# NTSB GAA17CA225: Weatherly 620B, N9004M, Lewistown, MT, April 10, 2017 - Type: NTSB accident investigation - Classification: Accident · No injuries · Data collection · Final report - Date: April 10, 2017, 17:00 local (23:00 UTC) - Place: Lewistown, MT - Coordinates: 47.033611, -109.463333 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=47.0336&lon=-109.4633&mi=10 - Phase of flight: [Maneuvering](https://openairdocs.org/ntsb-phases/450) (OpenAirDocs' grouping) - Defining event: [Maneuvering-low-alt flying: Loss of visual reference](https://openairdocs.org/ntsb-events/402) - Highest injury: No injuries - Injuries: 1 uninjured - Status: Completed, report June 20, 2017 - Report: Approved by Office Director - NTSB office: General Aviation Accidents - Parties: FAA - NTSB case key (mKey): 94988 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=GAA17CA225 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/94988/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=94988 - Page: https://openairdocs.org/ntsb/GAA17CA225 ## Probable cause The pilot’s failure to maintain altitude during low-altitude maneuvers in flat light conditions. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Use of equip/info: aircraft control](https://openairdocs.org/ntsb-findings/02063040): Personnel issues › Task performance · Pilot (probable cause); cases with it: [Weatherly 620B, 3 cases](https://openairdocs.org/ntsb-findings/02063040/cases.md?model=9630605), [All aircraft, 9,308 cases](https://openairdocs.org/ntsb-findings/02063040/cases.md) - [Performance/control parameters: altitude](https://openairdocs.org/ntsb-findings/01062012): Aircraft › Aircraft oper/perf/capability · Not attained/maintained (probable cause); cases with it: [Weatherly 620B, 5 cases](https://openairdocs.org/ntsb-findings/01062012/cases.md?model=9630605), [All aircraft, 1,311 cases](https://openairdocs.org/ntsb-findings/01062012/cases.md) - [Light condition: flat light](https://openairdocs.org/ntsb-findings/03036025): Environmental issues › Conditions/weather/phenomena · Effect on personnel (probable cause); cases with it: [Weatherly 620B, 1 case](https://openairdocs.org/ntsb-findings/03036025/cases.md?model=9630605), [All aircraft, 42 cases](https://openairdocs.org/ntsb-findings/03036025/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Maneuvering-low-alt flying: Loss of visual reference](https://openairdocs.org/ntsb-events/402) (defining event); cases with it: [Weatherly 620B, 1 case](https://openairdocs.org/ntsb-events/402/sequence.md?model=9630605), [All aircraft, 242 cases](https://openairdocs.org/ntsb-events/402/sequence.md) - [Maneuvering-low-alt flying: Collision with terr/obj (non-CFIT) ](https://openairdocs.org/ntsb-events/470); cases with it: [Weatherly 620B, 9 cases](https://openairdocs.org/ntsb-events/470/sequence.md?model=9630605), [All aircraft, 10,154 cases](https://openairdocs.org/ntsb-events/470/sequence.md) ## Analysis The pilot reported that, during a low-altitude, practice agricultural application flight, he lost depth perception between the snow-covered ground and overcast sky. He added that the airplane impacted the ground and came to rest after sliding about 500 ft. The airplane sustained substantial damage to the right wing and fuselage. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. The Federal Aviation Administration's pamphlet, "Flying in Flat Light and White Out Conditions," states the following: Flat light is an optical illusion, also known as "sector or partial white out." It is not as severe as "white out" but the condition causes pilots to lose their depth-of-field and contrast in vision. Flat light conditions are usually accompanied by overcast skies inhibiting any good visual clues. Such conditions can occur anywhere in the world, primarily in snow covered areas but can occur in dust, sand, mud flats, or on glassy water. Flat light can completely obscure features of the terrain, creating an inability to distinguish distances and closure rates. As a result of this reflected light, it can give pilots the illusion of ascending or descending when actually flying level. However, with good judgment and proper training and planning, it is possible to safely operate an aircraft in flat light conditions. ## Factual narrative The pilot reported that during a low altitude, practice aerial application flight, he lost depth perception between the snow-covered ground and overcast sky. He added that the airplane impacted the ground, and came to rest after sliding about 500 ft. The airplane sustained substantial damage to the right wing and fuselage. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. Federal Aviation Administration's pamphlet, "Flying in Flat Light and White Out Conditions", which states: Flat light is an optical illusion, also known as "sector or partial white out." It is not as severe as "white out" but the condition causes pilots to lose their depth-of-field and contrast in vision. Flat light conditions are usually accompanied by overcast skies inhibiting any good visual clues. Such conditions can occur anywhere in the world, primarily in snow covered areas but can occur in dust, sand, mud flats, or on glassy water. Flat light can completely obscure features of the terrain, creating an inability to distinguish distances and closure rates. As a result of this reflected light, it can give pilots the illusion of ascending or descending when actually flying level. However, with good judgment and proper training and planning, it is possible to safely operate an aircraft in flat light conditions. ## Aircraft - Registration: [N9004M](https://openairdocs.org/deregistered/N9004M-2019-12-17), registration cancelled 2019-12-17, S/N 1604, built 1995 (OpenAirDocs) - Reported make and model: WEATHERLY AVIATION CO INC 620B (UNDESIGNAT) - Registry model: [Weatherly 620B](https://openairdocs.org/model/weatherly-620b) (registry code 9630605) (OpenAirDocs) - Registry engine: [Pratt & Whitney R-985 series](https://openairdocs.org/engine/pratt-whitney-r-985-series) (registry code 52008, reciprocating) (OpenAirDocs) - Category: Airplane - Serial number: 1604 - Year built: 1995 - Airframe hours: 2534.5 - Engines: P&W R-985/AN14B 450 horsepower reciprocating - Damage: Substantial damage - Flight: Aerial application, Part 137 - Operator: Skyline Aviation LLC - Registered owner: Skyline Aviation LLC - Airworthiness certificate: Special, Restricted (Special) - Last inspection: Annual, March 23, 2017 - Departed: LWT, LEWISTOWN, MT - Destination: LWT, LEWISTOWN, MT - Airport: LWT, Lewistown Muni - Equipment: Stall warning system ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 51; injuries: none; commercial; airplane ratings: multi-engine land, single-engine land; instrument ratings: airplane; medical: class 2, with waivers/limitations; last flight review August 2, 2016; seat: center; flight hours: 2,601 total, 557 in this make and model, 2,403 as pilot in command, 90 in the last 90 days, 6.1 in the last 30 days, 1 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 220° at 13 knots, gusting 26 - Visibility: 10 statute miles - Lowest ceiling: 8,000 ft broken - Temperature: 39 °F, dew point 27 °F - Altimeter: 29.95 inHg - Observed at: KLWT, 1 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 20170410002289A](https://openairdocs.org/incident/20170410002289A), Apr 10, 2017: Weatherly Aviation Co Inc 620B, Lewistown, MT, Substantial damage. On April 10, 2017, at 1700 MDT, a Weatherly 620B, N9004M, registered to Skyline aviation LLC, collided with the ground at Lewistown Airport, Lewistown,… ## Docket (2 files) The files NTSB released with the investigation on May 25, 2017, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=94988. 1.7 MB in all. - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 1, .PDF, 1.4 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40452537&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Master.PDF - Accident Photo (item 2, .PDF, 1 photo, 278 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40452538&FileExtension=.PDF&FileName=Accident%20Photo-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.