Background
On May 11, 2021, at 1500 central daylight time, a Beech C23, registration N793Y, was substantially damaged during a forced landing near Merrill, Wisconsin. The pilot and passenger sustained minor injuries. The flight was operated as a personal flight under Title 14 Code of Federal Regulations Part 91.
Accident Sequence
The pilot reported returning to the airport after a one-hour flight. While approximately 5 miles from the airport and descending through about 1,300 feet above ground level, the engine began to run rough and gradually lose power. The pilot switched fuel tanks and cycled the magnetos, but engine power did not change. The pilot executed an off-airport forced landing. During landing, the landing gear collapsed, and the airplane slid to a stop, coming to rest upright. Both wings and the fuselage sustained substantial damage.
Examination Findings
Examination of the engine revealed metallic particles consistent with tappet material in the oil filter. Further inspection showed that 7 of the 8 tappets were destroyed. The tappets were sent to the National Transportation Safety Board Materials Laboratory for examination. The lab determined that the tappets fractured from overstress. Fractures initiated along the heads facing their corresponding camshaft lobes and propagated towards the back surface. No indications of pre-existing cracks, such as fatigue, were found; however, there was evidence of pitting and surface wear on many tappet head faces that contact the camshaft. Notably, the No. 2 exhaust tappet exhibited wear significant enough to alter its geometry. Its face was reflective and polished from repeated contact with the mating camshaft lobe, and the center depression showed circular wear marks. The fracture surface was circular, conforming to the stem radius. Widespread spalling indicated substantial material loss.
Maintenance History
Review of engine logbooks showed 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 accumulated about 838 hours since overhaul. No record of tappet replacement was found. The engine manufacturer recommends overhaul at 2,000 hours or 12 years in service, whichever comes first.
An oil sample collected before the accident flight on May 11, 2021, was sent for analysis. The vendor’s report found nothing remarkable.