Accident Overview
While in cruise on a night cross-country flight, the airplane's engine lost power. The pilot attempted a forced landing to a nearby airport, but the airplane touched down in a field short of the runway. The engine had accumulated 265.5 hours since factory remanufacture.
Engine Examination
During examination of the engine, the number 3 connecting rod was found separated from the crankshaft. The number 4 piston exhibited a hole burned through the crown structure of the piston. The hole was located along a crack that extended across the top of the piston, in line with the piston pin. Metallurgical examination determined that fatigue cracking of the piston initiated from a stamp mark "4" on the top surface of the piston. The stamp mark, which was made by the manufacturer during engine buildup, acted as a stress raiser and was responsible for initiation of the crack. Once the crack extended to the interior surface of the piston, hot combustion gases created the burn-through hole and escaped into the crankcase, leading to engine failure.
Previous Incident
The accident airplane had experienced a previous in-flight engine failure in which an engine teardown revealed a piston with a burned-through hole in the crown structure of the piston. That engine had accumulated 640 hours since factory remanufacture; however, that piston did not have any stamp marks on its top surface.
Thermocouple Leads
The Turbine Inlet Temperature (TIT) gauge, thermocouple, and probes were removed from the accident airplane, and the thermocouple leads were found spliced in two locations. According to the aircraft's maintenance manual, faulty thermocouple leads should be replaced. They should never be spliced, as this will change the resistance of the leads, which is critical for the proper operation of the TIT gauge. The pilot stated that he followed the leaning procedure in the airplane's operating handbook and leaned the engine by referencing the TIT gauge.