On August 13, 1998, approximately 1015 Pacific daylight time, a Cessna T337E, N78SC, impacted the terrain between the runway and the taxiway at Portland-Hillsboro Airport, Hillsboro, Oregon. The private pilot, who was the sole occupant, received minor injuries, and the aircraft, which was owned and operated by the pilot, sustained substantial damage. The 14 CFR Part 91 proficiency flight, which was being conducted in the traffic pattern, was being operated in visual meteorological conditions. No flight plan had been filed, and the ELT, which was set off by the impact, was turned off at the scene. According to the pilot, he had done a number of touch-and-go landings with no indication of any problem. But, while he was in the flare for another landing, the pilot's seat forward and aft movement locking pin disengaged, and his seat slid to its full aft position. As the seat began to move aft, the pilot grabbed the control wheel in his left hand and "...apparently moved the control wheel to the left." As the aircraft was rolling to the left, the pilot was able to move his seat forward, but he was unable to roll the aircraft back to a wings-level attitude prior to impacting the terrain. During the investigation it was determined that AD 87-20-03 R2 (seat track visual inspection) had been signed off as complied with on 12/18/97, as part of the last annual inspection. The seat track and locking pin were also inspected by the FAA Airworthiness Inspector who responded to the accident, with no discrepancies found.