On July 8, 2010, about 1305 Pacific daylight time, an amateur built experimental Johnson Helicycle helicopter, N62AJ, sustained substantial damage during a precautionary landing following an in-flight vibration while in the landing pattern at the Arlington Municipal Airport (AWO), Arlington, Washington. The helicopter was registered to and operated by the pilot under the provisions of Title 14 Code of Federal Regulations (CFR) Part 91. The private pilot, sole occupant of the helicopter, sustained minor injuries. Visual meteorological conditions prevailed and no flight plan was filed for the local personal flight. The flight originated from AWO about 5 minutes prior to the accident. The pilot reported in a written statement that after an uneventful flight, he entered a left downwind for his intended landing area at AWO. As the pilot lowered the collective to initiate a descent, he immediately experienced a "severe violent" vibration. The pilot stated that the vibration seemed to be "consistent with a once-per-main-rotor-revolution vibration. The entire aircraft was shaking so violently that it was impossible to read any instruments." The pilot reported that he raised and lowered the collective and noted no change in the vibration. He initiated an auto rotation to an open field adjacent to his intended landing area. Subsequently, the helicopter landed hard and came to rest upright. Examination of the helicopter by the Safety Board investigator-in-charge revealed that the tail rotor was separated from the tail boom. Additional damage was observed to the fuselage, tail boom, and main rotor blades. Flight control was obtained throughout the helicopters flight control systems. The source of the vibration was undetermined.