On April 16, 1997, at 1424 hours Hawaiian standard time, a single-place Duff Mini-500 homebuilt experimental helicopter, N13118, was destroyed when it impacted the ground in the vicinity of Lihue, Kauai, Hawaii. The student pilot was fatally injured. The flight departed from the pilot's Anahole residence for an unknown destination. No flight plan was filed for the personal flight and no en route communications were received by any Federal Aviation Administration (FAA) facility. An eyewitness, who was approximately 1/4 mile from the crash location, stated that the aircraft appeared to be about 500 feet agl, and that the engine did not sound as if it was maintaining a steady rpm. The aircraft was then observed to start yawing to the left and right, which was followed by a left roll to an inverted attitude, from which it descended nose down to the ground. At the time the final roll began, the tail rotor and gear box were observed to separate from the tail boom. The witness also stated that he heard a "tick-tick-tick" sound emanating from the aircraft at the time that it appeared to be in trouble. The helicopter was examined by an FAA airworthiness inspector from the Honolulu, Hawaii, Flight Standards District Office, with the technical assistance of the aircraft kit manufacturer's safety investigator. According to their report, which is appended to this report, the fuel system was intact and without obstructions. The system was equipped with a motorcycle fuel filter, but no debris or obstructions were found in the filter and there was no evidence of a fuel problem. All control system linkage breaks that were found appeared to have resulted from fracture and not fatigue, and all rod ends that were broken had first been bent. The engine exhaust manifold bolts and head bolts were found to be finger tight. The internal parts of the engine were in new condition and correctly assembled. One manufacturer's bulletin on the carburetor, which specified a change in jets, had not been complied with; however, the inspector stated that the engine had been running rich so the noncompliance was not considered significant. The electronic engine ignition system was intact and functional. The engine contained coolant and oil, and no evidence was found that would indicate an engine problem. The main transmission and tail rotor gear box contained lubricant. The tail rotor gear box was severed in half but the internal gears were in new condition. The tail rotor blades were bent and one main rotor blade had separated from the rotor head and was found 30 feet away from the aircraft. Black paint, the color of the tail boom, was evident on the leading edge of both main rotor blades, and four impact dents were found on the tail boom moving progressively towards the cockpit. The upper right-hand windshield was separated from the airframe with a smear of paint transfer identical to the main rotor blade color. The FAA inspector stated that the student pilot's records indicated that he had received 12 hours of helicopter flight instruction in 1991,of which a total of only 1 hour was logged for both autorotation and pattern flight. Five years later, and without any known additional instruction, the pilot obtained and assembled a Mini-500 helicopter and proceeded to train himself to fly it after a flight instructor endorsement for solo flights that the FAA stated was in violation of FAR's 61.59, 61.87, 61.189, and 61.195.