A crashed Indonesian passenger jet’s throttles showed an “anomaly” and had been repaired several times before the deadly accident, but the exact cause of last month’s fatal dive was still unclear, investigators said Wednesday.
A potential malfunction in the Sriwijaya Air Boeing 737-500’s engine control system was highlighted in a preliminary report on the January 9 crash which killed all 62 passengers and crew, including nearly a dozen children.
The 26-year-old plane — previously flown by US-based Continental Airlines and United Airlines — plunged around 10,000 feet (3,000 metres) and crashed into waters off Jakarta just minutes after takeoff.
On Wednesday, investigators said they were probing the autothrottle system as they published their interim report.
“The left (engine throttle) was moving backward too far while the right one was not moving at all — it was stuck,” said National Transportation Safety Committee investigator Nurcahyo Utomo.
“But what would have caused this anomaly? We can’t conclude anything just yet.”
Authorities have previously said the crew did not declare an emergency or report technical problems with the aircraft before its dive, and that it was probably intact when it hit the water.
They cited a relatively small area where the wreckage was scattered and details from a retrieved flight data recorder — one of two so-called “black boxes” — showing the engine was still running just before it crashed.
Communications with air traffic control were described as normal up until the moment that the plane sharply deviated from its intended course and crashed.
The crew, including an experienced captain, did not reply to questions about the plane’s change of direction.
Crews on previous flights had reported that the system was “unserviceable” and it had been repaired several times before the fatal crash, the report said.
But it did not pinpoint the system as the cause of the accident, and the probe would also look at other potential factors, including pilot error.
“The investigation is ongoing and focuses on, but not limited to, review (the) autothrottle system and related component, including its maintenance, and human and organisational factors,” the report said.
While the flight had been delayed due to bad weather, there was no indication that the conditions played a pivotal role in the accident, authorities said, adding that two other commercial planes flew the same route without incident just before and after the plane.
Divers were still hunting the seabed for a still-missing cockpit voice recorder, which tracks flight crew conversations, and could shed more light on the accident.
Black box data includes the speed, altitude and direction of the plane as well as flight crew conversations, and helps explain nearly 90 percent of all crashes, according to aviation experts.
A team from the US National Transportation Safety Board is taking part in the investigation, along with staff from Boeing and the Federal Aviation Administration.
Sriwijaya Air, which flies to destinations in Indonesia and across Southeast Asia, has previously had safety incidents — including runway overruns — but no other fatal crashes since starting operations in 2003.
Indonesia, an archipelago that relies heavily on air transport to connect its thousands of islands, has suffered a string of deadly plane crashes in recent years.
In October 2018, 189 people were killed when a Boeing 737 MAX jet from Lion Air plunged into the sea.
That accident — and another in Ethiopia — led to the worldwide grounding of the 737 MAX over a faulty anti-stall system.
The 737 that crashed last month was not a MAX variant.