Incident
During the climb-out from the departure airport, the pilot noticed that the engine felt off balance and began to misfire. The pilot initiated a return to the airport, but shortly thereafter the engine backfired and lost all power. The pilot then performed a forced landing, during which the airplane impacted trees and sustained substantial damage.
Examination
Post-accident examination of the engine revealed that a helicoil insert was cross-threaded in the number 3 cylinder head. Additionally, metallic particles were found adhered to the cylinder head and the top of the piston.
Maintenance Background
Thirteen days before the accident, the pilot had reported difficulty clearing one of the spark plugs and subsequently removed the number 3 cylinder bottom spark plug. A mechanic then cleaned the spark plug hole, assured that no steel particles remained in the cylinder, replaced the helicoil, installed a new spark plug, and ran the engine. The mechanic who performed the helicoil installation was non-certificated, and at the time of the work, no certificated mechanic was present.
According to Textron Lycoming Service Instruction No. 1043A, if a spark plug helicoil insert replacement is performed with the cylinder assembled on the engine, "some provision must be made for preventing the aluminum shavings from entering the combustion chamber."