On August 24, 2000, a Bell 206BII (registration N90307) was involved in an aviation accident near Newhalem, WA. Investigators recorded the probable cause as: The failure occurred over a long period of time and was probably due to inadequate surface preparation of the center plate to the elastomeric blocks, which resulted in the debonding of the material and allowing the center plate more movement than normal. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter experienced a main rotor gyration after landing due to a pylon isolation mount failure, causing the skids to lift off the ground and the helicopter to move backwards before shutdown.
Incident Description
After landing, the pilot reduced the engine to idle power. A 'snap' or 'click' was heard above and behind the pilot. The main rotor immediately pitched back and to the left, causing a violent gyration. This motion lifted the toe of the skids off the ground, and the helicopter moved backwards with each pitch up. The pilot shut down the engine, and the main rotors windmilled to a stop. The mast came to rest approximately 30 degrees off vertical, positioned back and to the left.
Post-Incident Findings
Inspection revealed that the pylon isolation mount had failed. Examination of the mount indicated that the failure occurred over a long period of time. The probable cause was inadequate surface preparation of the center plate to the elastomeric blocks. This led to debonding of the material, allowing the center plate more movement than normal.