An R44 made an autorotation to an emergency landing when the engine would not produce rated power.

Narrative

I departed on a City Tour. I was carrying 3 passengers with a combined weight of 520 LBS. I was carrying approximately 43 gallons of AvGas; putting the aircraft at maximum gross weight; and within the weight and balance limitations. Upon lifting off into a hover; I performed a power check to ensure I had enough power to takeoff. During the hover; I pulled approximately 25 inches of manifold pressure; which was expected considering the load. On climb-out; I recognized a slow rate of climb (approximately 300 FPM); and by the time I was 1.5 miles out; I was only at approximately 250 feet. At that point I began my turn in-land to continue my climb and cross the city. During the 90 degree right turn; I felt an immediate drop in my seat; followed by an immediate yaw-left of the helicopter and the sound of the low RPM horn. Instinctively; I entered an autorotation and headed towards the sand. Once in the autorotation; I looked down at my tachometer to notice the main rotor blades were pegged at the top of the gauge. I continued the autorotation; and landed safely in the sand. The passengers were unharmed and no physical damage was done to the aircraft. The engine was still running on impact so I immediately shut down the aircraft and cleared the area of any bystanders.