On April 10, 2000, approximately 1245 Pacific daylight time, an Aero Design Eleven model 12, N80XP, registered to and being flown by a commercial pilot, was substantially damaged during a loss of directional control on landing roll at Felts Field, Spokane, Washington. The pilot was uninjured. Visual meteorological conditions existed and no flight plan had been filed. The flight, which was personal, was to have been operated under 14CFR91, and originated from Spangle, Washington, approximately noon. The pilot reported that upon landing on runway 21R, and while decelerating through about 25 miles per hour the aircraft began to "hunt." In an effort to maintain directional control the pilot applied braking however, the aircraft ground looped coming to rest on the east side of the runway. When he exited the aircraft he noted that the tail wheel assembly had separated from the airframe. Post-crash examination by an inspector from the Federal Aviation Administration's Spokane Flight Standards District Office revealed that the tail wheel assembly was attached to a "stinger" (solid steel cylindrical rod). This stinger inserted into a cylindrical sleeve or hollow tube and was held in place by a single AN4 bolt and associated nut. The tube was attached at two points to the airframe; 1) the base of a vertical tube or rudder post, and 2) at the center of a laterally mounted cross tube (refer to DIAGRAM I). The sleeve in which the stinger had been bolted into was broken free from both its attach points and remained with the tail wheel assembly. Closer examination of the separation surfaces of the sleeve where it attached to the lateral cross tube revealed discoloration of a portion of the surfaces. The pilot/owner reported that upon "examining the weld on the broken end of (the) tailwheel mount (where the stinger tube is welded to the cross tube) only the top and bottom of the fish mouth joint had been welded at the point where it joins the (laterally mounted) cross tube between the lower longerons..." and the "failure at this joint allowed the tailwheel to break free of the airframe..." (refer to ATTACHMENT PS-I). The airframe total time at the time of the accident was 153.3 hours.