Incident Overview
An aircraft conducting aerial application experienced a propeller blade separation during flight. The resulting forced landing roll ended with the aircraft colliding with a tree.
Investigation Findings
Examination of the propeller revealed that a fatigue fracture had originated at a score mark in the hub threads of the blade. Deposits enriched with carbon and oxygen were found within the fracture. The lubricant used in the propeller assembly, graphite, is carbon-based. Further testing showed that the blade's hardness and tensile strength were below manufacturer specifications.
The safety board believes that the fracture was opened up during the last propeller overhaul, which occurred approximately 583 operating hours prior to the accident. Additionally, oil that is required in the hub to allow early detection of blade cracks was absent.
Conclusion
The investigation determined that the blade failure resulted from a pre-existing fatigue crack that propagated due to substandard material properties and the lack of lubricating oil to indicate the crack's presence.