History of the Flight
On 30 October 2003, at approximately 1450 UTC, a Yak-52 (registration G-BWOD) was engaged in private circuit flying near Conington Airfield, Cambridge. After an uneventful engine start and power checks, the pilot flew a series of touch-and-go landings. During the climb-out from the third touch-and-go at about 300 to 400 feet above ground level, the engine began to run roughly and lost power. The cylinder head temperature indication rose rapidly. The pilot initiated an immediate turn back toward the airfield, reducing power and announcing his intentions by radio. After completing the turn, he determined he was too high and too close to land on the reciprocal runway, so he continued downwind with the intention of landing on the take-off runway. When he opened the throttle to gain height, the engine again ran roughly, and he decided to land ahead in a field. With the landing gear down, the pilot saw a row of fence posts at the field's threshold. The aircraft struck three posts, causing extensive damage to both wings and separation of the outer end of one propeller blade. The aircraft remained intact and came to rest on its landing gear. The pilot later commented that the low altitude gave insufficient time to fully assess the symptoms before committing to the landing.
Aircraft Examination
The aircraft was recovered by a major UK importer of Yak aircraft. Examination established that the power reduction and vibration were caused by the break-up and subsequent blow-out of the insulator core of a spark plug in the No. 4 cylinder. The resulting loss of compression caused rough running, and venting of combustion gases through the spark plug body near a cylinder head temperature sensor produced the rising temperature indications. The damage sustained by the aircraft was significant, making the economic viability of repair questionable.
The failed spark plug carried markings identifying it as the standard Russian type used in the Vedeneyev M-14P engine. The casing showed a large melted and burnt-away sector; the ceramic inner core and central electrode were missing. Because the origin of the failure was no longer present, the underlying cause could not be established with certainty. The melted region's periphery was thickened and pitted, consistent with electrical arcing, and internal surface features near the burnt region also suggested arc damage. It could not be determined whether arcing was a primary cause or a secondary feature of another mechanical failure.
Additional Information
The UK importer reported that similar plug breakdowns—loss of the ceramic core and casing burn-through—had occurred previously on both UK- and US-based aircraft. During the incipient stages, after combustion gases begin leaking but before the core separates, progressive burn-through of the high-tension lead insulation typically occurs, often accompanied by a burning rubber smell in the cockpit. This interim period lasts about two minutes, during which cylinder head temperature rises progressively. No visible heat damage beyond the spark plug and harness was noted.
Anecdotal evidence indicates spark plugs have been found loose in cylinder heads of Yak-50 and -52 aircraft. Failures involving fracture of the body adjacent to the topmost thread, ejecting the upper part of the plug with core and electrode, also occur. One such failure in the United States in July 2002 resulted in loss of control and impact with the ground, causing serious injuries to the two occupants. Tests after that accident showed about a 13% reduction in static thrust with a single spark plug removed. Further anecdotal evidence suggests the engine can continue to produce significant power for an extended period under such conditions.
In summary, a body of anecdotal evidence exists of spark plug failures on Yak-50 and -52 aircraft involving ejection of the plug core, leading to loss of compression, rough running, and power reduction. These failures produce a combustion flame jet from the damaged plug and may indicate rapidly rising cylinder head temperature. The nature of these failures presents an unquantifiable but potentially serious risk of engine compartment fire, dictating that affected aircraft should land at the first available opportunity. However, anecdotal evidence also suggests the engine may continue to produce sufficient power for a safe powered landing at the earliest practicable airfield.