On March 15, 1986, a Cessna 404 (registration N408EX) was involved in an aviation accident near MC Rae, GA. Investigators recorded the probable cause as: Fatigue fracture of the connecting rod due to a surface inclusion in the rod shank, caused by oxides formed during heat treatment that were not completely removed prior to shot peening. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During cruise, a pilot heard a loud bang from the right engine, which feathered. The aircraft diverted and landed safely. Teardown revealed a fractured connecting rod due to a fatigue fracture from a surface inclusion caused by oxides formed during heat treatment.
Incident Overview
During cruise flight, the pilot of a single-engine aircraft heard a loud bang emanating from the right engine. The right propeller subsequently feathered. The pilot diverted the aircraft to an alternate airport, where a landing was performed without further damage and no injuries to the pilot.
Engine Examination
A subsequent teardown of the engine revealed a fractured connecting rod. Detailed examination of the rod identified a fatigue fracture that originated from a surface inclusion located in the rod shank. The inclusion was determined to have resulted from oxides that formed during the heat treatment process. These oxides were not completely removed prior to shot peening, leading to the initiation of the fatigue crack.
Findings
The investigation concluded that the engine failure was caused by a fatigue fracture of the connecting rod, which was attributable to a surface inclusion stemming from residual oxides left after heat treatment and insufficiently removed before shot peening.
Probable cause
Fatigue fracture of the connecting rod due to a surface inclusion in the rod shank, caused by oxides formed during heat treatment that were not completely removed prior to shot peening.