WPR25LA030

The pilot was conducting a local flight to try to replicate a fuel mixture leaning problem he experienced on the previous flight. The pilot experienced the leaning issues again and recognized unusual exhaust gas temperature (EGT) and fuel flow readings during the flight, so he elected to return to the airport. During the return to the airport the pilot heard two loud “booms” before the engine lost total power. Shortly after, smoke and flames appeared inside the cabin near the forward right floorboard. The pilot successfully extinguished the fire and performed a forced landing to an open field. The forward right area of the fuselage was substantially damaged. Examination of the engine revealed the No. 3 cylinder had separated from the engine case; the No. 3 connecting rod had fractured and mechanical damage was found to the engine case, camshaft, and crankshaft. Five of the eight No. 3 cylinder base nuts were not located. The examination revealed signs of fretting between both engine case halves, as well as on each of the six cylinder’s hold-down plates, consistent with a loss of torque of the engine case nuts and bolts and the cylinder base nuts. No other pre-impact anomalies were noted with the engine. Damage to the engine was consistent with the No. 3 cylinder base nuts losing their torque and the nuts coming off the studs, which enabled the cylinder barrel to come off the cylinder piston, resulting in mechanical damage to the studs, No. 3 connecting rod, engine case, camshaft, and crankshaft. The damage resulted in a fire originating in the engine compartment. Although the investigation did not determine why there was a loss of torque of the engine case nuts and bolts or the cylinder base nuts, the engine had operated about 72 hours since undergoing an annual inspection. FAA guidance for an annual inspection includes an inspection for visual evidence of excessive oil, fuel, or hydraulic leaks, and sources of such leaks, and directs inspection of engine studs and nuts for improper torquing. Accordingly, any improperly torqued engine studs or nuts should have been identified by the annual inspection.

On October 31, 2024, at 1620 mountain standard time, a Piper PA-32-300, N154DC, was substantially damaged when it was involved in an accident near Prescott Valley, Arizona. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 personal flight. According to the pilot, on the flight before the accident flight he leaned the fuel mixture less and the engine ran richer than he was accustomed to during climb out. On the accident flight, he elected to conduct a local flight to see if he experienced the same fuel mixture discrepancy. He reported no anomaly during engine startup, taxi, or pre-takeoff runup. During the takeoff climb, he noted that again he leaned the fuel mixture less and the engine ran richer than expected. The pilot noted unusual EGT and fuel flow readings during the flight and decided to return to the airport to have maintenance personnel look at the airplane before he flew it again. During the return to the airport the pilot heard two loud “booms” and then experienced a loss of total engine power. Shortly thereafter, smoke and flames appeared inside the cabin near the forward right-side floorboard area. The pilot successfully extinguished the fire with an onboard fire extinguisher and performed a forced landing to an open field. The forward right area of the fuselage sustained substantial damage. Postaccident examination of the wreckage showed the No. 3 cylinder had separated from the engine case. Outward deformation of both the upper and lower right engine cowlings was evident, consistent with impact from the separated No. 3 cylinder. Thermal damage was visible to the lower right engine cowling and the forward fuselage. Examination of the engine revealed the No. 3 connecting rod had fractured and mechanical damage was found to the engine case, camshaft, and crankshaft. Five of the eight base nuts for the No. 3 cylinder were not located. There was evidence of fretting at the engine case mating surfaces and on all six of the cylinders’ hold-down plates, consistent with reduced torque of the engine case nuts and bolts and the cylinder base nuts. At the time of the accident, the engine had a total time of 1,168.83 hours since its last overhaul. Engine logbook records revealed that the last overhaul was completed on October 9, 1997, during which internal engine components were replaced with Parts Manufacturer Approval (PMA) parts. The latest annual inspection of the engine was performed on June 27, 2024, about 72 operating hours before the accident flight. The pilot reported that he had identified an oil leak before the accident flight that concerned him, and he had intended to have maintenance personnel troubleshoot the leak. Title 14 CFR Part 43, Appendix D, Scope and Detail of Items (as Applicable to the Particular Aircraft) To Be Included in Annual and 100-Hour Inspections—Item (d)(1) directs an inspection for visual evidence of excessive oil, fuel, or hydraulic leaks, and sourcesof such leaks. Item (d)(2) directs inspection of engine studs and nuts for improper torquing. The pilot did not submit a Pilot Operator Aircraft Accident/Incident Report (NTSB Form 6120.1).