On August 17, 2004, at 1815 eastern daylight time, a Eurocopter AS B2, N631SD, operated by the Oakland Country Sheriff, impacted terrain while practicing low altitude autorotation near New Hudson, Michigan. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 public-use instructional flight was not operating on a flight plan. The private pilot and certified flight instructor (CFI) were uninjured. The flight originated from Oakland County International Airport, Pontiac, Michigan, at 1740. The private pilot stated the CFI pulled the throttle back and she entered an autorotation. The CFI rolled the throttle back to the full on position. The helicopter was leveled and collective was pulled. The helicopter landed hard. The CFI stated that he noticed an excessive sink rate and he applied the throttle with no response. He applied cyclic to help the private pilot level the helicopter and collective to cushion the landing.