Beech C23 N793Y accident near Merrill, Wisconsin

Probable cause

The loss of engine power due to tappet failure.

NTSB analysis

The pilot reported that he was returning to the airport after a 1-hour flight when the engine started to run rough and gradually lost power. The pilot switched fuel tanks and cycled the magnetos, with no change in the power loss. He executed an off airport forced landing. The landing gear collapsed during the landing 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 that were determined to be consistent with tappet material. Further examination revealed 7 of the 8 tappets were destroyed. A metallurgical examination of the tappets 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.

The engine was overhauled 19 years before the accident and had accumulated 838 hours since the overhaul. The engine manufacturer recommends overhaul of the engine at 2,000 hours, or 12 years in service, whichever comes first. No record of tappet replacement was found in the maintenance records.

Text from the NTSB. Open the NTSB docket for CEN21LA213 for the full record.