What happened
On 19 May 2014, a Cessna 152, registration G-BHYX, was conducting a private flight at approximately 13:27 UTC. While cruising at an altitude of roughly 2,300 ft, the pilot noticed the engine running roughly and a gradual reduction in RPM. The pilot attempted to rectify the issue by applying carburettor heat for approximately 15 to 20 seconds; while the RPM initially dropped during this application, it recovered once the heat was turned off.
Within a few minutes, the engine RPM decreased again, and the aircraft could no longer maintain its altitude. The pilot moved the throttle to the full forward position and verified that the magnetos were on both and the fuel primer pump was closed. After declaring a MAYDAY to Cranfield Approach, the pilot identified a suitable landing site. However, after spotting telegraph poles in the initial field, the pilot opted for a second field. During the landing, the aircraft touched down one third of the way into the field but could not stop in time, eventually striking bushes and entering a ditch at the field boundary. There were no injuries to the pilot or the passenger, though the aircraft sustained significant damage to the propeller, engine mounting frame, fuselage, and tail surfaces.
The investigation
Investigators examined the engine performance and environmental conditions leading up to the power loss. The pilot had been performing regular 'FREDA' checks, and the passenger, who was also a qualified pilot, had been assisting with the application of carburettor heat. The investigation looked into the possibility of carburettor icing, noting that the ambient temperature was 24ºC with a dew point of 12ºC. While the exact reason for the engine failure remained undetermined, the investigators noted that these atmospheric conditions presented a moderate risk of icing according to CAA guidelines.