Flight Details
On 12 April 2017, a Cessna C208 aircraft registered PK-FSO was operated by PT. Spirit Avia Sentosa (FlyingSAS) for unscheduled cargo flights. The aircraft completed multiple legs earlier that day: Mopah Airport (WAKK) to Tanah Merah Airport (WAKT) with two pilots and seven passengers, then Tanah Merah to Oksibil Airport (WAJO), back to Tanah Merah, and another round. The accident flight departed Tanah Merah at 1144 LT in daylight conditions with one pilot on board, carrying 1,225 kg of general cargo and 800 pounds of fuel, sufficient for about three hours. The intended cruising altitude was 7,000 feet. No aircraft system malfunctions were reported or recorded before departure.
Accident Sequence
After departure, the pilot advised Tanah Merah tower of an estimated arrival at Oksibil at 1224 LT. At 1149 LT, the pilot reported the aircraft was about 10 nautical miles from Tanah Merah, passing 3,500 feet. The controller acknowledged and advised monitoring frequency 122.7 MHz. About 29 nautical miles from Oksibil, PK-FSO passed a Cessna 208B flying in the opposite direction at 6,000 feet. The other pilot reported seeing PK-FSO, and the PK-FSO pilot responded that he was maintaining 7,000 feet direct to Oksibil. At 1230 LT, Oksibil tower received a call from the FlyingSAS officer in Jakarta asking if PK-FSO had landed; the tower had no communication with the pilot and no flight plan for the second flight. At 1240 LT, the officer called again, reporting that the flight following system had received an SOS signal from PK-FSO, with the last position at 04°48'47.7" S, 140°39'31.7" E, about 6 nautical miles north of Oksibil. The controller notified the Search and Rescue Agency.
Discovery
On 13 April 2017 at 0711 LT, the PK-FSO aircraft was found on the ridge of Anem Mountain, about 7 nautical miles north of Oksibil.
Official Findings
The official probable cause stated the possibility of pilot fatigue, physical and environmental conditions that increased pilot sleepiness, which might have caused the pilot to inadvertently fall asleep, indicated by no pilot activity. The absence of a GA-EGPWS aural alert and warning was unable to wake the pilot.