Background
On 2 October 2015, a DHC-6 Twin Otter, registered PK-BRM, was operated by PT. Aviastar Mandiri as a scheduled passenger flight with flight number MV 7503. The aircraft had a valid Certificate of Airworthiness and was within weight and balance limits. Both pilots held valid licenses and medical certificates. The flight was the sixth sector of the day for the aircraft and crew, with previous legs including Makassar, Tana Toraja, Masamba, and Seko.
Flight
The aircraft departed from Andi Jemma Airport, Masamba (WAFM) at 1425 local time (0625 UTC) with an intended destination of Sultan Hasanuddin International Airport, Makassar (WAAA). The flight was conducted under Visual Flight Rules (VFR) at a planned cruising altitude of 8,000 feet. On board were 10 persons: two pilots and eight passengers, including one company engineer. The Pilot in Command (PIC) acted as Pilot Flying, and the Second in Command (SIC) acted as Pilot Monitoring.
At 0630 UTC, the pilot reported passing 4,500 feet climbing to 8,000 feet. At 0633 UTC, air traffic control instructed the pilot to call when reaching 8,000 feet. At 0636 UTC, the pilot reported reaching 8,000 feet and was assigned squawk code A5616. At 0637 UTC, the pilots discussed flying direct to BARRU, a town about 45 nautical miles north of Makassar. Both agreed to deviate from the company's visual route and proceed directly to BARRU on a heading of 200°.
Accident
At 0651 UTC, the PIC told the SIC that he wanted to climb. One second later, the cockpit voice recorder (CVR) recorded the sound of impact. The aircraft was on a straight and level flight path with no indication of evasive maneuvers. The Enhanced Ground Proximity Warning System (EGPWS) did not produce aural cautions or warnings prior to impact; the reason for this absence could not be determined by the investigation. The terrain at the accident area included high terrain and cloud formations, as indicated by satellite imagery from BMKG (Indonesian meteorological agency) at 0700 UTC. Local villagers reported cloudy conditions at the time.
Investigation
The investigation identified 15 findings and contributing factors. Key findings included that the pilots' decision-making process did not show concern for environmental conditions ahead, which involved higher risks and required correct flight judgment. The flight deviated from the visual route toward an area with high terrain and cloud formation. The CVR data did not record EGPWS alerts. The search and rescue agency did not receive a signal from the aircraft's emergency locator transmitter (ELT), likely due to the antenna detaching during impact. Additionally, special briefing on EGPWS operation had been performed, but no special training; the TAWS (Terrain Awareness and Warning System) operational test was not in the pilot checklist, and some DHC-6 pilots had not been briefed on TAWS and EGPWS operation.
Probable Cause
The investigation determined the following contributing factors: Deviation from the company visual route without properly considering the elevated risks of cruising altitude lower than the highest terrain and instrument meteorological condition, in addition to the absence of the EGPWS warning, resulted in the omission of avoidance actions.