On March 3, 1994, approximately 1700 Pacific standard time (PST), an experimental Unrein 80-P51D, N51PL, experienced a gear collapse during the landing roll at Scappoose Industrial Airpark, Scappoose, Oregon. The private pilot, who was the sole occupant of the aircraft, was not injured, but the aircraft sustained substantial damage. The personal pleasure flight, which departed Kelso-Longview Airport about 15 minutes earlier, was in visual meteorological conditions at the time of the accident. There was no flight plan filed, and there was no report of an ELT activation. According to the pilot, the tailwheel locking mechanism accidently released either during flight or upon touchdown for a full-stop landing. With the tailwheel free to pivot, the pilot was unable to maintain directional control, and during the landing roll, the main gear was overloaded and collapsed. The pilot advised the FAA inspector who responded to the accident that he would redesign the tailwheel locking system so that it could not inadvertently unlock.