The pilot had flown the helicopter on the power line aerial observation flight for approximately 3 hours and 8 minutes when a power loss occurred. He initiated an autorotation to an open field. During the flare, he failed to maintain adequate rotor RPM and the main rotor blades severed the tailboom. The helicopter landed hard. The owner of the helicopter drained a small amount of fuel from the tanks and said that he was certain that lack of fuel caused the forced landing. The pilot performed a calculated fuel consumption check two days before the accident which he said showed a fuel burn rate of 13.7 gallons per hour or 3 hours and 47 minutes of flight time. He used a stop watch in the cockpit to account for flight time. The manufacturer states the fuel system holds 47.5 gallons of usable fuel, the engine's fuel burn rate is 19 gallons per hour at cruise, and lists the maximum cruise time as 3 hours and 18 minutes with the engine leaned.