On October 30, 2000, at 1130 mountain daylight time, a Bell 206L-1 helicopter, N613, sustained substantial damage when a tail rotor blade failed during approach to a helicopter landing pad at Mesa Verde National Park, Colorado. The airline transport certificated pilot, the sole occupant, was not injured. The flight was a public use flight operating under Title 14 CFR Part 91 and no flight plan was filed. Visual meteorological conditions prevailed. The flight originated from the pad about 0930. According to information provided by the Department of Interior, Office of Aviation Services, the flight was on approach to land at the completion of a seeding flight when the pilot noted an increase in vibration level. The pilot landed the helicopter without incident. An examination following shut down of the helicopter provided information that a counterweight and part of the trailing edge had separated from one tail rotor blade. The resulting vibration fractured three of the four tail rotor gearbox mounts. The tail rotor blades had accumulated approximately 30.8 hours time in service since new. All failed parts were recovered at the accident site. Following the accident, repairs to the helicopter were made. New tail rotor blades were required and when one of the two blades was unpacked from its shipping container, the repair station noted what appeared to be a faulty counterweight attachment. They rejected the blade. The failed tail rotor blade, its mate, and the new rejected blade were sent to the National Transportation Safety Board's Materials Laboratory for examination. Their report of the examination is attached and provides evidence that the failed 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. The rejected new tail rotor blade examination provided evidence of uneven bonding of the inboard counterweight and deformation due to the riveting process. Deformation was also noted on the outboard counterweight. According to information provided by the Federal Aviation Administration, Rotorcraft Directorate, Composite Structures, LLC, Monrovia, California, manufactured all the tail rotor blades.