Lion Air Crash: Key Aircraft Sensor Was Not Fixed Before Doomed Flight

Investigators have released a preliminary report on Lion Air Flight 610, which crashed in the Java Sea less than 13 minutes after takeoff on Oct. 29 and killed all 189 people on board.

No official cause of the crash was listed in the report, but Indonesian investigators focused on an automatic system on the 2-month-old Boeing 737 MAX 8 jet that continually pushed the plane's nose downward. Officials posed the question in the report whether erroneous information from aircraft sensors activated the automatic system.

Details from the plane's flight recorder included in the report, released by Indonesian officials on Wednesday, show how hard the pilots of the doomed jet fought to keep the plane in the air as an automatic system continually pushed the plane's nose downward 26 times. The system is meant to stop pilots from angling the aircraft nose too high (affecting the plane’s speed and lift, causing a stall) by automatically pushing the nose of the plane downward if it senses a stall is possible. It has become controversial in the crash's wake because pilots and airlines that operate Boeing's 737 MAXs say they weren't told the system existed. (Boeing denies withholding relevant information).

But at the center of the report was the potential malfunction of a sensor that reports the angle of the plane's nose to the cockpit crew — called an angle of attack sensor. The sensor, along with the jet's airspeed indicator, is said to have malfunctioned on the plane's penultimate flight. Pilots and passengers on that flight, the day before the fatal crash, describe a roller coaster of erratic speed and altitude changes similar to what Flight 610 experienced before plunging into the sea.

When the previous flight landed in Jakarta, mechanics examined the plane's sensors. According to the report, the technicians repaired other sensors and equipment but did not fix the angle of attack sensor, Bloomberg says. After the maintenance teams worked on the jet, it was deemed airworthy to fly the following day from Jakarta to Pangkal Pinang.

Data from the flight's so-called black box shows that the plane's "stick shaker" was warning of a stall from takeoff and stayed activated for most of short flight. The stick shaker vibrates the cockpit crew's controls indicating a potential stall.

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Former Boeing engineer Peter Lemme described in a blog post the pilots' struggle as a "deadly game of tag" in which the pilots would angle the nose up only to have the automated system push it down five seconds later. Lemme told the AP that the pilots failed to realize what was happening and didn't initiate the known procedure for correcting the erroneous activation of an automated system.

In a statement to TPG, a Boeing spokesperson re-iterated that the proper pilot response for recognizing and shutting down the system that automatically pushes the nose down, called "trim down," is listed in the flight crew's manual. "Trim down may be initiated by different airplane systems under certain circumstances on both the Next-Generation 737 and 737 MAX, and the appropriate flight crew response to uncommanded trim, regardless of cause, is contained in existing procedures," the emailed statement said.

Another worrisome aspect of the report, Lemme told the AP, was that there was no way to easily check if the sensors' information was faulty, the flight crew apparently wasn't told there were any issues on the preceding flight and that the plane was not fully repaired following those issues.

“Had they fixed the airplane, we would not have had the accident,” Lemme said. “Every accident is a combination of events, so there is disappointment all around here.”