What happened
On 26 July 2008, an Aeronca 7AC Champion, registration G-TECC, was conducting a private flight near Coningsby, Lincolnshire. Approximately 1 hour and 45 minutes into the flight, the engine suffered a complete loss of power at an altitude of 1,200 ft. At the time of the failure, the pilot had recently opened the fuel cock to transfer fuel from the left wing tank to the main tank, which was roughly three-quarters full.
The engine stopped almost immediately after the fuel transfer, without any prior signs of vibration or rough running. The pilot, managing the descent at best glide speed, selected a pasture field for a forced landing, choosing it over areas with standing crops or power lines. During the approach, the pilot navigated around livestock to align the aircraft for an upslope landing. Upon touchdown, the aircraft struck a transverse ridge, which caused the landing gear to collapse. There were no fatalities and no injuries to the pilot.
The investigation
The investigation examined the engine, the fuel system, and the landing site. The pilot considered several potential causes, including carburettor icing, fuel contamination, or vapour lock. While carburettor icing was a possibility based on environmental conditions, the pilot noted the absence of typical engine symptoms.
Post-accident inspections of the engine revealed that the engine had been operating with a very lean fuel/air mixture, as evidenced by the condition of the spark plugs and exhaust stacks. A critical finding was that the hose connecting the air intake duct to the No 3 cylinder inlet manifold was not properly secured. This defect allowed for a significant gap between the hose and the duct, which could permit unmetered air to enter the engine.
Findings
- The engine failure was caused by a lean fuel/air mixture resulting from an improperly secured air intake hose.
- The gap in the intake manifold allowed air to be entrained into the engine, destabilising the mixture.
- The landing gear collapse was caused by the aircraft striking a transverse ridge during the forced landing.
- The pilot's selection of the landing site avoided more hazardous obstacles such as power lines and crops.