On August 8, 2012, about 1930 central daylight time, an experimental amateur-built Ristine Avid Mark IV airplane, N1328L, exited runway 12 at the Poplar Grove Airport (C77), near Poplar Grove, Illinois, following a loss of directional control. The airline transport pilot, who was the sole occupant, reported no injuries. The airplane sustained substantial empennage damage. The airplane was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual flight rules (VFR) conditions prevailed for the flight, which did not operate on a VFR flight plan. The local flight originated from C77 about 1910. According to the pilot, he performed about ten landings during light and variable wind conditions on the hard surface runway at C77. The landings were on centerline with no drift or "tire protest" and all of the landings were three-point near stall landings. The last landing was on centerline, straight with no left drift tendency until there was a sudden, uncontrolled “jerk” to the left. The pilot indicated that the motion could not be stopped with opposite rudder and the aircraft rolled off the runway onto the smooth grass. The airplane continued and ultimately came to rest in the long grass about 150 degrees from the landing direction. A Federal Aviation Administration inspector examined the airplane. The inspector found that the lower right side longeron tail-spring support area failed due to excessive corrosion.