On January 4, 2000, about 1400 mountain standard time, an amateur built experimental Rotorway Exec 90 helicopter, N89678, sustained substantial damage when the main rotor blades contacted the tail boom during an autorotation at Memorial Airport near Chandler, Arizona. The autorotation followed a failure of a shaft in the engine to rotor drive system. Rotorway International operated the helicopter under the provisions of 14 CFR Part 91. The certified flight instructor (CFI) and private pilot student were not injured. The instructional flight departed Stellar Air Park, near Chandler, about 1320. Visual meteorological conditions prevailed and no flight plan was filed. The Safety Board was notified of the accident on January 12, 2000. The operator stated the flight was a familiarization for the private pilot, who was transitioning into this model of helicopter. An approach was being made to runway 03 when the instructor heard a loud bang from the engine compartment behind his back at an altitude of 100 feet. He stated that it felt like all engine power was lost. When the instructor saw the rotor rpm decreasing, he took control and initiated an autorotation to a soft sandy area. During the touchdown, the main rotor blades contacted the tail boom, and the tail rotor blades contacted the ground. The operator reported that a subsequent inspection determined that the secondary drive shaft fractured. Laboratory examination concluded that the shaft failed from fatigue, originating from galling sites introduced during the manufacturing process. The operator reported that a temperature strip on the bearing was noted to be darkened before the flight, but they were unsure if this information was determined independantly by either the CFI or the student. All other shafts in the operator's fleet were inspected and no additional suspect shafts were found.