What happened
The aircraft was operating at cruise altitude when a significant structural failure occurred in the propulsion system. A section measuring 17 inches of one propeller blade broke away from the main assembly, which had a diameter of 76 inches. The loss of the blade portion resulted in an immediate emergency situation requiring the pilot to execute a forced landing.
During the descent and subsequent touchdown phase, the aircraft impacted a fence post located on the ground. There were no reported injuries or fatalities associated with this impact.
The investigation
Post-accident examination of the propeller revealed extensive damage consistent with material fatigue and corrosion. Inspectors found filiform corrosion distributed across the backs of both blades. Metallurgical analysis of the fracture surface confirmed that the crack originated at a specific corrosion pit on the blade back.
Maintenance records indicated that the propeller had accumulated a total operating time of 2225 hours. The unit had been reconditioned by the manufacturer 1241 hours prior to the accident. At that time, the original reconditioning procedure required anodizing the propeller and applying a lacquer coating after repairs. Examination confirmed this process had been performed.
Findings
The investigation highlighted discrepancies between the maintenance history and current manufacturer recommendations. The manufacturer currently advises that this model of propeller be reconditioned every 1000 hours. Furthermore, the manufacturer has updated the reconditioning protocol to utilize a chemical conversion coating instead of the previous anodizing and lacquer method. Current guidelines also specify that the propeller should be frequently wiped with an oily rag between reconditionings to assist in corrosion prevention.
The failure was directly linked to the corrosion pit, which acted as the stress concentrator for the fracture.