THE HELICOPTER HAD BEEN CONDUCTING A SERIES OF DEMO FLTS DURING THE DAY & WAS ON THE LAST SCHEDULED FLT OF THE DAY. TO DROP OFF THE LOCAL PLT WHO HAD REQUESTED THE DEMO FLTS, WHEN THE LOSS OF POWER OCCURRED. THEY HAD MADE A TAKE OFF OVER THE TAXIWAY & TURNED TO THE NORTH AS DIRECTED BY THE TWR. THEY WERE OVER A HOUSING AREA NORTH OF THE ARPT WHEN THE ENG OUT AND LOW ROTOR WARNING SYSTEM SOUNDED. THE LOCAL PLT, WHO HAD MADE THE TAKE OFF & WAS AT THE CONTROLS, TOLD THE DEMO PLT THAT HE WAS TO TAKE CONTROL OF THE ACFT. HE STATED THAT THE START BUTTON WAS ON THE LEFT SET OF CONTROLS. THE DEMO PLT WAS ABLE TO CLEAR THE HOUSING AREA BUT THE ACFT HIT VERY HARD IN THE MEDIAN OF A DIVIDED HIGHWAY. THE ACFT CROSSED THE SOUTH LANES AND STOPPED IN THE GRASS AT THE EDGE OF THE ROAD. IT WAS DESTROYED BY FIRE. EXAMINATION REVEALED A LOOSE 'B' NUT ON THE P/C LINE TO THE FUEL CONTROL UNIT.