On July 18, 2016, about 1230 Alaska daylight time, a tailwheel-equipped Stinson 108 airplane, N343C, sustained substantial damage following a loss of control during the landing rollout at Haines Airport, Haines, Alaska. The certificated private pilot, and three passengers were not injured. The airplane was registered to and operated by the pilot, as a visual flight rules (VFR) personal 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. The flight departed the Skagway Airport (SGY), Alaska, at about 1200, destined for the Haines Airport (HNS). The pilot stated in the National Transportation Safety Board (NTSB) Form 6120.1, that he listened to the automated weather at HNS prior to arrival. He elected to land runway 26 with a quartering tailwind because the winds were light and variable. He performed a normal wheel landing, and during the roll out, when the airplane was about 20 mph, he started a right turn towards taxiway B, a high-speed exit taxiway. As he initiated the turn, the airplane abruptly turned right, which resulted in a 180° right ground loop. The pilot stated that he applied full left rudder and left brake during the turn, however the right turn continued. The left main landing gear collapsed and the fuselage and left wing impacted the runway surface. Substantial damage was sustained by the left wing, aileron, lift strut and lower fuselage. The pilot also stated that the winds were stronger and gusting from various directions after the accident. In an interview, a passenger who was in the front right seat stated that during the approach he heard the automated weather at HNS report wind at 4 knots, but he could not remember the reported direction. He said that the landing felt normal to him, however near the end of the landing roll, the airplane made an unexpected abrupt right turn, and then collapsed onto the left side during the ground loop. He stated that after the accident he noticed that the winds were gusting at times. Photographs revealed metal scrapes on the runway surface that were collocated with rubber wheel skid marks from the left main landing gear wheel. The ground scars began about 50 feet prior to the tight ground loop signatures. The left main landing gear wheel assembly separated from the left main landing gear leg at the axle weld, and came to rest about 15 feet behind the wreckage. The left main landing gear leg separated from the fuselage near the upper shock strut attachment points. There were no tailwheel abnormalities observed at the accident scene. The airplane was towed to a hangar on the right main landing gear wheel and tailwheel. During a post-accident examination of the airplane after recovery, the pilot discovered that the airframe tailwheel assembly attachment mount was fractured through the entire width. The bracket is one of two airframe attachments points for the tailwheel assembly. The tailwheel attachment mount and left main landing gear wheel hub and axle were sent to the NTSB Material Laboratory for detailed examinations. An NTSB materials engineer conducted a detailed examination of the fractured tailwheel assembly. Features consistent with corrosion intermixed with overstress failure were present at the bracket weld. The fractured surface of the left wheel axle was also examined. The features on this fracture surface were generally consistent with tensile overstress, such as microscopic dimple ruptures. The wheel axle also exhibited some areas with intergranular fracture and irregular weld material. A Materials Laboratory Factual Report is included in the public docket. The airplane had total time of 3,305 hours, and an annual inspection had been completed on August 14, 2015. According to the pilot's statement, the landing weight was estimated at 2200 lbs., which was 30 lbs. under the maximum gross weight limit for this airplane. The pilot originally stated the landing weight at 2240 lbs. on the NTSB form 6120.1, but then changed it via email. The pilot had logged about 1001 total flight hours and 900 hours of pilot in command time in the accident airplane. His last biennial flight review was conducted in the accident airplane on March 29, 2016. The closest weather reporting facility was HNS. At 1154, a HNS METAR reported in part: wind from 150° at 3 knots; sky condition, clear; visibility 10 statute miles; temperature 70° F; dew point 57° F; barometric pressure 29.90 inches of mercury.