ANC16LA046

The commercial pilot reported that he made a normal approach and landing to an unimproved landing strip and that the left main landing gear (MLG) wheel assembly separated from the landing gear leg at the axle bolts during rollout. The gear leg then dug into the gravel landing surface, which resulted in a sudden left turn, ground loop, and subsequent substantial damage to the left wing, left aileron, and elevator. The pilot and the passenger stated that it did not feel as if the airplane hit anything during the landing rollout. The pilot also stated that the initiating event may have been a crosswind landing on a hard-surfaced runway with large tundra tires earlier in the week, during which he inadvertently "put a side load on the landing gear." Postaccident examination of the left MLG axle assembly and bolts revealed that the left axle had separated from the landing gear leg due to a rearward overload event. The initial separation was due to the shearing of the forward bolt threads by tensile forces on the bolts, which was followed by the bending deformation of the aft bolts, consistent with a fore-to-aft overload.

On July 19, 2016, about 1030 Alaska daylight time, a tailwheel-equipped Cessna 185 airplane, N85CF, sustained substantial damage following a separation of the left main wheel and axle from the landing gear leg during the landing rollout at an unimproved landing strip about 20 miles northwest of Illiamna, Alaska. The commercial pilot, and three passengers were not injured. The airplane was registered to and operated by the pilot, as a visual flight rules (VFR) business flight, under the provisions of 14 Code of Federal Regulations (CFR) Part 91. Visual meteorological conditions prevailed at the time of the accident and no flight plan was filed. During an interview with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on July 20, 2016, the pilot stated that after a normal landing to a flat, level ridgeline, the left wheel and axle separated from the landing gear leg during the landing rollout. The gear leg then dug into the gravel landing surface which resulted in a sudden left turn and subsequent ground loop. The left wing, left aileron and elevator struck the ground and sustained substantial damage. The left wheel assembly came to rest about 25 feet from the airplane. A passenger of the accident flight stated during an interview that the she did not feel the landing was hard and did not feel the airplane hit anything on the landing roll out. The pilot stated on NTSB Form 6120.1, Pilot/Operator Aircraft Accident/Incident Report, that the landing was normal and he did not feel the aircraft hit anything. He stated that, while on large tundra tires, he inadvertently put a side load on the landing gear earlier in the week during a landing on a hard-surfaced runway with a strong crosswind. An NTSB metallurgist conducted an examination of the left axle connecting bolts. The bolts were identified as heavy duty and appropriate for installation per the Cessna 185 Illustrated Parts Catalog. Magnified examinations of the separations revealed fracture features and deformation patterns consistent with bending overstress separations. No indications of preexisting cracking or corrosion were visible. (Refer to the Materials Laboratory Factual Report in the public docket for further information regarding the fractured components.) The closest weather reporting facility is Illiamna Airport, Illiamna, Alaska, about 20 miles southeast of the accident site. At 0953, an Aviation Routine Weather Report (METAR) from the Illiamna Airport was reporting in part: wind from 320° at 10 knots gusting to 18 knots; sky condition, clear; visibility, 10 statute miles; temperature 63° F; dew point 48° F; barometric pressure 30.19 inHg.