What happened
On 9 May 2010, a Cessna 120, registration G-AJJS, departed Sywell Airfield in Northamptonshire for a flight to Popham Airfield. The aircraft was carrying a pilot and one passenger, utilizing Mogas fuel with both wing tanks fully replenished before departure.
While cruising at approximately 1,800 ft, the engine experienced a sudden loss of power. In response, the pilot attempted to use carburettor heat as a precaution. Although engine power was briefly restored, the loss of power recurred. The pilot attempted to navigate toward Bicester Airfield; however, as the aircraft descended to roughly 200 ft, the engine ceased functioning entirely. The pilot performed a forced landing in a field adjacent to the airfield. While there were no injuries to the occupants, the aircraft sustained significant damage to the engine cowlings, fuselage, one wing, and the landing gear.
The investigation
The AAIB examined the circumstances surrounding the engine failure, focusing on the fuel system and environmental conditions. The investigation noted that the pilot had maintained the mixture in a rich setting and had both magnetos selected. While the pilot confirmed fuel was available in the active tank, he did not switch to the alternative tank during the emergency.
Investigators analyzed the weather conditions along the flight path, noting an overcast layer with an ambient temperature of approximately 5°C and a dew point of 2.0°C. The investigation also considered the implications of using Mogas, which can lead to vapour lock, though this was deemed unlikely given the aircraft configuration and temperature.
Findings
- The engine power loss may have been caused by debris obstructing the fuel flow, a scenario that could have been mitigated had the pilot switched to the secondary fuel tank.
- The prevailing weather conditions, characterized by low temperatures and high humidity, created a significant risk of carburettor icing.
- It remains possible that an unidentified mechanical or fuel system issue contributed to the failure.
- The use of carburettor heat applied by the pilot may have been insufficient to clear ice once the power loss had already become critical.