What happened
While engaged in aerial application operations, one propeller blade separated from the aircraft. This mechanical failure necessitated a forced landing. During the landing roll, the aircraft collided with a tree.
The investigation
An examination of the propeller identified a fatigue fracture that originated at a score mark located within the hub threads of the blade. Investigators found deposits within the fracture area that were enriched by carbon and oxygen; graphite, which serves as the lubricant for the propeller assembly, is carbon-based. Further testing determined that the hardness and tensile strength of the blade were below the manufacturer's specifications.
Findings
Investigators concluded that the fracture likely opened during the previous propeller overhaul, which occurred approximately 583 operating hours before the accident. Additionally, the oil required in the hub to facilitate the early detection of blade cracks was absent from the assembly.