the flight data recorder, according to investigators.
Boeing stood by the aircraft’s safety record. “We are confident in the safety of the 737 MAX. Safety remains our top priority and is a core value for everyone at Boeing,” a spokesperson said.
The plane was intact with its engines running when it crashed, at more than 450 mph (720 kph), into the Java Sea, Soerjanto Tjahjono, head of Indonesia’s National Transportation Safety Committee, said at the time.
Tjahjono said that due to the small size of the debris found and loss of the plane’s engine blades, investigators determined that Flight 610 did not explode in the air, but was in “good shape” before it crashed 13 minutes after takeoff.
Authorities are still searching for the plane’s cockpit voice recorder (CVR), which is believed to be buried under deep mud. If found, it should add details to what happened in the cockpit in the final seconds of the flight.