# NTSB WPR23LA350: Cessna U206C, N3923G, Challis, ID, September 20, 2023 - Type: NTSB accident investigation - Classification: Accident · No injuries · Class 3 · Limited · Final report - Date: September 20, 2023, 14:45 local (20:45 UTC) - Place: Challis, ID - Coordinates: 44.979439, -114.73477 - Nearby events: https://openairdocs.org/map.md?lat=44.9794&lon=-114.7348&mi=10 - Phase of flight: [Takeoff](https://openairdocs.org/ntsb-phases/300) (OpenAirDocs' grouping) - Defining event: [Takeoff: Landing gear collapse](https://openairdocs.org/ntsb-events/094) - Highest injury: No injuries - Injuries: 4 uninjured - Status: Completed - Report: Approved by Office Director - Investigation class: Class 3 - NTSB office: Western Pacific Region - Parties: Federal Aviation Administration - NTSB case key (mKey): 193112 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=WPR23LA350 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/193112/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=193112 - Page: https://openairdocs.org/ntsb/WPR23LA350 ## Probable cause The failure of the left main landing gear spring assembly due to a fatigue crack. Contributing to the accident was the improper maintenance on the landing gear spring assembly which allowed corrosion to develop. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Landing gear system: main landing gear](https://openairdocs.org/ntsb-findings/01023210): Aircraft › Aircraft systems · Fatigue/wear/corrosion (probable cause); cases with it: [Cessna U206C, 1 case](https://openairdocs.org/ntsb-findings/01023210/cases.md?model=2073332), [All aircraft, 241 cases](https://openairdocs.org/ntsb-findings/01023210/cases.md) - [Maintenance: scheduled/routine maintenance](https://openairdocs.org/ntsb-findings/02062010): Personnel issues › Task performance · Maintenance personnel (probable cause); cases with it: [Cessna U206C, 1 case](https://openairdocs.org/ntsb-findings/02062010/cases.md?model=2073332), [All aircraft, 320 cases](https://openairdocs.org/ntsb-findings/02062010/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Prior to flight: Aircraft maintenance event](https://openairdocs.org/ntsb-events/040); cases with it: [Cessna U206C, 1 case](https://openairdocs.org/ntsb-events/040/sequence.md?model=2073332), [All aircraft, 484 cases](https://openairdocs.org/ntsb-events/040/sequence.md) - [Takeoff: Landing gear collapse](https://openairdocs.org/ntsb-events/094) (defining event); cases with it: [Cessna U206C, 1 case](https://openairdocs.org/ntsb-events/094/sequence.md?model=2073332), [All aircraft, 1,787 cases](https://openairdocs.org/ntsb-events/094/sequence.md) ## Analysis The pilot was in the process of taking off when the left main gear spring assembly fractured. The airplane veered off the runway and ground contact substantially damaged the left horizontal stabilizer and elevator. Metallurgical examination of the failed gear spring assembly revealed it failed due to fatigue cracking and overload stresses. Further examination revealed corrosion underneath the primer paint and inside the dimples of the shot-peened area, which likely initiated the development of the fatigue crack. A records review revealed that in March of 2010, the left and right main gear spring assemblies were inspected, heat treated, shot peened and painted with a zinc-rich primer. It is likely that corrosion existed on the landing gear before the application of the primer paint, and that the inadequate removal of the corrosion allowed fatigue cracking to propagate until the left assembly failed in overstress. ## Factual narrative On September 20, 2023, about 1445 mountain daylight time, a Cessna U206, N3923G, was substantially damaged when it was involved in an accident near Challis, Idaho. The pilot and three passengers were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 135 on-demand air taxi flight. The pilot reported that he had picked up the passengers and taxied for departure. During the takeoff roll, the left main gear spring assembly separated from the underside of the fuselage. The airplane then veered left off the runway. Contact with the terrain substantially damaged the left elevator and horizontal stabilizer. Figure 1 is an image of the fractured left main gear spring assembly. Figure 1. The two fractured sections of the Left main gear spring assembly. The two halves of the left main gear spring assembly were recovered and shipped to the National Transportation Safety Board’s Materials Laboratory for examination. The left main gear spring assembly was inspected using X-ray Fluorescence and was found to be consistent with an alloy steel. Hardness measurements were taken per American Society for Testing Materials (ASTM) E18. The average hardness was found to be 50 HRC, consistent with alloy steel in a quenched and tempered condition. Further cleaning and examination revealed the surface exhibited chevron marks radiating from an area on the lower surface. Three adjacent thumbnail-shaped features radiated from the lower surface. The largest of these features measured 0.18 inches (4.6 mm) across, with a depth of 0.054 inches (1.4 mm). This feature exhibited a series of parallel ratchet marks, indicative of multiple small cracks joining into a larger singular crack. The piece was then examined in a field-emission scanning electron microscope. The surface in the thumbnail features exhibited a mix of fatigue striations and intergranular decohesion features. Outside of these regions, the surface showed primarily ductile dimpling interspersed with intergranular facets consistent with overstress. The piece was again subjected to acetone cleaning in an ultrasound bath to strip paint and primer away from the bottom edge of the leg. The surface had a dimpled appearance, consistent with shot peening. Corrosion product was observed in the dimples. The piece was subjected to further cleaning in acetone and a soft scrubbing pad was used to remove primer that continued to adhere. Corrosion product was observed on the surface after the primer was removed. Review of the airframe maintenance logbook revealed a handwritten write up dated April 1, 2010, that stated in part, “Main gear sent to TD Aero for magna flux, retemper.” Two TD Aerospace Machine Inc. Form TD8130 (1/10) maintenance documents, dated March 18, 2010, one for each of the main landing gear, were reviewed. Each form stated, in part, “Main gear spring: clean and inspect per AC 43.13-1B, Chapter 4 Section 1 and 2. Chapter 5 Section 1,2,4 (Ref ASTM E1444-94A) 5,6,7,10, and 12.Repaired per the Cessna 206 Service Manual and MIL-H-6875 where applicable, Dimensions, Heat Treat and Shot Peened checking against Cessna drawing 142 1601. Magnetic particle inspection, per ASTM-E-1444-94A, CLASS B. Zinc-rich primer meeting TIP-1757A, MIL-P-6889A type 1 MIL-P-8585A type 1 applied. Primer coating meets corrosion/rust protection requirements of AC 43.13 1B Section 3 and the Cessna 206 service manual.” Attempts to contact TD Aerospace Machine were unsuccessful. According to the owner, the operator maintained the fleet in accordance with Federal Aviation Regulations 43, appendix D and inspection of the main landing gear springs was on that checklist and the inspection addressed corrosion, cracks or damage. A review of maintenance manuals revealed the last annual inspection occurred on August 15, 2023. No anomalies were noted in the maintenance logbook. ## Aircraft - Registration: [N3923G](https://openairdocs.org/aircraft/N3923G), registered now, S/N U206-0923, built 1967 (OpenAirDocs) - Reported make and model: CESSNA U206C - Registry model: [Cessna U206C](https://openairdocs.org/model/cessna-u206c) (registry code 2073332) (OpenAirDocs) - Registry engine: [Continental IO-520 series](https://openairdocs.org/engine/continental-io-520-series) (registry code 17032, reciprocating) (OpenAirDocs) - Category: Airplane - Serial number: U206-0923 - Year built: 1967 - Airframe hours: 10473.2 - Engines: Continental IO-550 300 horsepower reciprocating - Damage: Substantial damage - Flight: Business, Part 135 - Operator: Middle Fork Aviation Inc - Registered owner: Middle Fork Aviation Inc - Airworthiness certificate: Standard, Normal - Last inspection: Annual, August 15, 2023 - Destination: LLJ, Challis, ID - Flight plan: Company VFR - Airport: U54, Bernard USFS, runway 17/35, 1,900 × 150 ft, dirt - Equipment: ADS-B, Stall warning system ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 20; injuries: none; commercial, flight instructor, private; airplane ratings: single-engine land; instrument ratings: airplane; medical: class 2, without waivers/limitations; last flight review August 6, 2023; seat: left; restraint: 3-point; flight hours: 980.5 total, 204 in this make and model, 900 as pilot in command, 224.6 in the last 90 days, 80 in the last 30 days, 4.1 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 360° at 3 knots - Visibility: 10 statute miles - Lowest ceiling: 11,000 ft broken - Temperature: 66 °F, dew point 34 °F - Altimeter: 29.77 inHg - Observed at: KSMN, 37 nm away - METAR: KSMN 202156Z AUTO 36003KT 10SM BKN110 19/01 A2977 RMK AO2 SLP782 T01890006 ## 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 20230920012359A](https://openairdocs.org/incident/20230920012359A), Sep 20, 2023: Cessna U206C, Bernard, ID, Substantial damage. Aircraft on takeoff roll from a backcountry forest service turf airstrip when left main gear leg suffered a complete and clean fracture about mid-leg. Aircraft… ## Docket (6 files) The files NTSB released with the investigation on February 26, 2025, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=193112. 16.2 MB in all. - Pilot/operator aircraft accident report, NTSB form 6120.1 (item 1, PDF, 8 pages, 2.6 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=16212351&FileExtension=pdf&FileName=BRWC8940204B43C_001084_Redacted-Rel.pdf - Photos (item 2, PDF, 3 pages, 1.7 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17215223&FileExtension=pdf&FileName=Photos%20for%20WPR23LA350-Rel.pdf - Airframe and engine examination report (item 3, PDF, 9 pages, 2.0 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17770643&FileExtension=pdf&FileName=A%26E%20Exam%20Report%20for%20WPR23LA350-Rel.pdf - Operators statement (item 4, PDF, 1 page, 73 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17771681&FileExtension=pdf&FileName=MFR_P_Nelson-Rel.pdf - Maintenance records (item 5, PDF, 8 pages, 8.7 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17633717&FileExtension=pdf&FileName=Miantenance%20Records%20Excerpts_Redacted-Rel.pdf - 24-035 Materials lab factual report (WPR23LA350) (item 6, PDF, 9 pages, 7 photos, 1.1 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=17636863&FileExtension=pdf&FileName=24-035%20Materials%20Lab%20Factual%20Report%20(WPR23LA350)-Rel.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.