FINAL REPORT OF SJY182 ACCIDENT
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 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