Accident Details
On June 8, 2022, at approximately 1134 Alaska daylight time, a Cessna U206F (registration N592KB) was substantially damaged in an accident about 65 miles west-northwest of Kodiak, Alaska. The airplane was operating as a Title 14 Code of Federal Regulations Part 135 air tour flight for bear viewing. The pilot and four passengers were not injured.
According to the pilot, the airplane was the fourth company aircraft to land on a remote beach site known as Hallo Bay. The pilot configured for landing using the same tracks as the preceding airplanes. During the landing, the left main landing gear wheel touched down first, followed by the right wheel. When the right wheel contacted the ground, 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 point. The pilot reported that small leaves and seaweed debris were present in the landing area, noting no large debris.
Postaccident Examination
Examination of the airplane after the accident 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.
Records Review
A review of the airplane’s maintenance records indicated that in July 2019, the main landing gear legs were removed and repaired in accordance with the manufacturer’s service manual. Between that repair and the accident, the main landing gear legs accumulated about 875 hours and approximately 200 beach or off-airport landings.
Metallurgical Findings
The National Transportation Safety Board’s Materials Laboratory conducted a metallurgical examination of the fractured landing gear leg. The gear leg, 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.
Closer inspection revealed several thumbnail-shaped cracks with a large, vertically oriented 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 displayed fatigue striations, consistent with fatigue crack propagation. The remainder of the fracture surface showed 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 stress concentration from fatigue cracks that propagated inward from the lower surface of the leg. The fatigue cracks initiated from corrosion pits in an area of the leg lacking paint and primer.