THE PILOT-IN-COMMAND'S IMPROPER APPLICATION OF CYCLIC AND COLLECTIVE PITCH CONTROL DURING AN AUTOROTATIVE LANDING. THIS RESULTED IN A HARD LANDING, COLLISION OF THE MAIN ROTOR BLADES WITH THE TAILBOOM, AND SUBSEQUENT ROLL OVER OF THE HELICOPTER. CONTRIBUTING TO THE ACCIDENT WAS A PARTIAL LOSS OF ENGINE POWER CAUSED BY A TOTAL FAILURE OF THE ENGINE ASSEMBY EXHAUST VALVE.