On March 26, 2015, about 1455 central daylight time, an Air Tractor Inc., AT-400 airplane, N136DB, was substantially damaged during a precautionary landing on runway 35 at the Levelland Municipal Airport (KLLN), Levelland, Texas. The commercial pilot was seriously injured. Visual meteorological conditions prevailed for the flight, which without a flight plan.. The airplane was registered to and operated by Devil Dusters Inc., under the provisions of 14 Code of Federal Regulations Part 91 as a post maintenance test flight. The local flight was originating at the time of the accident.According to the pilot, he was asked by HSI Turbine to "test fly" his airplane which they had just rebuilt. During a ground run of the engine an issue with the throttle linkage was found and corrected. The pilot conducted a preflight of the airplane and did not note any issues. After restarting the engine, the pilot went through his preflight checklist including verification that the flight controls were free and correct. The pilot did not discover any discrepancies during the preflight check and the takeoff roll was uneventful. As the airplane leveled out on the crosswind leg of the traffic pattern, the flight controls felt "stiff." The pilot elected to land the airplane and have the flight controls examined. During the downwind and crosswind legs of the pattern the flight controls continued to feel stiff. After turning to the final leg in the traffic pattern, the airplane started to roll to the left. The pilot attempted to level the wings at which time the flight controls "became loose" and the pilot observed the left aileron "flopping" up and down. The right aileron continued to work correctly. While trying to land on runway 35, the airplane bounced and rolled to the left. On the second bounce the pilot added power to try and regain control of the airplane. The airplane immediately rolled to the left and the left wing impacted the ground. The airplane cartwheeled and came to rest to the west of runway 35. The empennage, fuselage, and both wings were substantially damaged. According to the pilot, he had purchased the airplane as a "wreck" in Minnesota. It did not have an engine or propeller and had damage to the right wing and landing gear. He brought the airplane to HSI Turbine during the summer of 2014 to have it repaired. According to the owner of HSI Turbine, the right wing was sent to Air Tractor to be repaired. Several mechanics and mechanic's assistants worked on the airplane and during interviews, they recalled that it was a long project that they worked on sporadically over the winter and spring. All of the mechanics and assistants reported using the maintenance manuals provided by Air Tractor to guide all of their work. All of the mechanics and assistants reported using the existing hardware on the airplane to reinstall the wings and flight controls. One assistant recalled helping to remount the wings on the airplane and recalled that was in October or November of 2014. Another mechanic recalled installing the ailerons in January of 2015. The maintenance on the airplane was signed off on just prior to the accident flight. Work orders for the airplane, provided by HSI Turbines, indicated that "new hardware" had been used for the installation of the horizontal stabilizer, elevator, rudder, and wings. Specific work on the ailerons was not documented in the work orders provided. An inspector with the Federal Aviation Administration and an investigator with Air Tractor examined the wreckage of the airplane. Control continuity to the elevator and rudder was confirmed. Separation points were consistent with overload. Control continuity was established to the right aileron control. All of the hardware was installed correctly and separation points were consistent with impact forces and methods used to recover the wreckage from the accident site. Control continuity was established from the left aileron control, inboard to the long aileron pushrod. The hardware to connect the long aileron pushrod to the aileron bellcrank was not present. The mounting surface on the aileron pushrod was not damaged or elongated, consistent with the hardware not being present prior to the impact sequence. An examination of the remaining wreckage and accident site did not locate the missing hardware. According to the Air Tractor AT-400 Owner's Manual and Parts Manual, an AN24-19A clevis bolt and an AN364-428 nut should be used to connect the pushrod to the bellcrank. The hardware should be torqued between 30 and 40 inch pounds.