ERA23LA361

The pilot reported that he was about 15 minutes into the cross-country flight when the engine made a “loud bang,” began to run rough, and eventually seized. Unable to glide to an open field, the pilot conducted a forced landing into trees. He and his passenger were not injured; however, the airplane was substantially damaged. Examination revealed several holes in the engine case, and one of the No. 2 crankshaft cheek counterweights was missing. Further examination revealed two of the counterweight retaining circlips were fractured. Examination of the fractured circlips revealed that one of the circlip legs failed from fatigue crack propagation. The engine had been disassembled and the crankshaft inspected due to a propeller strike about 130 operating hours before the accident.

On September 2, 2023, about 1100 eastern daylight time, a Beech V35 airplane, N5431U, was substantially damaged when it was involved in an accident near Rutledge, Tennessee. The airline transport pilot and one passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that the flight departed Gatlinburg/Pigeon Forge Airport (GKT), Sevierville, Tennessee, about 1045. The pilot proceeded northbound toward his destination, climbing to 2,500 ft mean sea level. About 15 minutes into the flight, over a forested area, the engine made a “loud bang” and started running rough. As the pilot turned to a southerly heading and an open field, the engine seized. Unable to land in the field, he performed a forced landing into trees. The wreckage came to rest inverted, and the pilot and passenger were met by first responders. Federal Aviation Administration inspectors responded to the accident site and examined the wreckage. There was no fire. The fuselage, wings, and empennage were substantially damaged. The external surfaces of the fuselage were covered with engine oil. After recovery of the wreckage, the engine was removed from the airframe for examination. Several small (about 0.75 inch) holes were found in the upper engine case, and one large hole was found under the area of the left magneto. The oil pan was removed and it contained engine oil and numerous metallic fragments. Further disassembly of the engine revealed that one of the No. 2 crankshaft cheek counterweights was missing. The largest hole in the engine case was directly adjacent to the area where the counterweight liberated. The retaining circlips and plates from the counterweight were found inside the engine. Two of the circlips were fractured, and two were bent but intact. The plates and circlips were forwarded to the NTSB Materials Laboratory for analysis. The circlips were arbitrarily numbered 1 though 4. Circlips 1 and 2 were intact and bent. Circlip 3 was fractured at both leg ends. An examination of both fracture surfaces revealed localized necking and cup-shaped fracture surfaces consistent with overstress. Circlip 4 also had both leg ends fractured. One of the fractures was similar to the circlip 3 fractures; however, the other fracture exhibited two thumbnail-shaped cracks along the inner surface. There were also fatigue striations consistent with fatigue crack propagation. A review of the aircraft maintenance records revealed that the engine was disassembled and inspected for a propeller strike on November 2, 2020, at 5,061 total airframe hours. The airplane had accrued about 130 hours since this inspection.