After establishing level flight with an external sling load, the pilot felt a bump from the rear of the helicopter. He corrected the accompanying pitch change and then a second bump was felt. The load was jettisoned and immediately the helicopter began yawing to the right. The pilot was unable to regain control before the helicopter struck the ground. The pilot had not flown helicopters for two years and although he had recently completed a check flight, slung operations were not covered. On this operation the pilot was substituted for another pilot who was unavailable. The load carried on this run was identical to loads previously carried, but much lighter. The pilot elected to use the same cruise speed as for the heavier loads. The load became unstable and encountered the tail boom. Upon its release the load slid back and was struck by the tail rotor, breaking off one blade and snapping the drive shaft.
