- Pilots of previous flights reported problems with the automatic throttle system
- The Boeing jet was 26 years old and was out of service for almost nine months last year
- Divers are still searching for a missing memory unit from the plane
National Transportation Safety Committee investigators said they were still struggling to understand why the jet nosedived into the water minutes after taking off from Jakarta on January 9, killing all 62 people on board.
Indonesia’s National Transportation Safety Committee (KNKT) has issued a preliminary report providing new details about the pilots’ struggle to fly the plane from almost the time it became airborne.
Lead investigator Nurcahyo Utomo said the left engine’s throttle lever had moved backward on its own while autopilot was engaged, reducing the power output of that engine.
He said pilots of previous flights had reported problems with the automatic throttle system on the 26-year-old jet.
“What we know is the left autothrottle moved backward,” Mr Utomo told reporters.
“We don’t know if it’s broken or not, but it’s an anomaly because the left moved far back, the right did not, as though it was stuck.”
At about 10,900 feet, the autopilot disengaged and the plane rolled more than 45 degrees to the left and started to dive, according to the report.
There had been two prior problems reported with the autothrottle system according to maintenance logs, but the issue was rectified on January 5, four days before the crash, KNKT said.
A working autothrottle is not required for a plane to be dispatched as pilots can control the thrust levers manually with their hands.
The report highlighted the importance of upset recovery training for pilots and the recognition of repetitive plane defects, just over six years after an AirAsia Indonesia crash where those were among the issues raised.
The preliminary report, as is standard, laid out factual information obtained to date but did not list the contributing factors to the crash.
Under international standards, the final report is due within a year of the crash.
Boeing said it would continue to support the investigation.
Memory unit of cockpit recorder still missing
Divers are still searching for the missing memory unit from the plane’s cockpit voice recorder.
Rafik Alaydrus, whose wife died in the crash, said investigators told family members that the cause had not been determined but that various factors, including the autothrottle system, were being investigated.
“This incident happened and the victims should get their right compensation.”
The Boeing 737-500 was out of service for almost nine months last year because of flight cutbacks caused by the coronavirus pandemic.
Regulators and the airline said it underwent inspections before resuming commercial flights in December.
The Sriwijaya accident is Indonesia’s third major airline crash in just over six years and has shone a spotlight on the country’s poor air-safety record.
Starting with just one plane in 2003, Sriwijaya has become one of the country’s biggest airline groups, aided by its strategy of acquiring old planes at cheap prices and serving routes neglected by competitors.