Indonesia's National Transportation Safety Committee said Wednesday that the auto-throttle system that automatically controlled the engine power of a B-737-500 jetliner that crashed into the sea off Jakarta last month showed "anomalies."
Sriwijaya Air Flight 182 with 62 passengers and crew on board, including 10 children, plunged into Java Sea on Jan. 9 when it was on the way from Jakarta to the West Kalimantan provincial capital of Pontianak. All of those on board were killed.
The left throttle lever "moved backward too far," while the right one "did not move at all as if it was stuck," the committee's investigator Nurcahyo Utomo told a press conference.
"We don't know exactly yet which (throttle lever) was in trouble -- the left or right one," he said.
He stressed, however, that the auto-throttle system of a plane is connected to 13 other components and received inputs from the components and the investigators have not been able to "determine which one of the 13 components caused the "anomalies."
Therefore, the investigators have not been able to find out what problems were suffered by the plane.
According to the preliminary report of the crash made public at the press conference, anomalies on the throttle levers were shown when the plane reached the 8,150-feet altitude after takeoff.
At 10,900 feet, the report says, the highest altitude of the aircraft recorded by the flight data recorder, the autopilot disengaged and the plane, piloted by an experienced former air force pilot, rolled to the left, beginning to nosedive to the sea.
The report also disclosed that since Dec. 25, two previous flights reported problems with the auto-throttle system, but "the problem was rectified" on Jan. 5, or four days before the crash, "and test results were good."
Utomo explained that a functioning auto-throttle "is not mandatory and that is why it is allowed not to get repaired for 10 days" and pilots can control their planes manually.
But "why the plane rolled and the situation became unrecoverable...hopefully, we can get the answer from the cockpit voice recorder," he said.
The flight data recorder, one of the black boxes of the plane, was retrieved from the sea a few days after the crash, but the cockpit voice recorder is still being searched for.
Efforts to find the cockpit voice recorder have been hampered by bad weather since mid-January. Heavy rainfall, causing floods on the northern coastal areas of Java Island, and strong underwater currents have sent mud to the sea that covers plane debris.
"If the CVR is not found...it will be a saddening condition for us...because we cannot produce any results or conclusions that are scientifically accountable," the committee's chief Soerjanto Tjahjono said.
The aircraft took off from Jakarta's Soekarno-Hatta International Airport at 2:36 p.m. Saturday, bound for Pontianak, and a minute later requested to ascend to 29,000 feet.
However, at 2:40 p.m., air traffic control asked the pilot why the plane was heading northwest instead of on its expected path. A few seconds later, it disappeared from radar.
Flight tracking website Flightradar24 showed that after taking off, the plane climbed to 10,900 feet in approximately 4 minutes, but then went into a steep descent over the next 21 seconds, with the last received data placing it at 250 feet above the water's surface.
Established in 2003, Sriwijaya Air is the country's third-largest airline and is known to have a good safety record. Before Saturday's crash, it had been involved in five minor incidents, none resulting in casualties.
The crash was the second involving a B-737 aircraft in Indonesia in recent years.
In October 2018, a B-737 MAX 8 operated by low-cost carrier Lion Air crashed off the northeast coast of Jakarta immediately after takeoff, killing all 189 passengers and crew.
The Lion Air incident, along with another involving an Ethiopian Airlines aircraft in 2019, caused the grounding of B-737 MAX airliners. The B-737-500 model involved in Saturday's crash was an earlier generation not affected by the grounding.