Indonesian investigators say doomed aircraft had trouble with the throttle

JAKARTA - The Indonesian passenger aircraft that plunged into the Java Sea shortly after take-off last month, killing all 62 people aboard, had a problem with its throttle control system, preliminary investigations by the country's National Transport Safety Commission (KNKT) showed.

Unequal engine thrust can cause a plane to roll onto its side and abruptly nosedive.

"The left throttle moved back too far, while the right throttle did not move at all as if it were stuck… The throttle system receives inputs from 13 other related components. We do not know which of these created anomalies (in both the left and right throttle)," Captain Nurcahyo Utomo, head of air safety investigation at the KNKT, told a virtual press briefing on Wednesday (Feb 10).

The plane had repeated problems with its automatic throttle system on two previous flights a few days prior to the crash on Jan 3 and Jan 4 and these were fixed, said the KNKT.

Captain Nurcahyo, however, said a problem with the throttle alone would not have caused the crash.

Investigations are continuing with the focus not only on the throttle system but on past maintenance, the human factor and the airline's organisation as well, said Capt Nurcahyo.

The final report of the KNKT investigation is due within a year of the crash in line with international standards.

The 27-year-old Boeing 737-500 belonging to Sriwijaya Air took off on Jan 9 in the afternoon from Indonesia's main gateway Soekarno-Hatta International Airport on the outskirts of Jakarta, en route to Pontianak, the provincial capital of West Kalimantan. It disappeared from radar four minutes after take-off.

On Jan 11, Capt Nurcahyo told The Straits Times the plane likely broke apart when it hit the water, noting that debris would have been scattered across a larger area if the aircraft had broken apart in mid-air.

Based on field data, the wreckage was found in an area between 300m and 400m in length and 100m in width.

The final piece of data received from the aircraft was when it reached 76m from the surface of the Java Sea.

Earlier, the head of KNKT, Mr Soerjanto Tjahjono, disclosed that the data indicated the aircraft system was functional. This, he said, apparently indicated that the engine was still operating before the plane hit the water.

Capt Nurcahyo said efforts were still underway to retrieve the memory unit of the aircraft's cockpit voice recorder (CVR), which would further help the investigation. So far, only the flight data recorder has been found.

The CVR would help investigators determine why the flight parameters, including altitude and direction, changed.

"Divers are being deployed again to the area on the seabed where the CVR is believed to be located, buried under thick mud," Capt Nurcahyo said.

He said the CVR would yield key information, including the conversation between the pilots and what happened in the cockpit during the flight.

"If we don't find the CVR, it would affect our investigation significantly," Capt Nurcahyo said.

Indonesia has seen two other major plane disasters in recent years - a Lion Air crash in October 2018, which claimed 189 lives; and another in 2014 when 162 people were killed after an an AirAsia jetliner went down in the Java Sea.