What happened
On 30 October 2003, a Yak-52, registration G-BWOD, was performing a series of touch-and-go landings at Conington Airfield, Cambridge, as part of a private flight. During the climbout from the third landing, at an altitude between 300 and 400 feet, the engine began to run roughly and experienced a loss of power. Simultaneously, the pilot noted a rapid increase in the cylinder head temperature.
In response to the engine issues, the pilot attempted to turn back toward the airfield. However, due to the aircraft's altitude and proximity to the runway, the pilot determined that a landing on the reciprocal runway was not feasible. After continuing downwind and attempting to regain height, the engine's performance deteriorated further. The pilot subsequently decided to land in a nearby field. During the final approach, the aircraft struck a row of fence posts, resulting in no fatalities or injuries to the two occupants. The impact caused extensive damage to both wings and the propeller, though the aircraft remained intact on its landing gear.
The investigation
An investigation into the aircraft's condition revealed that the engine's power loss and vibration were caused by the failure of a spark plug in the No 4 cylinder. Specifically, the insulator core of the plug had been lost, and the plug casing had been burnt through.
Examination of the damaged plug showed evidence of melting and pitting consistent with an electrical arc. This failure led to a loss of compression in the cylinder. The investigation established that the venting of combustion gases through the damaged plug body directed flames toward the cylinder head temperature sensor, explaining the rapid temperature rise observed by the pilot.