# NTSB ANC98TA125: Bell 206B II, N70711, Chickaloon, AK, August 15, 1998 - Type: NTSB accident investigation - Classification: Accident · No injuries · Final report - Date: August 14, 1998, 16:55 local (Aug 15, 1998, 00:55 UTC) - Place: Chickaloon, AK - Coordinates: 61.740692, -148.220703 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=61.7407&lon=-148.2207&mi=10 - Phase of flight: [Standing](https://openairdocs.org/ntsb-phases/150) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 2 uninjured - Status: Completed, report April 6, 2001 - Report: Approved by Office Director - NTSB office: Alaska Regional Office - NTSB case key (mKey): 43882 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=ANC98TA125 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/43882/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=43882 - Page: https://openairdocs.org/ntsb/ANC98TA125 ## Probable cause A fracture of the main rotor drag pin assembly. ## 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 - misc rotorcraft,transmission support tube/attchmnt - fractured (cause) ## Analysis The commercial helicopter pilot said that he had completed a normal, westerly approach, and subsequent landing atop a 4,000 feet msl mountain. The pilot stated that as he reduced the throttle to the flight idle position, in preparation for engine shut down, a severe airframe vibration occurred. A postaccident inspection revealed that the helicopter's main rotor transmission had been torn from the lower isolation mount assembly, and the helicopter's transmission deck sustained substantial damage. The main rotor drag pin assembly, main rotor driveshaft, forward coupling assembly, and associated fasteners, were sent to the NTSB Materials Laboratory for inspection. The inspection discovered no evidence of a progressive fracture mechanism, and all fracture faces displayed features typical of overstress separation. The main rotor transmission isolation mount was retained by the NTSB, and sent to the manufacture for evaluation, and testing. An FAA airframe certification engineer attended the evaluation process, and reported that the isolation mount operated slightly below acceptable stiffness limits. He added that since the isolation mount sustained damage during the accident, an accurate stiffness value was unattainable. ## Factual narrative On August 14, 1998, about 1655 Alaska daylight time, a Bell 206B II helicopter, N70711, sustained substantial damage after landing atop a 4,000 feet msl mountain, about 15 miles east of Chickaloon, Alaska. The helicopter was being operated as a visual flight rules (VFR) public use flight when the accident occurred. The helicopter was registered to the State of Alaska, and was operated by the Alaska State Troopers. The certificated commercial pilot/Alaska State Trooper, and the one passenger aboard were not injured. Visual meteorological conditions prevailed, and company flight following procedures were in effect. The flight originated at the Willow Airport, Willow, Alaska, about 0645. During a telephone conversation with the National Transportation Safety Board investigator-in-charge on August 18, the pilot reported that the accident helicopter was being utilized as part of a missing person search. He said that he had completed a normal, westerly approach, and subsequent landing atop the mountain. The pilot stated that as he reduced the throttle to the flight idle position, in preparation for engine shut down, a severe airframe vibration occurred. He said he made an emergency engine shut down in an attempt to minimize further damage to the helicopter. A postaccident inspection revealed that the helicopter's main rotor transmission had been torn from the lower isolation mount assembly, and the helicopter's transmission deck sustained substantial damage. The main rotor drag pin assembly, main rotor driveshaft, forward coupling assembly, and associated fasteners, were retained by the NTSB investigator-in-charge, and sent to the NTSB Materials Laboratory for examination. A metallurgical examination conducted by NTSB laboratory personnel discovered no indication of a progressive fracture mechanism. All components submitted displayed features typical of an overstress condition. The main rotor transmission isolation mount was retained by the NTSB, and sent to the manufacturer for evaluation. An FAA airframe certification engineer attended the evaluation, and reported that the isolation mount operated slightly below acceptable stiffness limits. He added that since the isolation mount sustained damage during the accident, an accurate stiffness value could not be attained. An examination of the helicopter's maintenance records disclosed that on May 4, 1998, 132.3 hours prior to the accident, the helicopter was involved in a hard landing incident. A subsequent hard landing inspection revealed damage to the isolation mount's upper plate assembly. A new isolation mount was installed, and the helicopter was returned to service. The pilot reported that wind conditions at the time of the accident were from the west at 10 knots, with peak gusts to 15 knots. ## Aircraft - Registration: [N70711](https://openairdocs.org/deregistered/N70711-2002-07-09), registration cancelled 2002-07-09, S/N 1331, built 1974 (OpenAirDocs) - Reported make and model: Bell 206B II - Registry model: [Bell 206B](https://openairdocs.org/model/bell-206b) (registry code 1181511) (OpenAirDocs) - Registry engine: [Allison 250 SER 400HP](https://openairdocs.org/engine/allison-250-ser-400hp) (registry code 03013, turbo-shaft) (OpenAirDocs) - Category: Helicopter - Serial number: 1331 - Airframe hours: 14159 - Engines: Allison 250-C20 314 horsepower turbo shaft - Damage: Substantial damage - Flight: Public aircraft, Part 91 - Operator: Alaska State Troopers - Registered owner: State Of Alaska - Airworthiness certificate: Standard, Normal - Last inspection: 100 hour, June 10, 1998, 107 hours before - Departed: UUO, WILLOW, AK - Destination: AK - Flight plan: Company VFR ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 52; commercial; airplane ratings: multi-engine land, single-engine land, single-engine sea; instrument ratings: helicopter, airplane; medical: class 2, valid medical--w/ waivers/lim; seat: right; flight hours: 3,000 total, 2,700 as pilot in command, 30 in the last 90 days, 10 in the last 30 days, 400 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 270° at 10 knots, gusting 15 - Visibility: 20 statute miles - Lowest ceiling: none - Temperature: 50 °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 19980814037989A](https://openairdocs.org/incident/19980814037989A), Aug 14, 1998: Bell 206B, Chickaloon, AK, Substantial damage. (.19) On August 14, 1998, about 1655 Alaska daylight time, a Bell 206B ii helicopter, N70711, sustained substantial damage after landing atop a 4,000 feet MSL… ## Docket (3 files) The files NTSB released with the investigation on May 12, 1999, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=43882. 701 KB in all. - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 1, .PDF, 584 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40464589&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Redacted.PDF - Supplement S - Aircraft Occupant and Injured Ground Personnel (item 2, .PDF, 41 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40464590&FileExtension=.PDF&FileName=Supplement%20S%20-%20Aircraft%20Occupant%20and%20Injured%20Ground%20Personnel-Redacted.PDF - NTSB Materials Laboratory Factual Report (item 3, .PDF, 77 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40464591&FileExtension=.PDF&FileName=NTSB%20Materials%20Laboratory%20Factual%20Report-Redacted.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.