What happened
On May 11, 2024, at approximately 08:25 UTC, a Cessna U206F, registration S5-DOT, was conducting skydiving operations at the Šentvid pri Stični airfield. While taxiing on the grass surface, the aircraft experienced a sudden failure of the right main landing gear spring strut. The failure caused the aircraft to tilt to the right, resulting in the right wing scraping the ground. The momentum caused the aircraft to tilt backward, leading to the deformation of the right horizontal stabilizer. The pilot successfully shut down the engine and evacuated the seven occupants (one pilot and six passengers) without injury. The incident caused damage to the landing gear, fuselage, right wing, and right horizontal stabilizer.
The investigation
The investigation focused on the structural integrity of the landing gear strut. Metallurgical analysis confirmed that the material used, AISI 6150 steel, met all international standards for high-strength spring steel. Chemical, microstructural, and hardness testing (Vickers) showed the steel was in a properly tempered and hardened state.
However, visual inspection of the strut revealed the presence of surface corrosion and areas where the coating had delaminated due to abrasion and corrosion. Investigators identified the crack origin as a corrosion pit measuring approximately 0.3 mm deep and 2 mm long. This pit acted as a stress concentrator, initiating a fatigue crack that progressed to a depth of 2.5 mm. The investigation noted that while a previous maintenance check in early 2022 had identified corrosion on both struts which was subsequently treated, the current failure originated from a pit exceeding the depth limits specified in the manufacturer's service manual.
Findings
- The direct cause of the incident was the sudden failure of the right main landing gear strut due to material fatigue originating from prior corrosion damage.
- An indirect contributing factor was the frequent operation of the aircraft on uneven grass surfaces, which accelerated fatigue and the progression of corrosion-related damage.
- The corrosion pit at the crack initiation site was approximately 0.3 mm deep, significantly exceeding the manufacturer's recommended threshold for shot peening repairs (0.0254 to 0.0508 mm).
- The aircraft's takeoff weight and center of gravity were within permissible limits, and the pilot was fully qualified and competent.