THE PILOT WAS RETURNING HOME, WHEN ACCORDING TO A WITNESS WHO SAW THE HELICOPTER ABOUT 800 FEET ABOVE THE GROUND, THE ENGINE SOUNDED NORMAL AT FIRST. SHORTLY THEREAFTER, THE ENGINE SPEED INCREASED FOLLOWED BY THE HELICOPTER 'DUTCH ROLLING' (BANKING) LEFT AND RIGHT FOR TWO CYCLES AND THEN ROLLED OVER ONE AND HALF TIMES NOSE FIRST AS IT DROPPED BEHIND THE TREES. THE HELICOPTER COLLIDED WITH POWER LINES BEFORE IMPACTING THE GROUND IN A NOSE DOWN ATTITUDE AND WAS DESTROYED BY POST-CRASH FIRE. EXAMINATION OF AIRCRAFT CONFIRMED CONTINUITY OF THE TRANSMISSION GEAR BOX, AND INTERNAL ROTATIONAL DAMAGE FOR THE ENGINE; HOWEVER, THERE WAS NO SIGNIFICANT ROTATIONAL DAMAGE TO THE MAIN ROTOR, TAIL ROTOR, AND ENGINE TO TRANSMISSION DRIVESHAFT (SHORT SHAFT). EXAMINATION OF THE SHAFT REVEALED IT HAD SEPARAED FROM THE BENDIX COUPLINGS AND WAS CRUSHED ALMOST FLAT. THERE WAS NO EVIDENCE OF ROTATIONAL DAMAGE BETWEEN THE SEPARATED HALVES. METALLURGICAL EXAMINATION REVEALED THE FUEL NOZZLE WAS CORRODED WITH FERROUS FIBERS.