What happened
On 31 July 2011, a Thruster T600N aircraft, registration G-CCRN, was conducting a private flight near Keal Coats, Lincolnshire. During the approach to land, the pilot attempted to adjust the engine power to maintain sufficient airspeed and height while turning onto the final approach. However, the engine failed to respond to the increased power demands.
Recognizing the loss of performance, the pilot opted to perform a forced landing in a stubble field directly ahead. Upon touchdown, the nosewheel struck soft, silty soil, causing it to break off. This impact caused the aircraft to flip onto its back. There were no injuries to the pilot or the single passenger, both of whom were able to exit the wreckage without assistance.
The investigation
The investigation focused on the engine's failure to respond to throttle adjustments. An engineer with experience in Jabiru engines examined the possibility of fuel delivery issues. It was determined that fuel had been observed leaking from the carburettor.
The investigation established that the engine's failure to maintain power was likely due to an over-rich fuel mixture. The engineer suggested that fuel may have overflowed from the carburettor bowl via the vent pipe. This overfilling could have been caused by excessive crankcase pressure resulting from an overactive mechanical fuel pump, which would create enough pressure to overcome the carburettor's float valve.