Investigation Summary
The accident was rooted in a long-standing, unresolved issue with the aircraft's autothrottle system. In the days leading up to the crash, multiple pilots had reported the autothrottle as unserviceable. Maintenance crews attempted to rectify the issue by cleaning electrical connectors and switches, but the underlying mechanical friction or binding in the thrust lever assembly remained undetected. During the accident flight, as the aircraft climbed through 8,150 feet, the left thrust lever began to retard while the right lever remained stuck. This created a significant thrust asymmetry. The Cruise Thrust Split Monitor (CTSM), designed to disengage the autothrottle in such events, failed to activate in time because it received an undervalued spoiler angle input. The crew, likely suffering from automation complacency and distracted by weather avoidance maneuvers, did not notice the developing asymmetry or the resulting left roll until the aircraft reached an extreme bank angle. Crucially, the crew suffered from spatial disorientation; when the aircraft was in a steep left bank, the Captain made a right-wing-up command (turning the yoke further left), which exacerbated the upset. Their subsequent recovery inputs were inadequate, and the aircraft entered a high-speed dive from which it did not recover.
Final Conclusions
The NTSC concluded the crash resulted from a combination of mechanical failure and human factors. The primary mechanical trigger was the friction in the right thrust lever system which the maintenance process failed to identify. The operational failure was attributed to the flight crew's lack of monitoring, which allowed the thrust asymmetry to develop into a full aircraft upset. Furthermore, the lack of national standards for Upset Prevention and Recovery Training (UPRT) in Indonesia meant the pilots were not adequately prepared to recognize and recover from the specific upset condition encountered.
Photographic Evidence (1)
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Crew Experience
Captain
54-year-old former Indonesian Air Force pilot with extensive experience.
First Officer
33-year-old Pilot Flying (PF) during the climb; failed to monitor the developing thrust asymmetry.
Systems & Failure Modes
Autothrottle (A/T)
A system that automatically controls engine power; a mechanical binding in the right lever caused a thrust asymmetry.
Cruise Thrust Split Monitor (CTSM)
A safety system designed to disengage the autothrottle if a significant thrust lever split occurs; it failed to trigger due to incorrect spoiler angle data.
Electronic Attitude Director Indicator (EADI)
The primary flight instrument for attitude; inadequate monitoring of this display led the crew to misidentify the direction of the roll.
Interesting Facts
- 01The aircraft had a history of autothrottle malfunctions on January 3, 4, and 5, 2021.
- 02The flight crew did not send a distress signal before the impact.
- 03The wreckage was found at a depth of approximately 16 meters, distributed over an 80 by 110 meter area.
- 04All 62 occupants were Indonesian nationals.
- 05The aircraft had accumulated 62,983 flight hours and 40,383 cycles at the time of the accident.
Safety Actions & Advisories
UPRT Standardization
The NTSC recommended the implementation of national standards for Upset Prevention and Recovery Training (UPRT) to ensure pilots can effectively handle aircraft upsets.
Maintenance Procedure Review
Recommendations were made to improve maintenance processes for identifying mechanical friction or binding within thrust lever systems that electrical BITE tests might miss.