Incident Description
During the approach to land on a helipad at the completion of a seeding flight, the pilot reported an unusual vibration. The pilot conducted a normal landing, and after shutdown, a post-flight inspection revealed that one tail rotor blade had shed its trailing edge counterweight and trailing edge skin. Additionally, three of the four mounting points for the tail rotor gearbox had fractured.
Examination of Tail Rotor Blades
Both tail rotor blades, which had accumulated approximately 30.8 hours since new, were sent to the NTSB Materials Laboratory for examination. The helicopter was subsequently repaired, and when a shipping container containing new tail rotor blades was opened, the repair station rejected one blade due to visual abnormalities in the area of the trailing edge counterweight. That rejected blade was also sent to the laboratory for examination.
Laboratory Findings
The laboratory examination provided evidence that the tail rotor blade failed due to fatigue in the inboard and outboard skins, initiating at the root near the trailing edge counterweights. The fatigue initiated due to buckling deformation of the inboard skin, which was found to be thinner than that specified in the manufacturer's drawings.
Examination of the rejected tail rotor blade provided evidence of uneven bonding of the inboard counterweight and deformation due to the riveting process. Deformation was also noted on the outboard counterweight.
Manufacturer Information
Composite Structures, LLC, of Monrovia, California, manufactured all the tail rotor blades.