ANC16LA024

The commercial pilot stated that, during the landing rollout in the tailwheel-equipped airplane, the left wheel axle fractured and separated from the airplane with the wheel attached. The left landing gear leg then dug into the runway surface, the airplane made a sudden left turn, and the left wing and left horizontal stabilizer struck the runway. An examination of the fractured left axle revealed features consistent with a bending overstress fracture. High magnification optical examination of the fracture also revealed several small fatigue cracks within the crack origin area. Although several fatigue initiation sites were present along the fracture origin area, none of them were large enough to have led to the failure or to have significantly lowered the ultimate strength of the axle enough for a bending overstress failure to occur under normal loading conditions. Therefore, it is likely the landing gear axle failed as a result of bending overstress due to a hard landing.

On May 3, 2016, about 1230 Alaska daylight time, a tailwheel-equipped Cessna 180 airplane, N6479X, sustained substantial damage following a separation of the left main wheel and axle from the landing gear strut during the landing rollout at the Naknek airport, Naknek, Alaska. The commercial pilot and sole occupant were not injured. The airplane was registered to Skol-Alaska, LLC and operated by the pilot under the provisions of 14 Code of Federal Regulations (CFR) Part 91 as a personal flight. Visual meteorological conditions prevailed at the time of the accident and a Federal Aviation Administration (FAA) visual flight rules (VFR) flight plan had been filed and activated. During an interview with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on May 3, 2016, the pilot stated that during the landing roll out, the left wheel axle fractured and separated from the airplane with the wheel attached. As the left landing gear leg dug into the runway surface, the airplane made a sudden left turn, and the left wing and left horizontal stabilizer struck the runway. The airplane sustained substantial damage to the left wing and left horizontal stabilizer. The fractured wheel axle was sent to the NTSB Materials Laboratory in Washington, D.C. for further examination. The fracture surface showed several areas of post-fracture damage. The undamaged portions of the fracture surface were optically, matte grey and highly textured consistent with a bending overstress fracture. Radial lines visible on the fracture indicated a wide area initiation at the outer diameter surface near the inboard end of the axle. High magnification optical examination of the fracture also revealed several small fatigue cracks within the crack origin area. A Materials Laboratory Factual Report is located in the public docket for this accident. The closest weather reporting facility is King Salmon Airport, King Salmon, Alaska, about 12 miles east of the accident site. At 1154, an aviation routine weather report (METAR) from the King Salmon Airport was reporting in part: wind from 080 degrees at 4 knots; sky condition, scattered at 2,700 feet AGL, broken at 5,000 feet AGL, broken at 7,500 feet AGL, broken at 20,000 AGL; visibility, 10 statute miles; temperature 52° F; dewpoint 36° F; barometric pressure 29.70 inHG.