On June 8, 2022, about 1134 Alaska daylight time, a Cessna U206F, N592KB, was substantially damaged when it was involved in an accident 65 miles west-northwest of Kodiak, Alaska. The pilot and four passengers were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 135 air tour flight.
According to the pilot, the airplane that he was flying was the fourth company airplane to land at a remote beach site, known as Hallo Bay, as part of a bear viewing flight. The pilot set up for a landing on the same tracks as the three preceding company airplanes. During the landing, the left main landing gear wheel touched down first, followed by the right main landing gear wheel. When the right wheel touched down, the pilot heard a “pop” sound and the airplane veered to the right. The airplane slowly came to rest upright about 150 ft from the initial touchdown location (see figure 1). The pilot reported that small leaves and seaweed debris were present in the landing area and that no large debris was noted.
Figure 1. Accident airplane in its resting location (Source: K Bay Air).
Postaccident examination of the airplane revealed substantial damage to the left wing and right horizontal stabilizer. The right main landing gear leg was fractured near the wheel attachment fitting and displayed fatigue signatures (see figure 2.).
Figure 2. Fractured right main landing gear leg (Source: K Bay Air).
A review of the airplane’s records revealed that, in July 2019, the main landing gear legs were removed and repaired per the manufacturer’s service manual. Between the time of repair and the accident, the main landing gear legs accumulated about 875 hours and about 200 beach/off-airport landings.
Metallurgical examination of the fractured landing gear leg by the National Transportation Safety Board’s Materials Laboratory found that the gear leg, which was composed of hardened alloy steel, had fractured through the cross-section, perpendicular to the longitudinal direction. River patterns on the fracture surface emanated from an area consistent with the start of the fracture sequence.
A closer view of the area revealed several thumbnail-shaped cracks with a large, vertically orientated ratchet mark on the fracture surface. The cracks exhibited crack arrest marks, consistent with crack propagation, and ratchet marks, consistent with multiple crack initiation sites. The end of the largest thumbnail crack on the fracture surface exhibited fatigue striations, consistent with fatigue crack propagation. The rest of the fracture surface exhibited dimpled rupture, consistent with overstress fracture.
The crack initiation sites contained iron oxides with elements typical of salt and marine environments. The fatigue crack initiation site near the larger ratchet mark contained a corrosion pit. These features indicated that the gear leg fractured due to the stress concentration of fatigue cracks that had propagated inward from the lower surface of the leg. The fatigue cracks initiated from corrosion pits in an area of the leg without paint and primer.