THE LOSS OF ENGINE POWER FOR UNDETERMINED REASONS, AND THE PILOT'S FAILURE TO MAINTAIN ROTOR RPM DURING THE FORCED LANDING AUTOROTATION, RESULTING IN A LOSS OF AIRCRAFT CONTROL AND COLLISION WITH TREES. ALSO CAUSAL TO THE ACCIDENT WAS THE PILOT'S LACK OF RECENT HELICOPTER EXPEREINCE AND HIS LACK OF EXPERIENCE IN THIS MODEL HELICOPTER.