Just after take-off, at about 30 feet above ground level, a loud bang was heard and the helicopter started rotating rapidly. The pilot manoeuvred the helicopter clear of fuel drums but it landed heavily while moving rearward. Initial inspection of the helicopter revealed that the tail rotor drive forward short shaft had become disconnected after the failure of the top left longeron. The longeron had failed as a result of high cycle, low stress fatigue. The failure had been developing for some time and should have been noted by maintenance personnel. The decision to take-off over fuel drums restricted the pilot's choices of suitable landing areas and complicated the emergency landing attempt.