Flight Preparation and Departure
The pilot, a Swiss private pilot with a PPL(A) and 161 total flight hours (18:21 on type), planned to fly a passenger from Langenthal (LSPL) to Môtiers (LSTO). The pilot had never flown to Môtiers before. Before departure, he performed an external check and measured fuel in the wing tanks using a dipstick: 35 L in the right tank and 30 L in the left. He calculated about 2.5 hours of fuel endurance; the flight was expected to take 40 minutes. During the engine run-up, he checked the electric fuel gauges and noted fuel in both tanks.
Flight and Engine Power Loss
At 14:00 LT, the aircraft departed Langenthal runway 05. The pilot climbed to 3,300 ft and flew west below the Bern terminal control area. Near Lake Biel, he checked the fuel gauges and saw fuel remaining. Over Colombier at 5,000 ft, visibility was good, but clouds prevented a higher altitude. Before Noiraigue, he began a descent for a direct approach to runway 23, reducing engine RPM to 2,000. Believing carburetor heat was unnecessary at this RPM, he did not apply it. The engine then lost power without unusual noises. Moving the throttle and applying carburetor heat did not restore power.
Emergency Landing
Judging his altitude insufficient to glide to Môtiers, the pilot turned back to conduct an emergency landing in the valley between Travers and Noiraigue. The engine continued rotating but produced inadequate power. During the final approach, the aircraft struck treetops and made a hard ground contact, resulting in substantial damage. Both the pilot and passenger exited the aircraft uninjured.
Fuel System Investigation
After the landing, the remaining fuel in the wing tanks was measured: left tank maximum 5 L, right tank maximum 3 L, total 7.2 L. According to the aircraft flight manual, non-usable fuel per tank is 6.5 L. The last refueling on 27 September 2014 had added 80 L. Including the accident flight, four flights totaling 3 hours 10 minutes had been flown since then. Based on the manual's fuel consumption of 23 L/h at 2,500 ft and 2,600 RPM, the calculated consumption since refueling was 72.8 L, which plus the 7.2 L accounts for the 80 L. The flight from Langenthal to the accident site lasted 48 minutes, consuming approximately 18.4 L, indicating a fuel quantity of about 25.6 L before departure.
Dipstick and Gauge Accuracy Tests
Tests were conducted on the accident aircraft with the wings attached and functioning electric gauges. With 30 L per tank, dipstick measurements were accurate. With 12.8 L per tank (the estimated pre-flight amount), measurements varied from 13 L to 25 L depending on dipstick movement or aircraft rocking. The electric fuel gauges also showed significant errors; for example, with 0 L in the left tank, the gauge indicated one needle width below E (empty), while with 12.8 L, it indicated 0.25 needle widths above E. The right tank gauge showed similar inaccuracies.
Summary
The investigation found no technical cause for the engine power loss; it was most likely due to fuel starvation. The electric fuel gauges were inaccurate, and dipstick measurements were prone to error, especially at low fuel levels.