What happened
During the climb-out phase from the departure airport, the pilot of the Lycoming powered aircraft experienced an engine imbalance and a noticeable misfire. Following these symptoms, the pilot attempted to return to the airport. Shortly after this decision, the engine backfired and suffered a total loss of power. The pilot subsequently performed a forced landing, during which the aircraft struck trees. The impact resulted in substantial damage to the airplane.
The investigation
An examination of the engine identified that a helicoil had been cross-threaded in the number 3 cylinder head. Investigators found metallic particles adhered to both the cylinder head and the top of the piston. Records indicated that thirteen days prior to the accident, the pilot had reported difficulty clearing a spark plug and subsequently removed the number 3 cylinder bottom spark plug. At that time, an uncertified mechanic cleaned the spark plug hole, replaced the helicoil, and installed a new spark plug after stating no steel particles were present in the cylinder.
Findings
- The engine failure was caused by metallic debris entering the combustion chamber.
- The work involving the helicoil replacement was performed by an uncertified mechanic without the supervision of a certificated mechanic.
- Contrary to Textron Lycoming Service Instruction No. 1043A, which requires provisions to prevent aluminum shavings from entering the combustion chamber when replacing a spark plug helicoil with the cylinder assembled, no such precautions were taken.