On July 31, 2017, about 1140 Pacific daylight time, an Air Tractor AT-802A, N70LA, was substantially damaged during a landing roll at Wells Municipal Airport/Harriet Field (LWL), Wells, Nevada. The commercial pilot was not injured. The airplane was registered to Custom Air, Inc., operated by Henry's Aerial Service, Inc., and under contract with the Department of the Interior to provide aerial firefighting services. Visual meteorological conditions prevailed and a company flight plan was filed for the cross-country flight that departed Battle Mountain, Nevada, about 1042 as a Public Aircraft flight. According to the pilot, after an uneventful flight he entered the airport through the left downwind leg of the traffic pattern for runway 26, which was the active runway at the time of the accident. He observed winds from the north about 10 mph based on the position of the midfield windsock and configured the airplane for a wheel landing. The airplane touched down on the main landing gear at approximately 85 mph on the runway centerline. As the airspeed bled off and the tailwheel began to settle down to the runway surface, the pilot lost all rudder and aileron authority. After the airplane entered a hard right turn and began to depart the right side of the runway, the pilot unlocked the tailwheel to regain directional control. Runway tire marks show the airplane begin to slowly veer to the right of the runway centerline, followed by a rapid departure to the right side of the runway. The left main landing gear collided with an imperfection in the asphalt surface and separated. The left wing then impacted the ground and the airplane rotated 180 degrees before it came to rest on the north side of the runway. The pilot reported that he had never experienced a loss of rudder and aileron control in his 3,787 total flight hours of experience in the airplane make and model. In a subsequent statement, the pilot reported that he did not encounter any mechanical anomalies with the powerplant or control system that could have precluded normal operation during the entire flight, except for the landing phase of flight when he lost rudder and aileron authority. The pilot further stated that he confirmed continuity and function of the rudder and part of the aileron control after the airplane came to rest. A representative of the Department of the Interior reported that he observed a build-up of fire retardant inside the porthole of the tailwheel lock. He further stated that he confirmed function of the rudder, elevator and partial movement of the aileron, which had been damaged at the accident site. The airplane manufacturer reported that landing with the tailwheel unlocked can manifest as a noticeable shimmy, but is not likely to result in a loss of directional control. Additionally, the pilot is not likely to observe any unusual flying characteristics or vibrations if the tailwheel remains unlocked during flight. If the airplane touches down with the tailwheel in the "unlocked" position, the pilot's attempt to move the lock lever to the "locked" position may or may not successfully lock the tailwheel. An NTSB weather study did not show any surface frontal boundaries near the accident site around the time of the accident. In addition, a 500-hectopascal (hPa) chart, around 18,000 feet msl, showed a large ridge of high pressure over the western United States at 0500 PDT on the accident day. These two charts indicated that with no strong surface, mid-, or upper-level features, the daily valley/mountain breezes and thermals would be the main weather interactions on the accident day. These daily circulations include variable winds during the morning hours, with a more consistent wind around lunchtime through sunset. Visible imagery from 1830 UTC and 1845 UTC showed isolated cumulus clouds around the accident site, likely indicating that the thermals were the biggest driver in up and down motions around the terrain of the accident site. The upper air sounding using a weather model for the accident site for 1100 PDT did not indicate any low-level wind shear or turbulence below 10,000 feet msl, with an east to northeast wind around 5 to 10 knots from the surface through 10,000 feet msl. According to the LWL airport manager, the airport is equipped with two windsocks; both located south of runway 26 about midfield. She further remarked that in the summertime the area is occupied with thermals and other weather phenomena. Each year she receives reports from pilots who experience unforeseen rapid yaw moments, and losses of directional control, both in the airport traffic pattern and on the ground, sometimes accompanied by an impact with a runway light. These sudden changes in flight performance are reported during times of wind circulation and thermal activity.