What caused the Indonesian Plane Crash?

In a preliminary report on Sriwijaya Air Flight 182, investigators said the crash might have been caused by several factors, including a problem controlling the engines’ thrust.

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. 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 the 9th January 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.