On October 16, 2009, at 1045 Pacific daylight time, a McDonnell Douglas Helicopter 500N, N521HB, main rotor contacted the helicopter's tail boom while landing during a practice autorotation at Los Alamitos Army Airfield, Los Alamitos, California. The Huntington Beach Police Department operated the helicopter as a public-use training flight under the provisions of Title 14 Code of Federal Regulations Part 91. The certified flight instructor and pilot under instruction were not injured, the helicopter was substantially damaged. Visual meteorological conditions prevailed, and no flight plan had been filed. The flight originated at Huntington Beach Police Department helicopter pad at 0945. The certified flight instructor stated in the NTSB Pilot Accident Report that he and the pilot he was instructing were practicing emergency procedures, in particular, power recovery autorotations. He said that at 700 feet above ground level (agl) he rolled the throttle off (idle) and entered the autorotation. At 50 feet agl he rolled the throttle on late, realized the ground was rapidly approaching, and decided to perform a running landing. The pilot reported that he inadvertently applied partial throttle, which did not allow the engine to regain full power. With the throttle positioned at 25 percent, the rotor rpm decayed, and when the helicopter contacted the ground, a rotor blade struck the tail boom. He shut down the helicopter and confirmed the damage to the tail boom. The pilot stated that the helicopter and engine had no mechanical failures or malfunctions during the flight.