NEWHALEM, WA, USA
N90307
Bell 206BII
The pilot reported that after landing the helicopter and reducing the engine to idle power, he heard a 'snap' or 'click' above and behind him. The main rotor immediately pitched back and to the left. This violent gyration resulted in the toe of the skids to lift off the ground and the helicopter moved backwards with each pitch up. The pilot shut the engine down. The main rotors windmilled to a stop and the mast came to rest back and to the left about 30 degrees off of vertical. Inspection of the helicopter found that the pylon isolation mount failed. Examination of the mount found that 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.
On August 23, 2000, approximately 1800 Pacific daylight time, a Bell 206 BII, N90307, registered to and operated by D & E Storage as a 14 CFR Part 91 business flight, experienced a component system failure shortly after landing at a remote helipad near Newhalem, Washington. Visual meteorological conditions prevailed at the time and no flight plan was filed. The helicopter was substantially damaged and the commercial pilot, the sole occupant, was not injured. The helicopter departed Brewster, Washington, about five minutes prior to the accident. The pilot reported that after landing the helicopter and reducing the engine to idle power, he heard a "snap" or "click" above and behind him. The main rotor immediately pitched back and to the left. This violent gyration resulted in the toe of the skids lifting off the ground by about a foot or two and the helicopter moving backwards with each pitch up. The pilot shut the engine down and held the collective full down. The main rotor windmilled to a stop and the mast came to rest back and to the left about 30 degrees off of vertical. The mast could also be moved forward and to the right without resistance. The pilot reported that the mast and transmission moved as a unit, indicating a transmission mount failure. The damage to the helicopter was originally reported to the National Transportation Safety Board as minor, however, on October 2, 2000, maintenance personnel found that the Pylon Isolation Mount experienced a catastrophic failure. It was at this time that the Board was notified of the substantial damage. Maintenance records provided by the operator indicated that a new transmission isolation mount was installed on July 26, 1992. At the time of the accident, the helicopter had accumulated approximately 2,011.1 hours since the installation. Maintenance personnel reported that the isolation mount does not have a life limit identified. Replacement of the mount usually occurs when the pilot either feels or reports a vibration, or when maintenance personnel during scheduled maintenance inspections moves the pylon beyond what he feels is normal. The purpose of the isolation mount is to dampen the vibration in the transmission (pylon motion). The isolation mount was sent to the National Transportation Safety Board, Materials Laboratory Division for examination. The mechanical engineer reported that the mount consisted of three plates (upper, center and lower) bonded to and separated by two molded elastomeric blocks (upper and lower). The upper and lower plates are screwed to side brackets connecting them together and allowing the center plate to move within the restrictions of the molded elastomeric blocks. The engineer reported that mechanical damage was noted to the upper surface of the mount and the front plate from contact with the transmission drive shaft. A lubricant had found it's way into the mount, but the effect was restricted to the forward part of the center plate. Rubbing was noted on the forward edges of the elastomeric blocks. The engineer reported that the rubbing would be consistent with a partial separation of the center plate from the elastomeric blocks allowing the center plate more movement than normal. The worn surfaces on the center plate indicated a firm contact by the molded gasket, and the amount of wear was indicative of long-term movement of the center plate. The adhesive bonding the center plate to the elastomeric blocks had remained on the blocks and indicated a problem with the bond to the center plate. Radial lines on the adhesive surface indicated individual stages in an adhesive failure and the crazing noted indicated flexing of the adhesive while separated from the center plate.
Debonding between the elastomeric material and the center plate of the transmission isolation mount.
Source: NTSB Aviation Accident Database
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