What happened
On 27 December 2004, a Yak-52, registration G-YAMS, was conducting a private flight near Osea Island when the engine began operating roughly. The pilot, who was flying solo, initially noticed an oily smell, increased vibration, and a change in engine noise. Shortly after, a puff of smoke was observed, and the engine RPM began to fluctuate cyclically.
Despite attempting emergency procedures, including using the primer and checking the magnetos, the engine performance continued to degrade. The pilot described the engine as running as if only five of the nine cylinders were functioning correctly. As the aircraft lost altitude, the pilot attempted to reach a nearby field but identified power lines at the edge of the chosen area. Consequently, the pilot elected to perform a forced landing in a series of paddocks.
During the landing, the aircraft struck the ground in a flat attitude. The left wing made contact with a fence post, causing the aircraft to rotate sharply to the left before coming to a halt against the fence. The impact caused significant damage to the wings, fuselage, landing gear, and propeller. The pilot sustained serious injuries, including head and chest trauma, though the use of a helmet and five-point harness likely prevented a fatal outcome.
The investigation
The AAIB examined the aircraft and the engine to determine the cause of the power loss. While investigators initially considered the possibility of fuel filters freezing due to water contamination, the filters were found to be clean and free of water.
Detailed inspection of the Ivchenko Vedeney .M-14P engine revealed that the high tension lead for the rear spark plug of the No 5 cylinder had been blown out of its fitting. Further examination of the plug itself showed that the ceramic insulating core had failed, and the inner electrode was missing. This failure allowed hot gases to escape, which caused significant localized heating to the plug's elbow fitting.
Findings
- The engine's rough running was caused by a failure of the No 5 cylinder rear spark plug, specifically the blowout of its ceramic insulating core.
- This failure resulted in a loss of compression in the fifth cylinder.
- The escape of hot gases from the cylinder through the damaged plug led to the disconnection of the high tension lead and heated the surrounding fitting.
- It is suggested that such failures may be caused by a mini hydraulic lock, where oil accumulation in the lower cylinders creates high stress on the plug cores during engine start.
- The spark plugs used were original Russian-manufactured components, which are noted to be less robust than some Western alternatives.