Accident Details
On September 20, 2023, at about 1445 mountain daylight time, a Cessna U206, registration N3923G, was substantially damaged during an accident near Challis, Idaho. The pilot and three passengers were not injured. The flight was operated as a Title 14 Code of Federal Regulations Part 135 on-demand air taxi flight.
The pilot reported that after picking up passengers and taxing for departure, the left main gear spring assembly separated from the underside of the fuselage during the takeoff roll. The airplane then veered left off the runway. Contact with the terrain substantially damaged the left elevator and horizontal stabilizer.
Maintenance History
Review of the airframe maintenance logbook revealed a hand-written entry dated April 1, 2010, stating in part, “Main gear sent to TD Aero for magna flux, retemper.” Two maintenance documents from TD Aerospace Machine Inc., dated March 18, 2010, one for each main landing gear, detailed cleaning, inspection, and repair of the main gear springs. The documents referenced inspection per AC 43.13-1B and the Cessna 206 Service Manual, including magnetic particle inspection, heat treating, shot peening, and application of corrosion-protective primer. Attempts to contact TD Aerospace Machine were unsuccessful.
According to the owner, the operator maintained the fleet in accordance with Federal Aviation Regulations Part 43, Appendix D. Inspection of the main landing gear springs was on that checklist and addressed corrosion, cracks, or damage. A review of maintenance records indicated that the last annual inspection occurred on August 15, 2023, with no anomalies noted in the logbook.
Examination of the Left Main Gear Spring Assembly
The two halves of the fractured left main gear spring assembly were recovered and shipped to the National Transportation Safety Board’s Materials Laboratory for examination. X-ray fluorescence determined the material was consistent with an alloy steel. Hardness measurements averaged 50 HRC, consistent with a quenched and tempered condition.
After cleaning and examination, the fracture surface exhibited chevron marks radiating from an area on the lower surface. Three adjacent thumbnail-shaped features radiated from the lower surface, with the largest measuring 0.18 inches across and 0.054 inches deep. This feature showed parallel ratchet marks, indicative of multiple small cracks joining into a larger crack. Scanning electron microscope examination revealed a mix of fatigue striations and intergranular decohesion features on the thumbnail features. Outside these regions, the surface showed primarily ductile dimpling interspersed with intergranular facets consistent with overstress.
Further cleaning revealed a dimpled appearance consistent with shot peening on the bottom edge of the leg, with corrosion product observed in the dimples. After additional cleaning to remove primer, corrosion product was also observed on the surface.