History of the Flight
The pilot, the sole occupant, departed from his private airstrip on the evening of 13 July 2005 for a club meeting at another airstrip approximately 20 minutes flying time away. After spending about an hour there, he flew to a friend's landing strip nearby and remained for half an hour. He then took off for the short return flight to his own airstrip. At a height of about 200 ft above ground level (agl), the engine stopped. Ahead were fields containing crops, so the pilot turned approximately 90° to the right to attempt a forced landing on a grass area adjacent to the strip. This turn also brought the aircraft into a headwind of about 5 kt. While clearing a hedge and passing upwind of a group of trees at about 30 ft, the aircraft suddenly stalled and impacted the ground nose-down at low speed. The aircraft sustained extensive damage, but the pilot suffered only minor injuries and was able to exit the wreckage unassisted.
Aircraft and Equipment Information
The aircraft was a Whitaker MW6 Merlin (Modified), three-axis microlight with a rigid wing mounted above a faired tricycle undercarriage. It was fitted with a two-cylinder, two-stroke liquid-cooled Rotax 582 piston engine driving a three-bladed propeller through a gearbox. The fuel system consisted of a main tank in the wing centre section, gravity-feeding to a header tank under the engine. The aircraft held a Permit to Fly based on inspections and recommendations by the Popular Flying Association (PFA).
Fuel and Refuelling Details
The aircraft was operated on motor gasoline (MOGAS) purchased from a local petrol station. The night before the accident, the aircraft was stored in a hangar with the fuel tank partially full. The next morning, the pilot refuelled using a different batch of fuel. The pilot stated that he did not test the fuel for water, as he had operated the aircraft for many hours without issue using the same practice.
Analysis of the Engine Failure
The engine was severely damaged by impact, preventing a definitive determination of whether the stoppage resulted from an engine malfunction or a fuel system problem. The pilot reported that colleagues had detected a paraffin-like smell in the exhaust and believed the failure was due to contaminated fuel. A fuel sample was sent for analysis, but the results were inconclusive at the time of the AAIB report. The possibility of water contamination was considered, but the aircraft had operated without problem on the same fuel for two previous flights, making significant contamination unlikely.
MOGAS has a higher vapour pressure than aviation gasoline, making engines susceptible to vapour lock—a condition where fuel vapourises due to low pressure or high temperature, blocking fuel supply. MOGAS use is restricted to altitudes below 6,000 ft and fuel tank temperatures not exceeding 20°C. Ambient temperatures on the accident evening were 24°C at the time of the accident and 27°C at the first takeoff. The fuel temperature likely exceeded 20°C while on the ground, especially if the aircraft was parked in direct sunlight, which could heat the wing tank well above ambient. Residual engine heat may have further raised the temperature of fuel in the header tank and supply lines.
Forced Landing Attempt
After the engine failure, the pilot chose not to land straight ahead in the crop field, fearing a rapid deceleration or overturn that could cause serious injury. Instead, he turned toward the grass area beside the airstrip. During the low-level turn, he believed that the trees shielded the aircraft from the wind, causing a loss of airspeed and leading to the stall.
Conclusion
The most likely cause of the engine failure was a vapour lock in the fuel supply, probably resulting from a combination of high ambient temperature, engine heating of fuel between flights, and possible solar heating of the wing fuel tank while parked. During the subsequent forced landing, the aircraft stalled at low altitude and impacted the ground.