History of Flight
On 27 December 2004, a private Yak-52 (registration G-YAMS) departed for a local flight after having been flown earlier that day without incident. The pilot, the sole occupant, intended to perform aerobatics but decided against it due to an oil level exceeding the recommended amount for such maneuvers. Engine start and power checks were normal, and the aircraft climbed to 2,500 feet for cruise. During cruise near Osea Island, the pilot detected an oily smell, a change in engine noise, and increased vibration. Engine instruments appeared normal initially. Shortly after, a puff of smoke appeared in peripheral vision, and engine RPM began cycling. The pilot used the primer to inject fuel and performed emergency checks, including magneto checks, but the engine continued running roughly.
Forced Landing
Unable to maintain altitude, the pilot notified Southend Approach of the rough-running engine and intended to return, but at approximately 700 feet south of the River Crouch, he decided on a forced landing. Selecting a field to the east, he turned but saw power lines late on approach. He then chose a nearby paddock. The aircraft touched down in a flat attitude; the left wing struck a fence post, spinning the aircraft left until it stopped against a fence. The left wing fuel tank was punctured, but no fire occurred. The pilot, seriously injured, secured aircraft systems and was assisted by the land owner, then taken to hospital.
Injuries and Damage
The pilot sustained serious injuries despite wearing a five-point harness and helmet. His head struck the instrument panel, splitting the helmet, and he suffered serious chest injuries from the harness. Damage to the aircraft included wings, fuselage, landing gear, and propeller.
Aircraft Examination
Examination by the AAIB revealed that the No 5 cylinder rear spark plug had failed. The high-tension lead was blown out of the elbow fitting, and the plug's ceramic core was damaged with the inner electrode missing. The remaining spark plugs were satisfactory. The damaged plug, marked SD-49SMM, was a Russian original equipment manufacturer (OEM) part installed for about 60 hours, last inspected 11 hours prior. Fuel filters were clean with no water contamination.
Probable Cause
The engine's rough running was directly attributed to the failure of the No 5 cylinder rear spark plug. The inner insulating core had blown out, allowing hot gases to escape, compromising compression on that cylinder and causing rough operation.