On September 6, 2000, at 1410 central daylight time, an Air Tractor 402B agricultural airplane, N6174H, was substantially damaged when it impacted an irrigation system while maneuvering near Seagraves, Texas. The airplane was registered to and operated by McIntyre Spraying Service Inc., of Seagraves. The commercial pilot, sole occupant of the airplane, was not injured. Visual meteorological conditions prevailed for the 14 Code of Federal Regulations (CFR) Part 137 aerial application flight. The local flight originated from the Seagraves Airport at 1300. According to the pilot, he was flying from east to west applying malathion to a cotton field when the airplane encountered a "large whirlwind." The aircraft impacted an irrigation system separating both main landing gear. The pilot continued flying the airplane approximately 1 mile before executing a forced landing in a plowed field. An examination of the airplane by the FAA inspector revealed that both main landing gear separated from the airframe, the main wing carry-through spar was damaged, and the right wing spar was structurally damaged.