History of the Flight
The pilot departed from a farmstrip near Hook Norton at 1430 hrs for a local flight. The weather was clear with a light wind from the north and a surface temperature of -2°C. At approximately 1515 hrs, while in level cruise at 2,500 ft and two miles south of the farmstrip, the engine stopped. The pilot attempted to restart the engine; it turned over and fired but would not restart. He then carried out a forced landing in a field.
Injuries and Damage
No injuries were sustained by the pilot, who was the sole occupant. The aircraft sustained damage to the fin, a crack in the left wing, broken engine mounting, and damage to the firewall. The nosewheel assembly and propeller were also damaged. During the landing roll on the uneven frozen surface, the nosewheel assembly failed, causing the aircraft to roll over. The pilot exited the aircraft without injury.
Engineering Investigation
The aircraft was removed to a repair agency where the engine and fuel system were inspected. Engineers observed rust in the gascolator, suggesting water had been present in the fuel at some stage. The fuel tanks were drained prior to transport without recording the volume remaining in each tank, and no sample was retained for analysis. The pilot reported that both tanks should have been approximately half full at the time of the power failure, and he had confirmed tank contents using a fuel level sight glass prior to departure. He routinely checked the aircraft fuel drains every second or third flight and rarely observed water in the fuel. However, he confirmed that fuel was not checked for water during his pre-flight routine for the accident flight. All other inspections of the aircraft revealed no abnormalities, and repair work was ongoing. The engine has conventional carburettors but no carburettor heat system, relying on ambient air temperature beneath the engine cowls to prevent ice formation. The pilot advised that this system worked well in his experience and that the engine had not demonstrated rough running symptoms prior to power failure. He also reported having the cabin air heat turned up high during the flight, which draws heat away from the exhaust system, potentially reducing the temperature of ambient air around the engine.
Analysis
In the absence of a confirmed defect on the aircraft, the most likely causes of the power failure are fuel starvation or carburettor icing. The traces of rust in the gascolator suggest that water may have been present in the fuel system, which was not drained prior to the accident flight. The weather had been particularly cold in the period preceding the flight, which may have resulted in increased formation of condensation within the fuel system. The air temperature during the flight was cold enough to cause any water in the fuel system to freeze, potentially creating a partial or complete blockage in the fuel supply, causing the engine to stop and preventing restart despite adequate fuel remaining. Ice formed from moisture in the air blocking the carburettor cannot be ruled out, though it is considered less likely in this accident.