Accident Overview
On July 31, 2017, at about 1140 Pacific daylight time, an Air Tractor AT-802A, registration N70LA, was substantially damaged during the landing roll at Wells Municipal Airport/Harriet Field (LWL) in 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 for aerial firefighting. Visual meteorological conditions prevailed, and a company flight plan was filed for the cross-country flight, which departed Battle Mountain, Nevada, about 1042 as a Public Aircraft flight.
Flight and Landing Details
According to the pilot, the flight was uneventful until landing. He entered the traffic pattern on a left downwind leg for runway 26, the active runway. Based on the midfield windsock, he observed winds from the north at about 10 mph 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 decreased and the tailwheel began to settle, the pilot lost all rudder and aileron authority. The airplane entered a hard right turn and began to depart the right side of the runway. The pilot unlocked the tailwheel in an attempt to regain directional control. Runway tire marks indicate that the airplane initially veered slowly to the right, then rapidly departed 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 coming to rest on the north side of the runway.
Pilot and Witness Observations
The pilot reported that he had never experienced a loss of rudder and aileron control in his 3,787 total flight hours in the airplane make and model. He stated that no mechanical anomalies with the powerplant or control system occurred during the flight, except during the landing phase when control was lost. After the airplane came to rest, he confirmed continuity and function of the rudder and part of the aileron control. A representative from the Department of the Interior observed a build-up of fire retardant inside the porthole of the tailwheel lock. He also confirmed function of the rudder, elevator, and partial movement of the aileron, which had been damaged at the accident site.
Manufacturer and Weather Information
The airplane manufacturer reported that landing with the tailwheel unlocked can produce a noticeable shimmy but is unlikely to result in a loss of directional control. Additionally, the pilot is unlikely to observe 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 be successful. An NTSB weather study showed no surface frontal boundaries near the accident site at the time. A 500-hPa chart indicated a large ridge of high pressure over the western United States. Daily valley/mountain breezes and thermals were the main weather interactions. Visible imagery showed isolated cumulus clouds around the accident site, likely driven by thermals. An upper air sounding for 1100 PDT indicated no low-level wind shear or turbulence below 10,000 feet, with east to northeast winds around 5 to 10 knots from the surface through that altitude. The LWL airport manager reported that two windsocks are located south of runway 26 about midfield. In the summertime, thermals and other weather phenomena are common. She receives reports from pilots experiencing unforeseen rapid yaw moments and losses of directional control in the traffic pattern and on the ground, sometimes accompanied by impacts with runway lights, particularly during wind circulation and thermal activity.