# NTSB WPR13TA205: Bell BHT407, N937GR, Big Fork, MT, April 25, 2013 - Type: NTSB accident investigation - Classification: Accident · No injuries · Final report - Date: April 25, 2013, 10:16 local (16:16 UTC) - Place: Big Fork, MT - Coordinates: 48.153057, -113.923889 - Nearby events: https://openairdocs.org/map.md?lat=48.1531&lon=-113.9239&mi=10 - Phase of flight: [Maneuvering](https://openairdocs.org/ntsb-phases/450) (OpenAirDocs' grouping) - Defining event: [Maneuvering: Loss of tail rotor effectiveness](https://openairdocs.org/ntsb-events/242) - Highest injury: No injuries - Injuries: 3 uninjured - Status: Completed, report July 30, 2014 - Report: Approved by Office Director - NTSB office: Western Pacific Region - Parties: FAA-FSDO - NTSB case key (mKey): 86704 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=WPR13TA205 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/86704/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=86704 - Page: https://openairdocs.org/ntsb/WPR13TA205 ## Probable cause The pilot’s failure to maintain helicopter control while maneuvering over mountainous terrain in strong up/downdrafts and possible mountain wave activity, which resulted in a loss of tail rotor effectiveness. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Performance/control parameters: yaw control](https://openairdocs.org/ntsb-findings/01062024): Aircraft › Aircraft oper/perf/capability · Capability exceeded (probable cause); cases with it: [Bell 407, 1 case](https://openairdocs.org/ntsb-findings/01062024/cases.md?model=1182206), [All aircraft, 149 cases](https://openairdocs.org/ntsb-findings/01062024/cases.md) - [Use of equip/info: aircraft control](https://openairdocs.org/ntsb-findings/02063040): Personnel issues › Task performance · Pilot (probable cause); cases with it: [Bell 407, 19 cases](https://openairdocs.org/ntsb-findings/02063040/cases.md?model=1182206), [All aircraft, 9,295 cases](https://openairdocs.org/ntsb-findings/02063040/cases.md) - [Terrain: mountainous/hilly terrain](https://openairdocs.org/ntsb-findings/03021010): Environmental issues › Physical environment · Contributed to outcome; cases with it: [Bell 407, 1 case](https://openairdocs.org/ntsb-findings/03021010/cases.md?model=1182206), [All aircraft, 329 cases](https://openairdocs.org/ntsb-findings/03021010/cases.md) - [Turbulence: terrain induced turbulence](https://openairdocs.org/ntsb-findings/03032010): Environmental issues › Conditions/weather/phenomena · Effect on operation (probable cause); cases with it: [Bell 407, 1 case](https://openairdocs.org/ntsb-findings/03032010/cases.md?model=1182206), [All aircraft, 70 cases](https://openairdocs.org/ntsb-findings/03032010/cases.md) - [Wind: downdraft](https://openairdocs.org/ntsb-findings/03034035): Environmental issues › Conditions/weather/phenomena · Effect on operation (probable cause); cases with it: [Bell 407, 2 cases](https://openairdocs.org/ntsb-findings/03034035/cases.md?model=1182206), [All aircraft, 178 cases](https://openairdocs.org/ntsb-findings/03034035/cases.md) - [Wind: updraft](https://openairdocs.org/ntsb-findings/03034030): Environmental issues › Conditions/weather/phenomena · Effect on operation (probable cause); cases with it: [Bell 407, 1 case](https://openairdocs.org/ntsb-findings/03034030/cases.md?model=1182206), [All aircraft, 14 cases](https://openairdocs.org/ntsb-findings/03034030/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Maneuvering: Loss of tail rotor effectiveness](https://openairdocs.org/ntsb-events/242) (defining event); cases with it: [Bell 407, 1 case](https://openairdocs.org/ntsb-events/242/sequence.md?model=1182206), [All aircraft, 100 cases](https://openairdocs.org/ntsb-events/242/sequence.md) - [Emergency descent: Loss of tail rotor effectiveness](https://openairdocs.org/ntsb-events/242); cases with it: [Bell 407, 1 case](https://openairdocs.org/ntsb-events/242/sequence.md?model=1182206), [All aircraft, 100 cases](https://openairdocs.org/ntsb-events/242/sequence.md) - [Landing: Off-field or emergency landing](https://openairdocs.org/ntsb-events/440); cases with it: [Bell 407, 8 cases](https://openairdocs.org/ntsb-events/440/sequence.md?model=1182206), [All aircraft, 3,365 cases](https://openairdocs.org/ntsb-events/440/sequence.md) - [Landing-flare/touchdown: Roll over](https://openairdocs.org/ntsb-events/097); cases with it: [Bell 407, 1 case](https://openairdocs.org/ntsb-events/097/sequence.md?model=1182206), [All aircraft, 261 cases](https://openairdocs.org/ntsb-events/097/sequence.md) ## Analysis The pilot reported that, while conducting a reconnaissance orbit over mountainous terrain, he crossed over a ridgeline and then lost directional control of the helicopter. He applied full left pedal and lowered the nose and collective in an attempt to regain control. The helicopter subsequently impacted the uneven snow-covered terrain and rolled onto its right side. Postaccident examination of the airframe and engine revealed no evidence of mechanical malfunction or failures that would have precluded normal operation. A weather study for the area around the accident site revealed that the temperature profile and windy environment supported strong up/downdrafts with the potential for mountain wave formation near the altitude at which the flight was maneuvering. It is likely that the pilot was unable to maintain control of the helicopter as a result of a loss of tail rotor effectiveness while operating in the high-altitude environment and in an area with strong up/downdrafts and possible mountain wave activity. ## Factual narrative On April 25, 2013, about 1016 mountain daylight time, a Bell 407, helicopter, N937GR, was substantially damaged when it landed on uneven snow covered terrain while maneuvering about 10 miles northeast of Bigfork, Montana. The airline transport pilot (ATP) and two passengers were not injured. The helicopter was registered to Two Bear Management LLC, and operated by the Flathead County Sheriff's Office as a public use flight under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and a company flight plan was filed and activated. The flight originated from Kalispell City Airport, Kalispell, Montana at about 0945.The pilot reported that he flew directly to a telecommunications repeater site located near the summit of Mt. Aneas and after reaching the site he performed a reconnaissance orbit of the area. Mt. Aneas has a summit of 7,528 feet mean sea level (msl). The reconnaissance orbit was to determine the wind direction and speed in the area. While maneuvering about 60 knots, he stated that he lost directional control after crossing over a ridge line. He applied full left pedal to correct the right turn and lowered the nose and collective control. All attempts to regain directional control were unsuccessful. During the descent he made several power increases to avoid colliding with rocky terrain. As the helicopter impacted the uneven snow covered terrain the helicopter rolled onto its right side. Examination of the helicopter revealed that the right side of the fuselage had crush damage and the tail boom was buckled aft of the horizontal stabilizer. The helicopter was relocated to a secure location and examined by a Federal Aviation Administration (FAA) inspector. The postaccident examination of the airframe and engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation. As reported by a Senior National Transportation Safety Board (NTSB) Meteorologist in a NTSB Weather Study, a plot of local observations and Remote Automated Weather Station (RAWS) data located about 15 miles east of the accident site revealed at 0921, winds were 29 miles per hour (MPH) with gusts of 41 MPH and a temperature of 44° F. At 1021, winds were 29 MPH gusting to 47 MPH and a temperature of 44° F. The meteorologist also reported that the temperature profile and windy environment in the area supported mountain wave formation with a potential wave near 7,000 feet msl with strong up/downdrafts. According to FAA's Advisory Circular 90-95, loss of tail rotor effectiveness (LTE) "is a critical, low-speed aerodynamic flight characteristic that can result in an uncommanded rapid yaw rate which does not subside of its own accord and, if not corrected, can result in the loss of aircraft control. LTE is not related to a maintenance malfunction and may occur in varying degrees in all single main rotor helicopters in some of the conditions conducive to LTE to include a high powersetting, low airspeed, and a tailwind or left crosswind." ## Aircraft - Registration: [N937GR](https://openairdocs.org/deregistered/N937GR-2017-04-12), registration cancelled 2017-04-12, S/N 53282, built 1998 (OpenAirDocs) - Reported make and model: BELL BHT407 - Registry model: [Bell 407](https://openairdocs.org/model/bell-407) (registry code 1182206) (OpenAirDocs) - Registry engine: [Allison 250-C47B](https://openairdocs.org/engine/allison-250-c47b) (registry code 03109, turbo-shaft) (OpenAirDocs) - Category: Helicopter - Serial number: 53282 - Year built: 1998 - Airframe hours: 1308 - Engines: Allison 250-C47B 813 horsepower turbo shaft - Damage: Substantial damage - Flight: Public aircraft - local, Public use - Operator: Flathead County Sheriff's Office - Registered owner: Two Bear Management LLC - Airworthiness certificate: Standard, Normal - Last inspection: Continuous airworthiness, April 10, 2013, 6 hours before - Departed: S27, Kalispell, MT - Destination: S27, Kalispell, MT - Flight plan: Company VFR - Airport: S27, Kalispell City ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 40; injuries: none; flight instructor, airline transport; airplane ratings: single-engine land, single-engine sea, multi-engine land; instrument ratings: helicopter, airplane; medical: class 2, with waivers/limitations; last flight review March 18, 2013; seat: right; flight hours: 5,000 total, 141 in this make and model, 4,000 as pilot in command, 61 in the last 90 days, 12 in the last 30 days, 0 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 170° at 9 knots - Visibility: 10 statute miles - Temperature: 52 °F, dew point 30 °F - Altimeter: 29.88 inHg - Observed at: GPI, 18 nm away ## 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 20130425005599A](https://openairdocs.org/incident/20130425005599A), Apr 25, 2013: Bell 407, Big Fork, MT, Substantial damage. Aircraft on public use mission for sheriffs office attempted to land on mt aeneas, MT. Elev 7254 MSL. During high recon pilot stated he was at 65 knots IAS in… ## Docket (5 files) The files NTSB released with the investigation on June 19, 2014, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=86704. 9.5 MB in all. - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 1, .PDF, 3.4 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40413466&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Redacted.PDF - FAA Inspector’s Statement (item 2, .PDF, 97 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40413467&FileExtension=.PDF&FileName=FAA%20Inspector%E2%80%99s%20Statement-Redacted.PDF - NTSB Brief Weather Study Correspondence (item 3, .PDF, 201 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40413468&FileExtension=.PDF&FileName=NTSB%20Brief%20Weather%20Study%20Correspondence-Redacted.PDF - Statement of Party Representatives to NTSB Investigation-Bell Helicopters (item 4, .PDF, 741 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40413469&FileExtension=.PDF&FileName=Statement%20of%20Party%20Representatives%20to%20NTSB%20Investigation-Bell%20Helicopters-Redacted.PDF - Photo 1 – View of Main Wreckage From Side (Photo Courtesy of Sheriff’s Office) (item 5, .PDF, 5.1 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40413470&FileExtension=.PDF&FileName=Photo%201%20%E2%80%93%20View%20of%20Main%20Wreckage%20From%20Side%20(Photo%20Courtesy%20of%20Sheriff%E2%80%99s%20Office)%0D%0A-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.