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On 9 January 2021, a Boeing 737-500 aircraft, registration PK-CLC, was on a scheduled domestic flight, from Soekarno-Hatta International Airport (WIII), Jakarta, to Supadio International Airport (WIOO), Pontianak, and departed at 0736 UTC (1436 LT).

During climbing, the autopilot (A/P) directional control was changed from LNAV to HDG SEL and subsequently the vertical control changed to Pitch V/S and MCP SPD. These changes required less engine thrust therefore the engine power reduced. The FDR recorded that left thrust lever moved backward and the left engine thrust decreased, however the right engine remained at its climb power setting, resulting in an asymmetric thrust condition. The investigation concluded that the autothrottle (A/T) system command being unable to move right thrust lever was a result of friction or binding within the mechanical system except the torque switch mechanism. The maintenance record showed that the A/T problem was reported 65 times since 2013 and the problem was unsolved and still exist on the accident flight.

The Cruise Thrust Split Monitor (CTSM) system delayed to disengage the A/T and the thrust asymmetry continued to increase. The investigation believed that the delay of CTSM was due to an error in the spoiler signal value.

As the thrust asymmetry became greater, the aircraft turned to the left instead of to the right as intended. The aircraft entered an upset condition, and the pilot was unable to recover the situation. Inadequate of upset prevention and recovery training contributed to the inability of the pilot to prevent and recover from the upset condition.

The investigation concluded several contributing factors based on the safety issues identified following the accident.

The Komite Nasional Keselamatan Transportasi (KNKT) acknowledged that the safety actions taken by the related parties were relevant to improve safety, however there are safety issues that remain to be considered. The KNKT issued safety recommendations to address the safety issues identified in this report.

This investigation involved the participation of the National Transportation Safety Board (NTSB) of the United States of America as the State of Design and the State of Manufacture, the Air Accident Investigation Branch (AAIB) of the United Kingdom and the Transport Safety Investigation Bureau (TSIB) of Singapore as States providing assistance. All agencies have appointed their accredited representatives and advisers to assist in this investigation in accordance with the provisions in ICAO Annex 13.