On November 14, 2004, about 1500 eastern standard time, an Enstrom 280C, N51739, was substantially damaged during a hard landing near Galloo Island, NY. The certificated commercial pilot and passenger were not injured. Visual meteorological conditions prevailed, and no flight plan was filed. The aerial photography flight, which originated from Syracuse Hancock International Airport (SYR), Syracuse, New York, was conducted under 14 CFR Part 91. According to the pilot, during the flight he entered an orbit to the right, about 75 to 80 feet above ground level. He recalled observing that the engine rpm was 2,700, the main rotor rpm was 280, and the indicating needles were "matched." He could not recall the manifold pressure. During the downwind portion of one orbit, the pilot heard a "noticeable" drop in engine performance. He responded by rolling out of the orbit, and performing the emergency checklist. A scan of the instruments revealed nothing out of ordinary, "except rpm's." He subsequently increased the throttle, then lowered the collective pitch while increasing the throttle. Both had no effect. The pilot attempted to enter an autorotation, but the main rotor would not "separate" from the engine, and both the engine and rotor rpm continued to decrease. He then pitched the helicopter forward in an attempt to gain more forward airspeed. The pilot attempted to arrest the descent with the remaining rotor energy, but the helicopter impacted the ground hard, then bounced. One main rotor blade struck a rock and was damaged. The damaged main rotor blade subsequently struck the tail rotor drive shaft, and sheared it. The pilot then disengaged the rotor system and shut down the engine. The helicopter was transported to a maintenance facility following the accident. According to the mechanic performing the repairs on the helicopter, the engine was operated while still attached to the airframe at an idle power setting for 3 minutes. The engine could not be operated at higher power settings due to damage sustained during the accident. Throughout the engine run, the engine operated normally. The engine was subsequently disassembled and inspected due to the sudden stoppage it sustained during the accident. A Federal Aviation Administration (FAA) inspector, who observed the engine disassembly and inspection, noted no anomalies. According to a representative of the airframe manufacturer, the clutch assembly was examined, and no anomalies were found. A portion of the video taken by the photographer during the flight was forwarded to the Safety Board and examined. A Sound Spectrum Study was conducted on the audio portion of the recording, and sound signatures were identified that corresponded to the rotational frequencies of the rotating reciprocal engine and the main rotor system rotation. According to the study, the engine and rotor system sound appeared to be in a normal range during the first part of the recording. At approximately 20 seconds elapsed time, the engine and main rotor system began a steady decrease in rotational speed. At approximately 30 seconds, the engine's speed began to decrease rapidly. During that time, the main rotor system did not decrease at the same rate as the engine. About 33 seconds of elapsed time, the engine speed leveled off, accompanied by a level off in the rotor system's decay. Ground impact occurred about 4 seconds later. After ground impact, the main rotor system stopped rotating while the engine continued to run for about 6 more seconds at a low, but measurable speed. The video also showed that prior to decrease in engine and rotor rpm, the helicopter was operating at an estimated altitude of 80 feet agl, and at an estimated groundspeed of 20 knots. Examination of the Height Velocity Diagram for the Enstrom 280C revealed that if the helicopter were loaded to maximum gross weight, at sea level pressure, at an altitude of 80 feet agl, and below 35 mph indicated airspeed, the helicopter would be operating in a region located outside of the "SAFE OPERATING AREA." The weather reported at Watertown International Airport (ART), Watertown, New York, located 18 nautical miles east, at 1456, included winds from 240 degrees at 13 knots, clear skies below 12,000 feet, temperature 44 degrees Fahrenheit, dewpoint 34 degrees Fahrenheit, and an altimeter setting of 30.63 inches of mercury.