What happened
During a night cross-country flight while in the cruise phase, the airplane's engine lost power. The pilot attempted to perform a forced landing at a nearby airport, but the aircraft touched down in a field located short of the runway.
The investigation
The engine had accumulated 265.5 hours since its factory remanufacture. Upon examination, investigators found that the number 3 connecting rod had separated from the crankshaft. Additionally, the number 4 piston showed a hole burned through the crown structure. This hole was situated along a crack that ran across the top of the piston, in-line with the piston pin.
Metallurgical analysis revealed that fatigue cracking of the piston began at a stamp mark "4" located on the top surface of the piston. This mark, applied by the manufacturer during engine buildup, functioned as a stress raiser that initiated the crack. As the crack reached the interior surface, hot combustion gases escaped through the hole into the crankcase, causing the engine failure.
Investigators also noted that the accident airplane had previously experienced an in-flight engine failure. A teardown of that engine revealed a piston with a burned-through hole in the crown, though that specific piston lacked any stamp marks on its top surface. This previous engine had 640 hours since factory remanufacture.
Furthermore, the Turbine Inlet Temperature (TIT) gauge, thermocouple, and probes were removed for inspection. The thermocouple leads were found to have been spliced in two locations. According to the aircraft's maintenance manual, faulty thermocouple leads must be replaced rather than spliced, as splicing alters the resistance of the leads, which is essential for the accurate operation of the TIT gauge.
Findings
- The fatigue crack in the number 4 piston was initiated by a manufacturer's stamp mark acting as a stress raiser.
- The presence of hot combustion gases escaping into the crankcase due to the piston failure led to the engine power loss.
- The pilot performed engine leaning procedures by referencing the TIT gauge, despite the thermocouple leads being improperly spliced.