On August 3, 1994, at 1530 central daylight time, a Piper PA-25 agricultural airplane, N6568Z, was substantially damaged when the pilot lost directional control during the landing roll at a private airstrip in Spiritwood, North Dakota. The pilot, who was not injured, had just returned from an aerial survey of a field. The 14 CFR Part 137 flight operated in visual meteorological conditions without flight plan. In a written statement, the pilot reported the airplane started to shudder and veered to the left upon landing on the private airstrip. He tried to regain directional control by applying the right brake and increasing power, but he states these actions only accelerated the turn. The airplane ground looped and came to rest on the left side of the runway. A postaccident investigation revealed eight inches of the vertical fuselage tube and the aft diagonal tube in the empennage near the tailwheel assembly were broken in several locations. Rust was observed on the inside of the tubing wall near the breaks.