Sensor failure under study in Indonesia air tragedy

JAKARTA, Indonesia — A crucial sensor was replaced on a Lion Air jet the day before it plunged into the Java Sea, and that sensor replacement may have exacerbated other problems with the plane, Indonesian investigators said Wednesday.

That sensor, known as the angle-of-attack sensor, keeps track of the angle of the aircraft nose to help prevent the plane from stalling and diving.

Earlier this week, Indonesian officials hinted that airspeed indicators played a role in the deadly Oct. 29 crash that killed all 189 people on board.

The jet's airspeed indicator malfunctioned on its last four flights, and that problem was related to the sensor issue, Soerjanto Tjahjono, chairman of Indonesia's National Transportation Safety Committee, said Wednesday.

Lion Air's first two attempts to address the airspeed indicator problem didn't work, and for the Boeing 737 MAX 8 plane's secondto-last flight on Oct. 28, the angle-of-attack sensors were replaced, Tjahjono said.

On the Oct. 28 flight, from Bali to Jakarta, the pilot and copilot's sensors disagreed. The 2-month-old plane went into a sudden dive minutes after takeoff, which the pilots were able to recover from. They decided to fly on to Jakarta at a lowerthan-normal altitude.

The next day, during the deadly crash, the plane hit the water at a very high speed just 13 minutes after takeoff from Jakarta. Its flight crew had requested permission to return to the airport several minutes after taking off.

Even if an angle-ofattack sensor on a jet is faulty, there's generally a backup system in place for the critical component, and pilots are trained to handle a plane safely if those sensors fail, airlinesafety experts said.

There are audio signals and physical warnings that can alert the pilot to malfunctioning equipment or other dangers, said Todd Curtis, director of the Airsafe.com Foundation.

Investigators are likely focused on how a single sensor's failure resulted in a faulty command that didn't take into account information from a second sensor, said John Cox, CEO of Safety Operating Systems.

Boeing, which manufactured the Lion Air plane, had previously circulated instructions about what flight crews should do if sensors fail.