History of Flight
The flight was intended for the re-validation of an IMC rating. The commander, a CAA PPL Examiner, was on board while the handling pilot, who was renewing his IMC rating, controlled the aircraft. The handling pilot performed a walk-round of the aircraft, paying particular attention to the engine, which had been replaced approximately four flying hours prior. No problems or fluid leaks were observed. After a normal engine start, the aircraft taxied to the run-up area. Engine run checks, with fuel selected from both tanks, were uneventful and showed normal engine operation.
Takeoff was planned from Runway 05, requiring a backtrack of about 8 km followed by a 180° turn to align with the runway centerline. The taxi was uneventful, but as the pilot began the turn, the engine suddenly stopped. The pilot immediately attempted a restart using the hot engine start procedure, which was unsuccessful. A second attempt also failed. Controllers in the tower then informed the crew that flames were visible from the engine area, although neither pilot had noticed. The commander ordered an evacuation, and both pilots exited via the cockpit door. The airfield fire service, already dispatched, extinguished the fire upon arrival.
Engine Examination
Post-incident inspection at a local maintenance facility revealed that the fire was concentrated around the carburettor heat box, with subsequent fire and heat damage to the lower engine cowlings and components mounted on the rear of the engine. No evidence of fuel leakage, loose fuel connections, or split fuel supply lines was found in the engine compartment. A test of the fuel system with the fuel pumps operating showed free fuel flow and no leakage. The engine rotated freely, and spark plugs indicated the fuel/air mixture was correctly set.
Discussion
The fire's concentration around the carburettor heat box, located directly below the carburettor, suggests it started from fuel dripping from the carburettor. When the throttle is advanced, the accelerator pump normally injects fuel into the venturi, relying on airflow to move it into the engine. Without airflow—as with the engine stopped—fuel can drip and pool in the carburettor heat box. It is possible that during the attempted restarts, excess fuel pooled there, and heat from the hot exhaust system provided the ignition source for the fuel vapour. The reason for the initial engine stoppage was not determined. The maintenance organisation plans to rebuild and test the engine in the future; any significant findings will be reported in a later bulletin.