History of the Flight
The pilot taxied the aircraft from its parking position to the fuel pumps with the intention of refueling prior to a local flight. He reported that the aircraft started at the first attempt and noted that he had experienced difficulty starting the engine the previous week, for which an instructor advised to continue cranking until it started. After refueling, with two passengers on board, the pilot attempted to start the engine while it was hot from the recent run. The engine did not start, so the pilot continued to crank it using the starter motor while advancing and retarding the throttle. He could not recall how many times the throttle was advanced or whether the engine was cranked continuously throughout the attempt. The engine still did not start, and the pilot noticed smoke issuing from the engine bay, followed by flames. He set the fuel cock to OFF and supervised the successful evacuation of his passengers. The airfield fire service arrived shortly after and extinguished the fire.
Aircraft Examination
The forward lower section of the engine cowling and the inside of the engine bay were extensively fire damaged, with the seat of the fire appearing to have been located at the bottom of the engine, close to the carburettor. An engineering inspection of the fuel system within the engine bay did not reveal loose fittings, split fuel hoses, or other pre-existing defects.
Discussion
The source of fuel for the fire was likely fuel pumped into the carburettor venturi by the carburettor accelerator pump, which operates when the throttle lever is advanced swiftly. The induction of fuel relies on airflow to move it into the engine inlet manifold. However, if the engine is stopped with the throttle lever advanced, excess fuel can drip downward from the carburettor and pool in the carburettor heat box. The heat radiated by the hot exhaust system can then cause the pooled fuel to vaporize and can also act as an ignition source. The Piper PA-28-151 owner's manual contains advice against over-priming and specifies a procedure for restarting after a failed start. In the absence of evidence of a pre-existing defect, it is likely that the engine fire was caused by over-priming the hot engine.