BANGKOK — A preliminary investigation into the crash in Indonesia last month of Sriwijaya Air Flight 182 has found that a difference in the level of thrust between the plane’s two engines may have contributed to the aircraft rolling over before it plunged into the Java Sea, investigators said Wednesday.
As in most crashes, the investigators are looking at a range of factors that may have contributed to the cause. They are assessing information from the flight data recorder and the plane’s maintenance records, but searchers have not recovered the cockpit voice recorder, which would tell them what the pilots were saying in their final minutes.
A difference in the level of thrust — the force of the engines that propels the aircraft forward — can make planes difficult to control. It is not clear, at this point, why that problem may have occurred during the Sriwijaya flight.
Many questions remain unanswered, including why the pilot and co-pilot were unable to recover control of the plane before it plummeted more than 10,000 feet in less than a minute, said Nurcahyo Utomo, chief of the investigation team.
“What happened? We don’t know,” he said after the 31-page preliminary accident report was released by Indonesia’s National Transportation Safety Committee. “What are the problems? We cannot answer that yet. We still cannot explain if there was an auto-throttle malfunction.”
The Boeing 737-500 crashed minutes after taking off on Jan. 9 from Soekarno-Hatta International Airport in Jakarta, the Indonesian capital. The crash killed all 62 people aboard, including six active crew members.
One focus of the investigation has been the auto-throttle, which is a system that controls the thrust of a plane’s engines and is separate from the autopilot function. In the event of an auto-throttle malfunction, it is typical for a pilot or co-pilot to simply turn the system off. The auto-throttle uses data entered by the pilot to automatically regulate the two levers in the cockpit that control the thrust of the engines.
The auto-throttle system on the Sriwijaya aircraft had malfunctioned twice in the weeks before the crash and had been fixed after both incidents, the plane’s repair records show.
Flight data indicate that when the aircraft reached an altitude of 8,150 feet during the fatal flight, the thrust lever for the left engine was reduced while the right engine’s thrust lever remained unchanged. At 10,600 feet, the differential between the thrust of the two engines may have caused the plane to start rolling to the left, the report said.
Despite the problems with the plane’s auto-throttle on earlier flights, Mr. Nurcahyo said there was no clear evidence that it malfunctioned on this flight. In addition, there are many airline systems that can affect the engines’ thrust.
The auto-throttle itself receives input from 13 aircraft components, he added, any of which could have contributed to the problem.
“So why the anomaly occurred in the throttle, which components caused it, we still cannot determine,” he said.
Prita Widjaja, a senior investigator with the transportation committee, said one critical question that remained was how far the lever for the left engine moved and how much the aircraft’s power was reduced.
Indonesian investigators were required by international convention to release a preliminary report 30 days after the air disaster.
Because of the coronavirus pandemic, the 26-year-old aircraft had not been flown for nine months before going back into service in December. It then flew 132 flights in the weeks leading up to the crash. Investigators did not cite any evidence that the hiatus contributed to the disaster.
Three days after the crash, divers recovered the airplane’s flight data recorder, one of the so-called black boxes, which records information about the jet’s mechanical operation. It has provided important evidence in the investigation.
But the inquiry has been hindered by the inability of searchers to locate the crucial memory module of the other black box, known as the cockpit voice recorder. That device records sounds in the cockpit, such as engine noises and conversations between the pilot and co-pilot.
Divers retrieved most of the cockpit voice recorder, but the memory unit broke loose from the casing and searchers fear it is buried in a layer of seafloor mud that is two to three feet thick. They are still searching for the module in waters about 60 feet deep, but the effort has been hampered by rough weather and poor visibility.
The relatively small size of the plane’s debris field — about 260 feet by 360 feet — indicates that the plane was intact when it hit the water and broke up on impact, not in the air.
The Sriwijaya aircraft was the third to crash into the Java Sea in just over six years after departing from airports on Java, one of Indonesia’s five main islands.
In December 2014, Air Asia Flight 8501 crashed into the Java Sea off the coast of Borneo with 162 people aboard as it flew from the Indonesian city of Surabaya to Singapore. Investigators eventually attributed the disaster to the failure of a key component on the Airbus A320-200 and an improper response by the flight crew.
And in October 2018, Lion Air Flight 610 nose-dived into the Java Sea northeast of Jakarta minutes after taking off for Pangkal Pinang with 189 aboard. Investigators concluded that the anti-stall system malfunctioned on the Boeing 737 Max, a newer model than the Boeing that crashed last month.
A law firm in Illinois filed suit against Boeing last month on behalf of an Indonesian family that lost three relatives in the Sriwijaya crash. The suit claims that parts of the aircraft malfunctioned during the brief flight, possibly including the auto-throttle, causing the crash.