On October 3, 1997 at 1620 eastern daylight time, a Hughes 269A, N269SS, was substantially damaged when it collided with terrain during a forced landing near Campbellsville, Kentucky. The certificated flight instructor and passenger were not injured. Visual meteorological conditions prevailed and a flight plan was not filed. The local, introductory flight originated at Campbellsville, and was conducted under 14 CFR Part 91. The pilot reported that he picked up a passenger for an introductory flight in the helicopter. He said they flew around for about 30 minutes and landed to refuel the helicopter. He had called the airport earlier and told the Fixed Based Operator (FBO) that he needed a "quick turn." Upon his arrival, he parked the helicopter near the fuel pump, and was met by the an employee of the FBO. The round nozzle of the black hose was inserted into the helicopter, and 18 gallons were pumped into the tank. The pilot went into the building, signed the ticket, and got into the helicopter. Approximately 3 minutes into the flight, at approximately 800 feet AGL, the engine sound changed along with a decrease in engine RPM, and increase in manifold pressure. He said he immediately started a descent into an open field. With the collective down the engine RPM was still decreasing along with the rotor RPM. He opened the throttle wide, but there was no change in engine RPM. At approximately 300 feet AGL, he autorotated to the field, but as he got closer he realized that the field was a downhill slope. He moved the cyclic forward to put the toes of the skids down first, but the left skid contacted the ground, followed by the right skid, and the helicopter cartwheeled onto its nose. Examination of the helicopter by a Federal Aviation Administration (FAA) Inspector, revealed that the fuel tanks had been fueled with jet A fuel, instead of 100 LL Avgas. The fuel slip signed by the pilot indicated jet fuel. The pilot said he does not recall if he or the employee of the FBO pulled the hose from the pipe. He said the accident could have been prevented if the marking on the fuel pumps were more visible, and there should be regulation for the airport to remove the small nozzle on Jet fuel hoses to prevent them from going into Avgas tanks.