Bell UH-1B N64RA accident near Iliamna, Alaska

Probable cause

The failure of company maintenance personnel to ensure the helicopter was properly maintained, resulting in a severe in-flight vibration due to collective bounce. Contributing to the accident was loose and worn bearings on the helicopter's stabilizer bar, swashplate, flight control tube assemblies, and the right and left hydraulic servos.

NTSB analysis

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..."

Text from the NTSB. Open the NTSB docket for ANC08LA083 for the full record.