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.