On August 29, 1996, at 1930 central daylight time, an Air Tractor AT-401, N1558N, registered to a private owner, operated by South Plains Aero as a Title 14 CFR Part 137 aerial application flight, was substantially damaged following a loss of control while maneuvering near Plains, Texas. Visual meteorological conditions prevailed, and a flight plan was not filed. The commercial pilot sustained minor injuries. The local flight originated from Terry County Airport, Brownfield, Texas, at 1900. According to the pilot he was spraying a field oriented northwest to southeast. During a right turn on the west side of the field, "the high wing (lt. wing) very violently came over the top like a snap roll." He reported having the same experience on the previous two right turns on the west side. On the third right turn he made an extended turn with "plenty" of airspeed, and prevented the wing from going all the way over by applying full left forward stick, and left rudder. The airplane descended rapidly. The pilot also reported that the airplane "stalled" just prior to impact; however, he stated it did not stall during the right turn. The pilot reported that the wind was out of the northeast at 5 knots gusting to 10 knots, and he possibly encountered wind shear. Examination of the accident site by the FAA inspector revealed that the airplane was in an upright position with its engine inverted. The left main landing gear was separated, and the fuselage was wrinkled. Examination of the airplane and engine did not disclose any mechanical anomalies. Control continuity was established to all flight controls.