On March 25, 1999, approximately 1630 mountain standard time, a Robinson R22 Beta helicopter, N8354Z, operated by HL Helicopters of Boulder, Colorado, was substantially damaged while maneuvering 7 miles south of Englewood, Colorado. The airline transport rated flight instructor and his private pilot-certificated student were not injured. The aircraft was being operated under Title 14 CFR Part 91, and no flight plan had been filed. Visual meteorological conditions prevailed for the local area instructional flight that originated from Centennial Airport, Englewood, Colorado, 30 minutes before the accident. According to the flight instructor, he was teaching autorotations to his student along a large, flat plateau. After completing one straight and one 180 degree autorotation, the student was attempting a second 180 degree autorotation. The entry was made at 700 feet above the ground (agl), and the turn was completed at approximately 200 agl. He stated that a large collective increase was required during the turn due to the high bank angle necessitated by the strong wind. As the aircraft rolled out of the turn, the collective was lowered "but not enough," and the rpm continued to decay. At the end of the flare, approximately 20-25 feet agl, a high sink rate began to build. The instructor said he took control of the helicopter and rolled the throttle to recover the rpm, but did not have sufficient altitude or rpm. The helicopter impacted the ground "very hard" in a level attitude. Upon impact, the skids buckled outward and the main rotor blades flexed downward, severing the tail boom.