WHILE MANEUVERING AT LOW ALTITUDE IN SEARCH OF ANIMALS, THE PILOT EXPERIENCED A SEVERE VIBRATION THROUGH THE FLIGHT CONTROLS. THE HELICOPTER BEGAN AN UNCONTROLLED DESCENT AND ATTEMPTS TO STOP THE DESCENT BY USING THE COLLECTIVE WERE UNSUCCESSFUL. THE HELICOPTER CONTACTED THE GROUND AND WAS SUBSTANTIALLY DAMAGED. AN EXAMINATION REVEALED THAT AN AN-320-5 NUT HAD COME OFF OF A PIN ON THE ROTATING SWASHPLATE RING. THE PURPOSE OF THIS NUT WAS TO SECURE THE LINK AND SCISSORS TO THE SWASHPLATE PIN. WITH THE NUT OFF, A LOSS OF CONTINUITY OCCURRED IN THE ROTATING CONTROLS OF ONE MAIN ROTOR BLADE. A COTTER PIN WAS REQUIRED TO SECURE THE NUT. THE ABSENCE OF A COTTER PIN WOULD HAVE ALLOWED THE NUT TO EVENTUALLY WORK ITS WAY OFF. THE HELICOPTER HAD BEEN OPERATED ABOUT 26 HRS SINCE A 1200 HR INSPECTION HAD BEEN PERFORMED. DURING THAT INSPECTION, THE ABOVE MENTIONED PARTS AND COMPONENTS HAD BEEN REMOVED, INSPECTED AND REINSTALLED.