On August 9, 1996, at 1200 central daylight time (cdt), a Grumman-Schweizer G-164B, N6966K, registered to Wallace Flying Service, Inc., of Wallace, Nebraska, piloted by a commercial pilot, impacted terrain, while maneuvering during the aerial application of a herbicide. The airplane was destroyed. The pilot reported no injuries. The local CFR Part 137 flight was operating in visual meteorological conditions. No flight plan was on file. The flight originated from Wallace, Nebraska, at 1150 cdt. According to the pilot's written statement, after making six agriculture spraying passes over rolling terrain, the pilot turned downwind over trees and experienced a violent nose down followed by a left wing down. He recovered from the left wing down and experienced a right wing down and nose down attitude. The airplane struck the terrain with the right wing. A Federal Aviation Administration Principal Maintenance Inspector (PMI) represented the NTSB on-scene. The PMI said the pilot reported no mechanical deficiencies or operational problems prior to impact. The pilot believes he may have experienced a severe wind shear condition.