On July 31, 1998, about 0930 eastern daylight time, a Hughes TH-55A helicopter, N998MC, operated by the Chatham County Mosquito Control Commission, crashed on Elba Island near Savannah, Georgia, while on a Title 14 CFR Part 91 public-use, aerial application flight. Visual meteorological conditions were reported and no flight plan was filed. The helicopter was destroyed. The commercial-rated pilot/mechanic was not injured. The flight originated from the Commission's heliport the same day, at 0900. According to the pilot, this was the second flight. The first flight had lasted 1.3 hours with "no discrepancies noted." The pilot said, "...the second flight was normal...for [the] first .5 of flight. I completed five passes...and on the last pass at 40 feet Agl [above ground level] with RH [right hand] quartering headwind, I had an engine partial power loss. The symptoms were main rotor started to droop below 490 RPM (2900 engine RPM). I lowered collective, rolled in full throttle, raised collective to stop decent. Rotor continued to droop." The pilot said, "...I tried to maneuver the helicopter past trees, [a] mound of dirt, to flat land on [the] far side of [a] ditch. I could not maintain enough altitude or rotor RPM to clear [the] ditch. I contacted far side of ditch with the unoccupied side [left] of [the] aircraft." The pilot told the NTSB investigator-in-charge and FAA Inspector, that the helicopter was functioning properly before the accident, and he had no explanation for the loss of rotor rpm. An engine run at idle rpm did not reveal any discrepancies. A compression check while the engine was hot revealed low compression on cylinders No. 1 and 3. Both cylinders were removed, and both cylinders were found to have exhaust valves that were not seated completely. The calculated density altitude, based on a temperature of 31 degrees C (88F), elevation 25 feet msl, altitude 65 feet msl, and an altimeter setting of 30.05 inches of mercury, all obtained from the Savannah 0953 weather observation, was 1,868 feet. The pilot stated that the weight of the helicopter was "...1590 [pounds] at the time of the accident." The maximum gross weight of the helicopter was 1,670 pounds.