On June 15, 1996 at 1628 eastern standard time (est), a Enstrom 280C, N557H, was substantially damaged, when the main rotors impacted with the tail boom, during an off airport landing. The pilot reported a total loss of engine power, shortly after setting cruise power. The commercial rated pilot and the two passengers were uninjured in the accident. The 14 CFR Part 91 aerial observation flight was not operating on a flight plan, and visual meteorological conditions prevailed at the time of the accident. An investigation by a Federal Aviation Administration Principal Maintenance Inspector (PMI) revealed the following. The helicopter's fuel gauge was indicating that the fuel tank on the helicopter was between 1/4 and 3/8 full of fuel. The actual amount of fuel in the fuel tank was measured at approximately one gallon. The PMI said that the ground wire for the fuel gauge was loose, and when it was moved around following the accident, the fuel gauge indicator went to a zero fuel indication. The helicopter's transmission had been removed from the helicopter less than ten hours before the accident, and the fuel gauge wires were disconnected during the transmission removal. After the accident the fiber lock nuts which secure the ground wire for the fuel gauge showed signs similar to lack of torque at installation. The helicopter was equipped with a wooden dipstick, which can be used during preflight to verify the amount of fuel in the fuel tank. The helicopter's main rotor blades were damaged. The tail rotor gear box and tail rotor had separated from the helicopter during the off airport landing.