19980814037989A

(.19) 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 is 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 an extensive missing person search. He said that he had completed a normal, easterly 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. The pilot noted that 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 pilot reported that wind conditions at the time of the accident were from the east at 10 knots, with peak gusts to 15 knots. (.4) the commercial helicopter pilot said that he had completed a normal, westerly approach, and subsequent landing atop a 4,000 feet MSL 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. 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 sent to the NTSB materials laboratory for inspection. The inspection discovered no evidence of a progressive fracture mechanism, and all f racture faces displayed features typical of overstress separation. The main rotor transmission isolation mount was retained by the NTSB, and sent to the manufacture for evaluation, and testing. An FAA airframe certification engineer attended the evaluation process, 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 was unattainable. (-23) on August 14, 1998 at 1655 adt, a Bell 206B helicopter operated by the Alaska troopers as a public use aircraft, sustained substantial damage after landing atop a 4000 feet MSL mountain. The pilot reported the helicopter was being operated on search mission and landed to place a radio repeater. The pilot stated he made a normal landing, and as he rolled the power off to idle, heard a grinding noise from the top cabin section of the helicopter. He initiated emergency engine shutdown and he and his passenger exited the aircraft. He noted the rotor mast was canted forward about 15', and noted damage to the top fairing. The helicopter was recovered by sling operati on.