On September 24, 1998, at 1540 hours mountain standard time, a Boeing 369E helicopter, N685F, collided with the ground during a practice 180-degree autorotation at the Falcon Field Airport, Mesa, Arizona. The aircraft, operated by Boeing Helicopters, sustained substantial damage. The instructor pilot and dual student were not injured. The local area instructional flight conducted under 14 CFR Part 91 departed the Boeing Helicopter ramp to perform the recurrency training at 1500. Visual meteorological conditions prevailed at the time of the accident. The instructor pilot stated that he and the student flew for about 1 hour demonstrating various landing procedures to include about 15 straight-in autorotations to Falcon Field's north turf area, which was damp and soft. The instructor pilot demonstrated a 180-degree autorotation, which he stated "appeared to finish with a normal deceleration flare and touchdown." He stated that right after the touchdown, a loud rattling sound was heard coming from the tailboom. The aircraft then had a short slide to a stop with a slight right curve. Ground personnel indicated to the flight crew that the aircraft should not lift off because they could see the tail rotor was slowing to a stop. The instructor pilot said he opened his door and saw the tail rotor was not turning, and he proceeded to shut the aircraft down. Prior to moving the aircraft, it was discovered that the aircraft's tail rotor blades had hit the ground causing damage to the tail rotor blades, tailboom, and severing the tail rotor drive shaft. Witnesses stated they thought the aircraft hit harder than usual and bounced slightly with a short slide.