JAKARTA – A faulty automatic engine throttle system that was not properly monitored by pilots led to the deadly January 2021 crash of a Sriwijaya Air 737-500 jet, a final report by Indonesia’s air accident investigator said on Thursday.

Flight 182 nosedived 3,000m into waters off the capital Jakarta into the Java Sea just minutes after take-off on January 9, 2021.

All 62 people on board were killed.

In its final report, Indonesia’s National Transportation Safety Committee (KNKT) pointed to several factors that contributed to the crash, starting with mechanical issues.

The twin-engine airliner’s autothrottle system suffered a malfunction, which caused it to tilt off course, KNKT said in a press release.

The autothrottle system automatically controls engine power. Problems with the system had been reported 65 times in the 26-year-old jet’s maintenance logs since 2013 and were still unresolved before the accident, the agency said in its 202-page report.

A working autothrottle is not required for a plane to be dispatched because pilots can control the thrust levers manually.

However, KNKT said in this case they did not appear to have closely monitored the asymmetrical thrust situation involving the left engine throttle lever moving back to as low as 34 per cent speed after take-off while the right lever stayed in its original climb setting at about 92 per cent.

“There were several indications available that the pilots could have checked to identify the aircraft anomalies, such as engine parameters, thrust levers position, and roll angle,” the agency said.

“Complacency and confirmation bias led to limited monitoring” by the pilots, who failed to immediately rectify the situation, lead investigator Nurcahyo Utomo said at a briefing.

Sriwijaya did not respond immediately to a request for comment. Boeing, the manufacturer of the 737-500 jet, declined to comment.

At about 3,260m, the autopilot disengaged and the plane rolled to the left more than 45 degrees and started its dive into the sea.

The first officer said “upset, upset” and “captain, captain” before the recording stopped, but the captain’s channel was not working, making it more difficult for investigators to analyse events.

An upset situation involves an aircraft operating outside normal flying parameters like speed, angle or altitude.

Mr Utomo told reporters there had been no regulations and guidelines on upset prevention training by Indonesian airlines that ensured a pilot’s ability to stop unwanted situations from occurring, with a key part of that being monitoring.

“There was nothing wrong with the design or with the machine, the problem was with the control mechanics,” said Mr Utomo. “The pilots could have addressed the problem” had they monitored it properly.

Sriwijaya has since carried out such training for its pilots, he said.

KNKT had raised the lack of upset recovery training after the 2014 crash of an AirAsia Indonesia jet that killed all 162 people on board.

Upset recovery training was then mandated in Indonesia in 2017, according to the final report, but Mr Utomo said the aviation regulator did not update the requirement after the UN aviation body, the International Civil Aviation Organisation, said in 2018 the course should also include upset prevention.

Indonesia is putting in place updated upset prevention and recovery training, KNKT said in the report.

Indonesia is a vast archipelago with a poor air safety record despite relying heavily on air transport to connect its thousands of islands.

The Sriwijaya Air crash was Indonesia’s third major commercial plane crash in just over six years. The other two were an AirAsia Indonesia plane crash in 2014, and a Lion Air Boeing 737 MAX crash in October 2018, where the plane plunged into the sea and killed 189 people.

The Lion Air crash – and another in Ethiopia – led to the worldwide grounding of the 737 MAX model over a faulty anti-stall system. AFP, BLOOMBERG, REUTERS