A malfunctioning sensor at the center of the investigation into the Oct. 29 crash of a Lion Air jetliner into the Java Sea wasn’t repaired before the fatal flight even though it had failed on the plane’s previous trip, according to a preliminary investigative report.
A mechanic worked on other sensors and equipment during a night shift before the early morning departure, but not on the so-called angle-of-attack vane, according to Indonesia’s National Transportation Safety Committee. The investigators also asked Lion Air to take corrective steps to improve the safety culture as they released the report in Jakarta on Wednesday.
The report doesn’t offer a cause for the accident that killed all 189 on board but provides the most detailed look so far into the chaotic minutes before the crash and into the steps that were taken to address malfunctions that occurred on the plane the previous night. On both flights, pilots reported that they had difficulty figuring out basic information such as speed and altitude.
The pilots on Flight 610, which plunged into the Java Sea more than 11 minutes after it took off from Jakarta, appeared not to understand what was happening to them as they radioed air traffic controllers asking for their altitude and speed. They said they had an unspecified “flight control problems,” according to the report.
Lion Air Director Zwingly Silalahi said the airline was still studying the safety recommendations made by the investigators in the report for implementation. Boeing said in a statement that its 737 Max series of jets are safe and it is working closely with investigators to assist the Indonesian probe.
The Boeing Co. 737 Max 8’s angle-of-attack sensor, which measures how high or low the plane’s nose was pointed relative to the oncoming air, had malfunctioned on the previous flight as well as in the minutes before the crash, according to the report. The sensor erroneously concluded the nose was pointed too high and the aircraft was in danger of losing lift, prompting a stall warning in the cockpit and triggering safety software that attempted to put them into a dive.
The two sets of pilots reacted differently to the multiple errors messages and malfunctions. On the previous flight, the pilots were able to shut off the motor that was trying to push down the nose relatively soon after taking off.
For reasons that haven’t been explained, the pilots on Flight 610 didn’t take that step – which is part of a long-standing emergency procedure. The plane’s crash-proof cockpit recorder hasn’t been recovered, so investigators don’t have much insight into what they were thinking as they responded to the emergency.
“Boeing is taking every measure to fully understand all aspects of this accident, working closely with the U.S. National Transportation Safety Board as technical advisers to support the NTSC as the investigation continues,” the aircraft maker said in a statement.
Earlier this month the manufacturer issued a bulletin to operators of the Max reminding them that such a cascade of failures could be addressed by an existing emergency procedure. The manufacturer has said it’s confident in the safety of the latest version of its 737 model.
On the Oct. 28 flight that landed safely in Jakarta, the captain told investigators that he scanned cockpit instruments and determined that the copilot’s readings matched a third standby system and were accurate. He turned over control of the plane to the copilot.
By contrast, the captain on the flight that crashed radioed a controller about a minute before the plane disappeared from radar to say that all of the plane’s altitude gauges were different and they couldn’t determine how high they were.
Investigators will focus on how the airline performed maintenance on the plane.
The captain on the Oct. 28 flight reported problems with speed and altitude readings and with a system called Feel Differential Pressure, which controls the force pilots need to push or pull the control column that raises and lowers the nose, according to the report. There was no indication that the angle of attack sensor was malfunctioning.
An angle of attack sensor had been replaced and tested just before the Oct. 28 flight. According to flight data from the plane, the sensor on the left was reading about 20 degrees differently from the one on the right.
On both flights, a device known as a stick shaker – a warning that the wings are about to lose lift, which vibrates the control column and also makes a loud thumping noise – was activated shortly after takeoff for the captain but not the copilot, according to the data. It was triggered by the faulty sensor.
It also triggered safety software newly added to the Max. The Maneuvering Characteristics Augmentation System is designed to ensure the wings don’t lose lift, which is known as an aerodynamic stall. It was programmed to automatically lower the nose and was doing so for most of the 11-minute flight even though the plane was not close to stalling.
The pilots on the flight that crashed responded by hitting a switch on their control column that temporarily raised the nose, but it only stopped the MCAS temporarily. They apparently never shut off the motor activating the movement as the pilots did on the previous flight.
Indonesian authorities met with passenger families Tuesday in Bangka and Jakarta to provide a snapshot of preliminary findings. Those who attended the briefing say it didn’t indicate a cause for the crash.
“There was no explanation related to Boeing,” said Ari Priawan, whose brother, sister-in-law and their child were on board Flight 610.
Investigators say they hope to resume the search for the plane’s voice data recorder when equipment is obtained. A ship that can hold its position in one spot without casting anchor is needed because of the presence of many gas pipelines in the area, said Nurcahyo Utomo, the lead investigator.
“The search for the cockpit voice recorder will resume,” Utomo said. “We are hoping to start this week and will put in our maximum efforts.”
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