On September 7, 1994, about 1124 central daylight time, a Schweizer 269C helicopter, N61245, sustained substantial damage when it landed hard during a practice autorotation near Grand Forks, North Dakota. The certificated flight instructor and student pilot aboard reported no injuries. The local, instructional flight operated without flight plan in visual meteorological conditions. The flight instructor reported he and the student had practiced several successful autorotations. The final, accident autorotation was to be with a power recovery. The instructor said he adjusted the throttle to 3100 RPM, and told the student to recover. When the rate of descent was not arrested, the instructor joined the student on the controls and attempted to autorotate just prior to ground collision. Postaccident inspection of the helicopter discovered no evidence of preimpact mechanical anomaly. The helicopter's engine was started and accelerated without any observed deficiencies.