Indonesian accident investigators on Thursday submitted their final report to parliament on last year's deadly crash of a Sriwijaya Air jet, citing mechanical problems, the pilot's complacency and confirmation bias as factors behind the tragedy.
The report by the National Transportation Safety Committee will be issued to the public soon, according to chief investigator Nurcahyo Utomo.
Sriwijaya Air Flight 182, with 62 passengers and crew on board, plunged into the Java Sea on Jan. 9 last year when it departed Jakarta bound for the West Kalimantan provincial capital of Pontianak. All on board were killed.
Nurcahyo told a hearing with parliamentarians that troubles were earlier found on the auto-throttle system that automatically controlled the engine power of the B-737-500 aircraft.
"Normally, the auto-throttle will move both thrust levers backward to reduce engine power," he said, but when the plane was climbing to a high altitude after takeoff, the right throttle lever did not move at all, forcing the left one to continuously move backward to reduce engine power to compensate for the power cut needed.
"It caused asymmetry between the right and left engines," he said, adding that as the asymmetry was getting bigger, the plane yawed and rolled to the left, beginning to nosedive to the sea.
During the probe, the chief investigator said, some previous flights of the plane reported problems with the auto-throttle system, and some components had been replaced.
"Following Boeing's procedures, the components were replaced one by one to check which component had problems, but the procedures had not reached its mechanical system," Nurcahyo said.
After checking seven components in the United States and the United Kingdom, he said, "We believed that the trouble on the right thrust lever had been caused by problems on the mechanical system."
According to the investigation, the pilot was also apparently "unaware" of what was happening during the flight, including the position of the thrust lever and the plane, as well as other indicators in the cockpit.
"We don't know what happened, but we assumed that the pilot trusted the automatization system of the plane, or what we frequently say complacency," Nurcahyo said, adding that the pilot might not use his headset as his voice was not heard on the cockpit voice recorder.
"Confirmation bias, or seeking information that supports his opinion, had also likely made the pilot pay less attention in monitoring the plane's instruments and the ongoing situation," he said.
During the first four seconds of his recovery efforts, according to him, the pilot likely thought that the plane was turning right because the rudder was going to the right. He, therefore, turned the plane, which had been yawing continuously to the left.
Established in 2003, Sriwijaya Air is the country's third-largest airline and is known to have a good safety record. Before the crash, it had been involved in five minor incidents, none resulting in casualties.
