KNKT Investigation Results In Sriwijaya Air SJ 182 Accident: Mechanical System Disorder

Screenshot of the Head of the KNKT Aviation Accident Investigation Sub Committee, Nurcahyo Utomo in a Hearing Meeting with Commission V DPR RI in Jakarta/ANTARA

The National Transportation Safety Committee (KNKT) submitted the results of an investigation into the Sriwijaya Air plane crash coded flight SJY 182 on the Jakarta - Pontianak route, which occurred on January 9, 2021, to Commission V of the Indonesian House of Representatives. Chairman of the KNKT Aviation Accident Investigation Sub Committee Nurcahyo Utomo said the investigation team believed there was a disturbance in the mechanical system of the Boeing 737-500 aircraft with the PK-CLC registration. "From the Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR) that we downloaded the data, when the plane boarded there was a change in the auto pilot mode, which previously used computers, moved using panel control mode," said Nurcahyo in a Hearing Meeting with Commission V DPR RI in Jakarta, Antara, Thursday, November 3. Nurcahyo explained that in normal operations, auto-throttle will drive both thrust levers backwards to reduce engine power. But what happened with the US manufacturer was that the auto-throttle could not move the right thrust lever. The NTSC, he continued, had examined seven components so that it was confirmed that there were mechanical disturbances on the aircraft, not on the computer system. "Due to the tight flight that day and coincidentally there was a plane with the same destination, the SJY182 flight was asked by Air Traffic Controller (ATC) to stop at an altitude of 11,000 feet," he said. Furthermore, Nurcahyo revealed that before an altitude of 11,000 feet, the engine power was decreasing because the right thrust leak did not move, which caused the difference in the power of the left and right engines to get bigger, or referred to as as as asymmetry. He said the asymmetry raises a difference in engine power that produces a force that deflects the plane to the left. The force to the left is larger than the force that deflects to the right by the aileron and flight spoiler so that the plane turns left. The delay in the Cruise Thrust Split Monitor (CTSM) to cut auto-throttle during asymmetry due to flight spoilers gives a lower value resulting in a larger asymmetry. "The lack of monitoring of tilted steering instruments and positions may have led to the assumption that the plane is tilted so that recovery measures are not appropriate. This recovery cannot be carried out effectively and on time," he said. He added that the investigation process was led by the NTSC and carried out in accordance with the provisions of the International Civil Aviation Organization (ICAO), involving aircraft manufacturing countries, namely Boeing from the United States, the Singapore Transport Safety Investigation Bureau, the UK Air Accident Investigation Branch, to the General Electric engine factory.