IAKARTA, Indonesia (AP) – An important sensor was replaced in a Lion Air jet the day before it sank into the Java Sea, and sensor replacement may have aggravated other airplane problems, Indonesian investigators said on Wednesday.
This sensor, known as the "angle of attack" sensor, monitors the angle of the aircraft's nose to prevent the plane from stopping and diving.
Earlier this week, Indonesian officials have hinted that air speed indicators played a role in the deadly crash of October 29, killing 189 people.
The jet airplane's airspeed index did not work on its last four flights, and this problem is related to the sensors issue, Soerjanto Tjahjono, chairman of Indonesia's National Transportation Security Committee, said on Wednesday.
Lion Air's first two attempts to tackle the speed indicator problem did not work, and for the second flight of the Boeing 737 MAX 8 aircraft on 28 October, the angle of attack sensors was replaced, Tjahjono said.
On the October 28th flight from Bali to Jakarta, the pilot and pilot's sensors disagreed. The 2-month-old plane suddenly took off after takeoff, which the pilots managed to recover. They decided to fly to Jakarta at an altitude lower than normal.
The following day, during the fatal crash, the plane struck the water at high speed just 13 minutes after taking off from Jakarta. His crew had requested permission to return to the airport several minutes after takeoff.
"The point is that after replacing the AOA (sensor), the problem is not solved, but the problem can grow even more, is it deadly?" NTSC (National Transportation Safety Committee) wants to investigate it ".
Even if a jet angle sensor is defective, there is generally a backup system in place for the critical element and the pilots are trained to operate a plane safely if these sensors fail, aviation security experts said.
There are audible signals and physical warnings that can alert the pilot about equipment malfunctions or other hazards, said Todd Curtis, director of the Airsafe.com Foundation.
"They should have been fully involved in what was happening in the cockpit, and any warning that came, it would be wise to watch it," Curtis said.
Researchers are probably focusing on how failure of a single sensor resulted in a faulty command that ignored information from a second sensor, said John Cox, Chief Operating Systems Security Officer.
"We do not know what the crew knew and did not know yet," Cox said. "We will."
Boeing, which manufactures the Lion Air, issues safety-related bulletins and has previously released instructions on what flight crew should do if the sensors fail.
The Indonesian National Transportation Safety Committee said it had agreed with Boeing on procedures that the airplane manufacturer would have to distribute globally about how flight crews could face "angle of attack" sensor problems.
However, a Boeing statement said a security bulletin, sent to airlines on Tuesday, directs flight crews to existing instructions on how to respond to incorrect "angle of attack" data. It was not immediately clear if he plans an update, although comments from Indonesian officials show they expect one.
Indonesian researchers said the recommendations for the Boeing flight procedure were based on how the flight crew responded to Bali-Jakarta flight problems.
"The plan to be transported by Boeing this morning was presented to us," said Nurcahyo Utomo, a plane crash researcher.
"There are some things that we are asking for explanations and some that we are asking to abolish and there was agreement between NTSC and Boeing to release a new process for all Boeing 737 MAX users in the world," he said.
Indonesia's search and rescue organization expanded its search for a second time on Wednesday, saying it would continue until Sunday. Body parts are still recovering and the researchers continue to chase for the cockpit chatter.
The crash of Lion Air is the worst air disaster in Indonesia since 1997, when 234 people died on a Garuda flight near Medan. In December 2014, an AirAsia flight from Surabaya to Singapore sank into the sea, killing all of them 162.
Lion Air is one of Indonesia's newest airlines but has grown rapidly, bringing to dozens of domestic and international destinations. It has spread aggressively to Southeast Asia, a rapidly growing region of more than 600 million people.
Business writer Cathy Bussewitz contributed to this report from New York.