ANC08LA083

The commercial pilot said that as the helicopter approached the destination airport during a Part 91 repositioning flight, he elected to make a practice autorotation to the runway. As he lowered the collective to enter the autorotation, the helicopter began to vibrate violently. He continued the autorotation to runway, and stated that during the landing flare the helicopter's tail boom contacted the runway, the helicopter rocked forward, and the landing gear skids struck the ground. The hard landing resulted in substantial damage to the fuselage, tail boom assembly, and main rotor transmission supports. A maintenance technician that did a postaccident inspection of the helicopter discovered loose and worn bearings on the helicopter's stabilizer bar, swashplate, flight control tube assemblies, and the right and left hydraulic servos. In a written report the maintenance technician reported that the mechanical discrepancies he discovered during his inspection were, in part: "noted as being the root cause of the vertical vibration..."

On June 29, 2008, about 2230 Alaska daylight time, a restricted category Bell UH-1B helicopter, N64RA, sustained substantial damage during a hard landing at the Iliamna Airport, Iliamna, Alaska. The helicopter was being operated as a visual flight rules (VFR) positioning flight under the provisions of 14 Code of Federal Regulations Part 91, when the accident occurred. The commercial pilot, and the one pilot-rated passenger, were not injured. Visual meteorological conditions prevailed, and a VFR flight plan was filed. The flight originated from a remote site near Platinum, Alaska, about 2100. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on July 3, the pilot reported that the purpose of the flight was to reposition the helicopter to Iliamna, and provide the pilot-rated passenger with a familiarization flight in a Bell UH-1B helicopter. The pilot said that as the flight approached the Iliamna Airport, he instructed the passenger that he was going to demonstrate a practice autorotation to runway 07. The pilot reported that as he lowered the collective to enter the autorotation, the helicopter began to vibrate violently. He said that the vertical vibration, which he described as "collective bounce" was so severe that at one point he was unable to read the airspeed indicator and engine toque meter. The pilot said that he elected to continue the autorotation to runway 07. He said that during the landing flare the helicopter's tail boom contacted the runway, the helicopter rocked forward, and the landing gear skids struck the ground. The hard landing resulted in structural damage to the fuselage, tail boom assembly, and main rotor transmission supports. The Operator's Manual for the Bell UH-1B helicopter, states in part: "Collective bounce is a pilot induced vertical oscillation that may be encountered in any flight condition by a rapid buildup of vertical bounce at approximately three cycles per second. The severity of this oscillation is such that effective control of the aircraft may become difficult to maintain." According to the operator, a postaccident inspection of the helicopter was accomplished by a Bell Helicopter service center technician, which revealed loose and worn bearings on the helicopter's stabilizer bar, swashplate, flight control tube assemblies, and the right and left hydraulic servos. In a letter dated July 18, 2008, the maintenance technician reported that the mechanical discrepancies he discovered during his inspection were, in part: "noted as being the root cause of the vertical vibration..." A copy of the maintenance technician's letter is included in the public docket for this accident.