On May 11, 2021, at 1500 central daylight time, a Beech C23, N793Y, was substantially damaged when it was involved in an accident near Merrill, Wisconsin. The pilot and passenger received minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that he was returning to the airport after a 1-hour flight. When the airplane was about 5 miles from the airport and descending through about 1,300 ft above ground level, the engine started to run rough and gradually lost power. The pilot switched fuel tanks and cycled the magnetos, with no change in the engine power. He executed an off-airport forced landing. The airplane touched down, the landing gear collapsed, and the airplane slid to a stop coming to rest upright. The airplane sustained substantial damage to both wings and fuselage. Examination of the engine found that the oil filter contained metallic particles consistent with tappet material. Further examination revealed 7 of the 8 tappets were destroyed. The tappets were removed for further examination. Examination of the tappets by the National Transportation Safety Board Materials Laboratory revealed that the tappets fractured from overstress. The fracture started along the heads facing their corresponding camshaft lobes, which proceeded towards the back surface. While there were no indications of pre-existing cracks, such as from fatigue, there was evidence of pitting and surface wear on many of the tappet head faces that contact the camshaft. Of note was the No. 2 exhaust tappet, the only tappet that underwent wear on the tappet face significant enough to change its geometry. Figure 1 shows the No. 2 exhaust tappet face, which was reflective and consistent with polishing from repeated contact with the mating camshaft lobe. The depression at the center of the tappet head corresponding to the stem exhibited circular wear marks. The fracture surface on the bottom of the figure was circular, conforming to the geometry of the stem radius. Figure 2 shows the widespread spalling of the remaining surface, demonstrating the degree of material loss on this part. Review of the engine logbooks revealed that the engine was overhauled on February 19, 2002, and installed on the accident airplane on February 1, 2019. The last annual inspection was completed on February 24, 2021. At the time of the accident, the engine had about 838 hours of operation since overhaul. No record of tappet replacement was found in the maintenance records. The engine manufacturer recommends overhaul of the engine at 2,000 hours, or 12 years in service, whichever comes first. An oil sample collected on May 11, 2021, before the accident flight, was sent in for analysis. The vendor who conducted the oil sample analysis provided an oil sample report. Nothing remarkable was found in the oil sample report.

Figure 1. View of the #2 exhaust tappet head.

Figure 2. Worn areas of the #2 exhaust tappet head, showing spalling and pitting.