THE HELICOPTER WAS AIR TAXIED TO THE FUEL PUMP FOR REFUELING. THE AIRCREW REMAINED IN THE HELICOPTER WHILE THE LINEMAN SERVICED THE FUEL TANK. AFTER REFUELING AND STARTING THE ENGINE, THE INSTRUCTOR WAS AT THE CONTROLS WHILE IN CLOSE PROXIMITY OF THE PUMPS. HE REPORTED HEARING A "THUD" WHEN THE HELICOPTER HAD MOVED ABOUT 10 FEET FROM THE PUMPS AND THOUGHT SOMETHING HAD JAMMED. THE PILOTS THOUGHT THEY HAD EXPERIENCED SOME MALFUNCTION OF THE FLIGHT CONTROL SYSTEM. THEY REPORTED THAT THE HELICOPTER GYRATED LEFT AND RIGHT BEFORE CRASHING AFTER THE MAIN ROTOR BLADES STRUCK THE GROUND. AN INVESTIGATION REVEALED THAT DURING REFUELING OPERATIONS, A GROUND WIRE WAS NORMALLY CLIPPED TO A TAB ON THE LANDING SKID. THE PRINCIPAL EXECUTIVE OFFICER FOR THE FLIGHT SCHOOL BELIEVED THAT THE GROUND WIRE MAY NOT HAVE BEEN REMOVED. HOWEVER, THE LINEMAN WAS POSITIVE HE HAD REMOVED THE GROUND WIRE, AND A WITNESS STATED NO WIRE WAS ATTACHED. NO PREIMPACT FAILURE WAS FOUND.