The helicopter collided with a powerline transmission tower while towing cable attached by means of a cargo hook. The helicopter was moving laterally toward the tower when it hit the tower with the M/R blades. The blade tip weight separated and penetrated the tail boom in two places, damaging the T/R drive shaft. The helicopter spun twice to the right in a slow descent tethered by the steel cable. During the second turn, parts of the T/R separated. Witnesses did not observe the cable being jettisoned. The pilot was newly hired, and did not have any prior Class C external load experience. He had received training by the company chief pilot, who then left the company 3 days after the pilot was hired, leaving the pilot to work alone. The FAA office holding the operator's certificate had been notified of the chief pilot's departure. The office did not, nor was it required to, notify the local FAA district office of the operator's external load operations within that office's geographic responsibility.