Right from the start of last month’s fatal flight of a Lion Air 737 MAX jet from Indonesia, pilots were struggling with an automatic flight control system that had caused problems during a flight the day before, according to a preliminary investigative report.
The report — based on an analysis of readings from the jet’s flight data recorder, or “black box” — says the pilots on the Oct. 29 flight fought for 10 minutes to keep the Boeing jet’s nose from being pushed downward. But they lost the struggle against the automatic system, known as the Maneuvering Characteristics Augmentation System or MCAS. The 737 MAX 8 crashed into the Java Sea, killing all 189 people aboard.
Boeing had added the MCAS capability to the 737 MAX as a safeguard to keep the plane from stalling under extreme aerodynamic conditions. Investigators say the system on the Lion Air jet apparently was responding to spurious information from angle-of-attack sensors that measure the air flow over the wings.
Read the preliminary report: Indonesian investigators trace what went wrong on Lion Air Flight 610
The pilots repeatedly used manual controls to push up the jet’s nose, and the automatic system repeatedly sent commands to push it back down, according to the report. “In our view, the plane was not airworthy,” Reuters quoted Nurcahyo Utomo, an investigator for Indonesia’s National Transportation Safety Committee, as saying at a news conference in Jakarta on Wednesday (Tuesday night PT).
Boeing says the remedy for the angle-of-attack sensor problem is to switch off the automatic anti-stall system, but the Lion Air pilots didn’t take that step.
One of the sensors had been replaced two days before the crash, on Oct. 27, but the system acted up nevertheless during an Oct. 28 flight, the report says. During that trip, the plane’s flight crew experienced similar problems with the anti-stall system and switched it off, as recommended. After landing, the pilot made note of the issues. Maintenance workers conducted some corrective procedures and reported that the issues were resolved.
The report raises a number of questions about the anti-stall system, and about Lion Air’s maintenance procedures in the days before the crash. Investigators called on Lion Air to improve its “safety culture” and to tell its pilots to discontinue flights as soon as possible if they encounter conditions that aren’t airworthy.
Another question has to do with the pilots’ state of mind on the final flight: Why didn’t they switch off the automatic control system, as the previous day’s crew had done? Further clues for answering that last question might have been contained on the 737’s cockpit voice recorder, but that black box has not been recovered.
There’s been a debate about how much pilots are expected to know about the automatic control system and what to do if it malfunctions. Boeing says the procedure is covered in flight operations manuals for the 737, but some pilots have said they didn’t hear about the MCAS or about the potential control issue until after the Lion Air crash.
After the report’s release, Boeing issued a statement saying that the company was “deeply saddened by the loss of Lion Air Flight 610.” It expressed condolences to those whose loved ones were lost in the crash.
“Safety is a core value for everyone at Boeing, and the safety of our airplanes, our customers’ passengers and their crews is always our top priority,” Boeing said in its statement. “As our customers and their passengers continue to fly the 737 MAX to hundreds of destinations around the world every day, they have our assurance that the 737 MAX is as safe as any airplane that has ever flown the skies.”
Boeing said it was “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 [Indonesian] NTSC as the investigation continues.”
This is an updated version of a report originally published at 3:01 p.m. PT Nov. 27.