On February 2, 2007, a Republic RC-3 (registration N6248K) was involved in an aviation accident near Julian, CA. Investigators recorded the probable cause as: The failure of the propeller to crankshaft attachment bolts due to fatigue. A contributing factor was the improper installation of the propeller assembly by maintenance personnel. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During cruise flight, the propeller assembly detached from the crankshaft flange and fell from the airplane, prompting a forced landing to an open field. Examination found that all six attachment bolts fractured due to fatigue cracking, with no material deficiencies detected.
Event Description
During cruise flight, the propeller assembly separated from the crankshaft flange and departed the airplane. The pilot subsequently executed a forced landing to an open field. The propeller assembly had recently been overhauled and had been installed on the engine for approximately 8 hours prior to the separation.
Examination Findings
Examination of the airframe revealed that the six propeller-to-crankshaft attachment bolts were fractured. All bolts failed as a result of fatigue cracking. No chemical composition or other material deficiencies were found on the bolts. The presence of equal amounts of fatigue damage on all six bolts suggested that the bolts were equally affected. While bolt fatigue is commonly associated with insufficient preload from under-torquing, improper installation, or loss of preload during service, the investigation did not establish a specific cause for the fatigue in this instance.
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