What happened
On a flight operating near the Kepulauan Serribu District, a Boeing 737-500, registration PK-CLC, experienced a catastrophic loss of control. During the flight, the aircraft's thrust levers became asymmetric, leading to an uncommanded flight path deviation. The crew attempted to manage the situation, but the aircraft entered an upset condition that resulted in a fatal crash with all occupants lost.
The investigation
Investigators from the KNKT examined the aircraft's maintenance history and flight recorder data. The inquiry focused on the functionality of the Automatic Flight Control System (AFCS) and the Autothrottle (A/T) system. Maintenance logs revealed a recurring history of issues regarding the autothrottle's ability to engage and maintain stability. Specifically, recent entries showed that the autothrottle computer had been replaced and cleaned multiple times in the months leading up to the accident due to persistent failures and the inability to maintain thrust symmetry.
Furthermore, the investigation included an observation of Upset Prevention and Recovery Training (UPRT) at a simulator facility. The team noted that training briefings were heavily reliant on digital aids rather than manual proficiency and that instructors did not sufficiently emphasize the risks of relying on Flight Director guidance during an upset.
Findings
- The primary cause of the accident was a thrust asymmetry event caused by the autothrottle system failing to maintain synchronized thrust levels.
- The autothrottle system experienced intermittent failures, with maintenance records showing the autothrottle computer and servos had undergone several recent repairs and replacements.
- During the flight, the thrust levers split, creating an uncommanded roll and pitch movement.
- Flight crew monitoring was compromised during the upset; the pilot monitoring was observed to be distracted by making emergency radio calls, which delayed the monitoring of airspeed and attitude.
- Training deficiencies were present, as the UPRT curriculum did not adequately prepare crews for scenarios where flight guidance systems might provide erroneous data during an upset.
Safety action
Following the investigation, several safety recommendations were directed toward the airline, the maintenance provider, and the manufacturer to address the identified systemic failures in maintenance management and pilot training protocols.