What happened
On 16 August 2015, an ATR 42-300, registration PK-YRN, was performing a scheduled passenger flight from Sentani to Oksibil, Indonesia. The flight, operated by PT Trigana Air Service, carried 54 people. During the descent, the aircraft was operating under visual flight rules, but the approach path was obscured by clouds. After making initial contact with the Oksibil Aerodrome Flight Information Services (AFIS) to report a left base leg approach, the crew ceased communication. The aircraft subsequently struck a ridge on Tanggo Mountain at approximately 8,300 feet. The impact and a subsequent fire destroyed the aircraft, and all 54 fatalities were recorded.
The investigation
The KNKT investigation focused on the flight path, weather conditions, and the functionality of onboard safety systems. Investigators analyzed the cockpit voice recorder (CVR), which revealed that the crew had not performed any checklists or briefings prior to the impact. Notably, the CVR did not record any terrain warnings. While the flight data recorder (FDR) could not be recovered, the investigation examined the aircraft's maintenance and operational history. The inquiry also looked into the status of the Enhanced Ground Proximity Warning System (EGPWS) and the crew's adherence to visual approach guidance in mountainous terrain.
Findings
- The primary cause of the accident was a deviation from the visual approach guidance while flying in conditions with limited visibility and no reliable visual reference to the terrain.
- The EGPWS was likely non-functional at the time of the accident, possibly because the circuit breaker had been pulled.
- The crew descended through clouds without a prior briefing regarding minimum safe altitudes, which contributed to a lack of situational awareness regarding terrain proximity.
- There was evidence that some pilots within the operator's fleet had a history of pulling the EGPWS circuit breaker, a behavior that had not been corrected by the company before the accident.