The pilot intended to land the helicopter near a water tank so that he could free sheep bogged in the muddy ground. When the aircraft was at about 80 feet agl on descent, the pilot applied power and raised the collective to reduce the rate of descent. He heard a bang, followed by a noise he described as clutch growl and the helicopter began to vibrate severely. The collective was lowered momentarily and power was reapplied, however, the clutch growl and vibration continued. The pilot lowered the collective but the helicopter struck the ground firmly at a low forward speed and the main rotor severed the tail boom. Subsequent investigation revealed that the clutch had suffered an overload failure, however the reason for the failure was not determined. The associated loss of power occurred at a height and speed combination which precluded the pilot from establishing an autorotative descent.
