On August 14, 1998, about 1655 Alaska daylight time, a Bell 206B II helicopter, N70711, sustained substantial damage after landing atop a 4,000 feet msl mountain, about 15 miles east of Chickaloon, Alaska. The helicopter was being operated as a visual flight rules (VFR) public use flight when the accident occurred. The helicopter was registered to the State of Alaska, and was operated by the Alaska State Troopers. The certificated commercial pilot/Alaska State Trooper, and the one passenger aboard were not injured. Visual meteorological conditions prevailed, and company flight following procedures were in effect. The flight originated at the Willow Airport, Willow, Alaska, about 0645. During a telephone conversation with the National Transportation Safety Board investigator-in-charge on August 18, the pilot reported that the accident helicopter was being utilized as part of a missing person search. He said that he had completed a normal, westerly approach, and subsequent landing atop the mountain. The pilot stated that as he reduced the throttle to the flight idle position, in preparation for engine shut down, a severe airframe vibration occurred. He said he made an emergency engine shut down in an attempt to minimize further damage to the helicopter. A postaccident inspection revealed that the helicopter's main rotor transmission had been torn from the lower isolation mount assembly, and the helicopter's transmission deck sustained substantial damage. The main rotor drag pin assembly, main rotor driveshaft, forward coupling assembly, and associated fasteners, were retained by the NTSB investigator-in-charge, and sent to the NTSB Materials Laboratory for examination. A metallurgical examination conducted by NTSB laboratory personnel discovered no indication of a progressive fracture mechanism. All components submitted displayed features typical of an overstress condition. The main rotor transmission isolation mount was retained by the NTSB, and sent to the manufacturer for evaluation. An FAA airframe certification engineer attended the evaluation, and reported that the isolation mount operated slightly below acceptable stiffness limits. He added that since the isolation mount sustained damage during the accident, an accurate stiffness value could not be attained. An examination of the helicopter's maintenance records disclosed that on May 4, 1998, 132.3 hours prior to the accident, the helicopter was involved in a hard landing incident. A subsequent hard landing inspection revealed damage to the isolation mount's upper plate assembly. A new isolation mount was installed, and the helicopter was returned to service. The pilot reported that wind conditions at the time of the accident were from the west at 10 knots, with peak gusts to 15 knots.