What happened
On 23 June 2000, a Piper J3C, registration G-BSYO, was conducting a private VFR flight from Pent Farm in Kent to Cranfield Airport. The flight began with the pilot having confirmed a full main tank and an estimated ten to twelve gallons in the auxiliary wing tank. During the initial climb, the pilot activated the fuel transfer selector to begin topping up the main tank from the auxiliary supply.
Roughly 45 minutes into the journey, the pilot observed that fuel was no longer visible in the transparent transfer pipe. Although the pilot could not verify the exact volume remaining in the auxiliary tank, the main tank's float gauge indicated it was still full, leading the pilot to believe the transfer had simply ceased while the main tank remained replenished. However, upon reaching Royston, the pilot noticed the main tank float gauge had become stuck, failing to show the expected drop in fuel level.
While established on the downwind leg for landing at Cranfield, the engine began to lose power. Despite attempts to restore power by pumping the throttle, the engine failed entirely at approximately 1,100 feet. The pilot managed to maneuver the aircraft into the wind and perform an emergency landing in a wheat field. The impact caused the aircraft to nose over and come to rest upside down, resulting in substantial damage to the engine, fin, rudder, and port axle. Both occupants escaped without injury.
The investigation
The investigation examined the fuel system and the pilot's monitoring of fuel levels. It was established that the pilot had visually confirmed fuel levels during pre-flight checks and observed fuel moving through the transfer pipe during the early stages of the flight. The investigation focused on why the fuel transfer stopped and why the pilot was unable to detect the decreasing fuel levels in the main tank.