On February 25, 2002, at 1430 mountain standard time, a Rotorway Exec 90 experimental helicopter, N9876D, made a hard landing following a loss of power during cruise about 5 miles south of Phoenix, Arizona, on the Gila River Indian Reservation. The helicopter was operated by Cobb International, Inc., under the provisions of 14 CFR Part 91 as an instructional flight, and sustained substantial damage. The certified flight instructor (CFI) and student pilot were not injured. Visual meteorological conditions prevailed for the local area flight that departed the Chandler Stellar Airpark (P19), Chandler, Arizona, at 1410. The flight was scheduled to terminate at P19. A flight plan had not been filed. In the CFI's written statement, he stated that he and his student were on a training flight to practice autorotations. During the third practice autorotation from 500 feet agl, the student performed the maneuver "without problems until the aircraft was straight and level after flaring" about 30 feet agl. Attempts were made to reintroduce power without success. The CFI stated he was on the controls "lightly" with the student, when the student told the CFI he could not reintroduce power. The CFI then came fully onto the controls and attempted to roll on the throttle only to discover it would not roll any further. The collective was raised to cushion the landing at 10 feet agl; however, the aircraft made hard contact with the ground, collapsing the left skid, and rolling onto its left side. The Federal Aviation Administration Inspector, who responded to the accident site, informed the National Transportation Safety Board that the mechanical stop for the throttle was bent. He stated once the linkage is driven "over center," it can't be brought back, and it remains at ground idle. The engine was started after the accident and ran "with no problems."