Cessna T210N N1925U accident near East Sparta, Ohio

Probable cause

A total loss of engine power due to a fatigue failure of the No. 2 cylinder exhaust valve.

NTSB analysis

The pilot reported that during cruise flight the engine began to run rough with a sudden decrease in engine manifold pressure and exhaust gas temperature for cylinder Nos. 2 and 4. The engine continued to run erratically for another 100 seconds before it lost total power. The pilot made a forced landing on hilly terrain, during which the nose gear collapsed. The airplane’s engine mount and lower fuselage structure were substantially damaged.

Postaccident examination determined that the No. 2 cylinder exhaust valve fractured near the stem-to-fillet transition. The fracture surface exhibited features consistent with a fatigue failure. Additionally, the clearance from the exhaust valve stem to the valve guide exceeded the manufacturer’s service limit.

The No. 2 cylinder exhaust valve failed in fatigue likely due to the wear of the valve guide and rocker arm bearing systems. The wear of the guide allowed the exhaust valve to move so that it could no longer consistently contact the valve seat. The asymmetric load induced a bending moment on the exhaust valve stem that eventually resulted in the initiation of a fatigue crack along the stem edge. The wear of the rocker arm shaft and rocker arm bearing likely contributed to the exhaust valve failure by tilting the rocker arm and changing the angle at which it engaged the tip of the exhaust valve stem.

Text from the NTSB. Open the NTSB docket for CEN21LA472 for the full record.