# NTSB SEA00LA197: Bell 206BII, N90307, Newhalem, WA, August 24, 2000 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: August 23, 2000, 18:00 local (Aug 24, 2000, 01:00 UTC) - Place: Newhalem, WA - Coordinates: 48.589134, -122.129013 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=48.5891&lon=-122.1290&mi=10 - Phase of flight: [Standing](https://openairdocs.org/ntsb-phases/150) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 1 uninjured - Status: Completed, report July 2, 2001 - Report: Approved by Office Director - NTSB case key (mKey): 51335 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=SEA00LA197 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/51335/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=51335 - Page: https://openairdocs.org/ntsb/SEA00LA197 ## Probable cause Debonding between the elastomeric material and the center plate of the transmission isolation mount. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Airframe/component/system failure/malfunction, standing - idling rotors - miscellaneous,bolt/nut/fastener/clamp/spring - debonded (cause) ## Analysis 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. ## Factual narrative 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. ## Aircraft - Registration: [N90307](https://openairdocs.org/aircraft/N90307), registered now, S/N 1747, built 1975 (OpenAirDocs) - Reported make and model: Bell 206BII - Registry model: [Bell 206B II](https://openairdocs.org/model/bell-206b-ii) (registry code 1181505) (OpenAirDocs) - Registry engine: [Allison 250-C20 series](https://openairdocs.org/engine/allison-250-c20-series) (registry code 03020, turbo-prop) (OpenAirDocs) - Category: Helicopter - Serial number: 1747 - Engines: Allison 250-C20 420 horsepower turbo shaft - Damage: Substantial damage - Flight: Business, Part 91 - Registered owner: D & E Storage - Airworthiness certificate: Standard, Normal - Last inspection: Annual, May 31, 2000 - Departed: S97, BREWSTER, WA ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 58; commercial, flight instructor; airplane ratings: single-engine sea, single-engine land, multi-engine land; instrument ratings: airplane; medical: class 2, valid medical--w/ waivers/lim; seat: right; flight hours: 28,711 total, 386 in this make and model, 119 in the last 90 days, 26 in the last 30 days ## Weather - Conditions: Visual (VMC), Day - Wind: calm - Visibility: 50 statute miles - Lowest ceiling: none - Temperature: 61 °F ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Accident 20000823041179A](https://openairdocs.org/incident/20000823041179A), Aug 23, 2000: WA, No damage. (-23)According to pilot report, after landing and reducing power to idle, a noise was heard immediately followed by the main rotor pitching back and to the… ## Other investigations on N90307 (1) From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. - [NTSB SEA91FA124](https://openairdocs.org/ntsb/SEA91FA124), May 28, 1991: Bell 206BII (N90307), Brewster, WA, substantial damage. Probable cause: THE PILOT-IN-COMMAND'S SELECTION OF UNSUITABLE TERRAIN FOR THE EMERGENCY SIMULATION. ## Docket (5 files) The files NTSB released with the investigation on May 22, 2001, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=51335. 11.8 MB in all. - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 1, .PDF, 490 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40467559&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Master.PDF - Materials Laboratory 15 - Factual Report of Group Chairman (item 2, .PDF, 9 photos, 10.9 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40467558&FileExtension=.PDF&FileName=Materials%20Laboratory%2015%20-%20Factual%20Report%20of%20Group%20Chairman-Master.PDF - Pilot Statement (item 3, .PDF, 99 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40467560&FileExtension=.PDF&FileName=Pilot%20Statement-Master.PDF - Maintenance Records (item 4, .PDF, 118 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40467561&FileExtension=.PDF&FileName=Maintenance%20Records-Master.PDF - Illustrated Parts of the Isolation Mount (item 5, .PDF, 204 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40140001&FileExtension=.PDF&FileName=Illustrated%20Parts%20of%20the%20Isolation%20Mount-Master.PDF From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.