On February 2, 2000, at 1437 hours Pacific standard time, a Robinson R44, helicopter, N999EV, was substantially damaged during a practice autorotation at Palo Alto, California. Neither the commercial rated pilot nor the passenger was injured. The personal flight was operated by the pilot under 14 CFR Part 91. No flight plan was filed. Visual meteorological conditions prevailed for the operation that originated at San Carlos, California, at 1427. The pilot reported that during the autorotation the engine had been at idle for an extended period. He said he did not think he was going to make the runway so he added power. The engine failed to respond and the tail stinger contacted the ground, resulting in the separation of the tail boom. The pilot pulled collective pitch; the helicopter briefly became airborne and spun violently to the right. The pilot maintained the helicopter in a level attitude and the landed hard. After the main rotor stopped rotating, the pilot and passenger exited the helicopter. The pilot told the Federal Aviation Administration inspector on scene that he did not use carburetor heat during the autorotation. According to the Palo Alto METAR, the temperature was 64 degrees Fahrenheit and the dew point was 52 degrees Fahrenheit. Reference to a carburetor icing probability chart revealed that this temperature and dew point were in an area of the graph annotated "moderate icing-cruise power or serious icing-glide power."