This February 2007 accident report is provided by the National Transportation Safety Board. Posted as an educational tool, it is intended to help pilots learn from the misfortunes of others.
Aircraft: Republic Seabee.
Location: Julian, Calif.
Aircraft damage: Substantial.
What reportedly happened: The airplane’s propeller was replaced prior to the accident flight. During cruise flight the propeller fell off. It had been in service for eight hours. The pilot executed a forced landing in an open field.
The post-accident examination revealed that six propeller to crankshaft attachment bolts were fractured and had failed as a result of fatigue cracking. Bolt fatigue is most commonly the result of improper torquing the fastener, or improper installation. The equal amounts of fatigue on all six bolts suggested that all bolts were equally affected.
Probable cause: 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.
For more information: NTSB.gov/ntsb/brief.asp?ev_id=20070209X00167&key=1.