THE PILOT PERFORMED A VERTICAL TAKEOFF. AT APRX 30 FT AGL, WHILE TRANSITIONING TO FORWARD FLIGHT, THE ENGINE LOST POWER AND THE PILOT PARTIALLY LOWERED THE COLLECTIVE TO MAINTAIN ROTOR RPM. THE HELICOPTER LANDED HARD ON SLOPING TERRAIN AND BOUNCED LATERALLY TO THE RIGHT COLLIDING WITH TREES. DURING THE COURSE OF THE INVESTIGATION THE HELICOPTER WAS DEFUELED. APRX 14.6 GALS OF FUEL WERE DRAINED FROM THE REAR MAIN FUEL CELL AND APRX 4.7 GALS WERE DRAINED FROM THE RIGHT FRONT TANK. EXAMINATION OF THE FUEL SYSTEM REVEALED CONTAMINATION OF THE RIGHT AND LEFT IN-LINE FUEL FILTER ASSEMBLIES (PART NO. 206-063-001). THE CONTAMINATION WAS IDENTIFIED AS DIRT, PAINT CHIPS, LINT TYPE FUZZ, AND METAL SHAVINGS. ACCORDING TO BELL HELICOPTER THE FUEL TRANSFER SYSTEM CAN BE RENDERED INOPERATIVE BY CONTAMINANTS RESULTING IN ENGINE FLAMEOUTS.