At approximately 11:18 PST, a Hiller UH-12E suffered an inflight separation of a control rotor, after which the helicopter struck power lines and crashed. The pilot and one passenger were seriously injured; a third occupant was not injured. The pilot said he was on a fish-spotting mission at about 400 feet when the helicopter began vibrating. He raised the collective and tried to increase RPM, but the control rotor then departed the aircraft and the vibration became so severe that he thought the engine would vibrate off of its mounts. The pilot reported that he was about 75 feet agl when he saw the power lines, but did not feel he had sufficient control authority to evade them. He maintained the flight path and went through the lines. The lines stopped the helicopter partially and the helicopter then fell vertically from about 50 feet. AD 97-10-16, which requires inspection of UH-12E control rotor cuffs and spar tubes at 100-hour intervals, was last complied with on the accident helicopter on September 6, 1999, about 56 flight hours prior to the accident.