In the days before a Lion Air jet dove into the sea off Indonesia last month, there were multiple chances to break the chain of errors that led to the deaths of all 189 people aboard.
Sensors were misfiring on the nearly new Boeing Co. Max 8 jet on its previous four flights. The night before the accident, pilots on the same plane continued to their destination despite significant malfunctions that investigators now say made the plane “unairworthy.” And, after that flight, pilots failed to cite all the breakdowns when they requested maintenance.
A blunt preliminary report by Indonesian investigators issued Wednesday doesn’t say what caused the Oct. 29 crash, but it leaves little doubt that the disaster could have been averted at many points.
“Why is the airplane flying passengers?” said John Cox, president of consulting company Safety Operating Systems and a former airline pilot who participated in multiple accident probes. “There’s some organizational issues within Lion Air.”
Accident investigators often speak of the “Swiss cheese” theory of airline accidents. For a crash to occur, they say, a string of issues have to line up as if holes in a block of cheese had aligned from one end to the other. All it takes to prevent a crash is to block the hole at any point.
In the case of Lion Air, there were multiple such opportunities, according to the preliminary report issued by the Indonesian National Transportation Safety Committee. They revolve around maintenance, pilots’ response to the malfunctions and a small software change by Boeing that reacted to a faulty sensor by repeatedly trying to dive the plane.
While it will take months to sort out whether mechanics acted properly making repairs on the jet, sensors that measure speed and altitude were repeatedly failing on the four previous flights dating back to Oct. 26, according to logs released in the report.
And the problem appeared to get worse in spite of attempts at repair. The sensor failures expanded to include what’s known as angle of attack, a measure of whether the plane’s nose is pointed above or below the airflow, on the day before the crash.
Cascading Problems
It was that angle-of-attack sensor that triggered multiple other issues on the fatal flight. A computer program Boeing added to the new Max models interpreted the erroneous angle-of-attack signal as a warning that the plane was in danger of losing lift and began commanding the nose to drop.
The report also raises questions about how the problems on the flight the night before the crash were reported to mechanics.
As soon as the Oct. 28 flight from Bali to Jakarta lifted off, a device known as a “stick shaker” -- which is designed to alert pilots to an imminent aerodynamic stall -- activated with its unmistakable vibrations on the control column and loud thumping noise. Initially the pilots had difficulty controlling the plane, but after running three emergency checklists they were able to manually fly it.
Still, the stick shaker on the captain’s side of the cockpit continued its alert. Such an issue was significant enough that pilots should have landed under Indonesia regulations, even though the checklists never instructed the crew to do so, according to the NTSC. The report said the plane was “unairworthy” as a result of the stick shaker.
Nevertheless, the pilots completed the 90-minute flight. The NTSC issued a recommendation that the airline encourage pilots to divert flights in those conditions.
When they reached Jakarta that night, they logged various problems with the plane, but didn’t say the shaker had been activated. While maintenance workers checked various sensors, they didn’t work on the angle-of-attack system, which would have been an obvious item to examine because it had been replaced in work on Oct. 27 and could have been related to the plane’s failures.
Since the accident, Lion Air has beefed up its maintenance procedures for recurring problems, according to the report. At carriers in the U.S. and other nations, it is routine to take additional steps to monitor and repair issues that persist. They include bringing in more seasoned mechanics or conducting a test flight before a plane carries passengers.
Successful Work-Around
Even though the angle-of-attack sensor seems to be at the center of the confusing alarms and the final dive before the crash, the pilots on the Oct. 28 flight showed that it could be overcome. The captain, following a longstanding emergency procedure, had flipped a pair of switches to halt the 737 Max’s automatic attempts to push the plane down.
On the plane’s 11-minute final flight, the Boeing safety software attempted to push the nose down more than 30 times, but the pilots -- a different crew than had flown the jet the day before -- never used those switches.
Lion Air President Director Edward Sirait bristled at some of the preliminary report’s findings. Sirait told reporters in Jakarta the airline has always upheld a safety culture.
“We are really shocked today when someone quoted that the plane was not airworthy,” he said. “Is this true? If it’s like that, it seems that the report is finished. We look guilty now.”
Boeing’s Responsibility
While the report doesn’t dwell on the Boeing design, the consultant Cox said that was also an area that should be examined for how to prevent future accidents. The safety software known as Maneuvering Characteristics Augmentation System, which lowers the nose in some conditions without pilot inputs, apparently relied on a single angle-of-attack sensor. That makes it more prone to failure, he said. U.S. pilot unions also have raised concerns that they weren’t told about this new feature on the Max models.
“It’s a pretty serious issue for Boeing,” Cox said.
After the Indonesian report, Boeing issued a statement noting that the pilots on the Oct. 28 flight avoided a serious emergency by running the proper checklists, and pledged to work with investigators.
“Safety is a core value for everyone” at the company, it said.
--With assistance from Harry Suhartono, Julie Johnsson, Fathiya Dahrul and Tassia Sipahutar.
To contact the reporter on this story: Alan Levin in Washington at alevin24@bloomberg.net
To contact the editors responsible for this story: Jon Morgan at jmorgan97@bloomberg.net, Susan Warren
©2018 Bloomberg L.P.
