What happened
On February 12, 2017, a Scheibe SF25C Falke motor glider, registration JA233 and 0, operated by the Okayama Glider Club, experienced a total engine failure while conducting a flight training session. The flight, which included an instructor and a trainee, departed from Kohnan Aerodrome on runway 27.
After an initial period of flight using updrafts, the trainee successfully restarted the engine to prepare for landing. However, while the aircraft was holding at approximately 2,000 feet northwest of the aerodrome, the engine failed again as the trainee advanced the throttle. Despite attempts to restart the engine, the power lever failed to increase engine RPM. The instructor subsequently decided to perform a gliding landing, touching down on runway 27 and coming to a halt on the runway.
The investigation
The Japan Transport Safety Board (JTSB) investigated the incident, focusing on the atmospheric conditions and engine operation. Investigators noted that the aircraft was flying in an environment characterized by low temperatures and high humidity, with outside temperatures near 0 ºC and the presence of snow clouds.
Technical analysis revealed that the engine's oil temperature was below 40 ºC, which is significantly lower than the 70 ºC threshold recommended in the flight manual to mitigate icing risks. The investigation also examined the specific mechanics of the aircraft's carburetor, noting that unlike standard Venturi-type carburetors, this model's structure makes it difficult for pilots to detect icing until a change in throttle position disrupts the air/fuel ratio.
Findings
- The primary cause of the engine failure was carburetor icing.
- The icing occurred because the aircraft was operating at a low oil temperature while flying through high-risk airspace without the use of the carburetor heater.
- The pilot did not activate the carburetor heat in advance, likely because the engine appeared to be operating normally during the initial descent and holding phase.
- The engine was unable to restart because the continuous presence of ice prevented the achievement of a proper air/fuel ratio.