On April 30, 2015, about 1615 eastern daylight time, an American Champion Aircraft 7AC, N85510, was substantially damaged while landing at Dillant-Hopkins Airport (EEN), Keene, New Hampshire. The sport pilot and passenger were not injured. The airplane was privately owned and operated. The personal flight conducted under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no flight plan was filed for the local flight, which originated from EEN about 1610.The sport pilot reported that he had completed one touch-and-go landing uneventfully at EEN and was planning a second landing to a full stop. He performed a three-point touchdown on runway 2, a 6,201-foot-long, 100-foot-wide, asphalt runway. The pilot added that during the touchdown, he held the control stick completely aft, when the tailwheel began to "shimmy" and the airplane departed the left side of the runway. The airplane traveled over grass and impacted an approach light indicator, before coming to rest upright. The pilot further stated that the tailwheel on that particular model was supposed to remain locked during landing, but had unlocked at touchdown. The sport pilot had accumulated a total flight experience of 115 hours; of which, 85 hours were in the same make and model as the accident airplane. He had flown 2.5 hours during the 90-day period preceding accident, and those hours were in the same make and model as the accident airplane. The two-seat tandem, high-wing, tailwheel airplane was manufactured in 1946. It was powered by a Continental Motors, C-85, 85-horsepower engine. The airplane's most recent annual inspection was completed on September 19, 2015. At that time, the airplane had accumulated 3,715 total hours of operation. The engine had accumulated 2,345 total hours of operation; of which, 545 hours were since its last major overhaul. The airplane had flown about 7 hours from the time of the last annual inspection, until the accident. Examination of the wreckage by a Federal Aviation Administration inspector revealed damage to the fuselage and right wing. The inspector also observed that the tailwheel locking mechanism appeared worn. The inspector added that the condition of the tailwheel locking mechanism should have been checked during the last annual inspection, but that the locking mechanism could have worn during the 7-month period from the time of the most recent annual inspection, until the accident. The recorded weather at EEN, at 1615, included wind from 210 degrees at 7 knots, visibility 10 miles, and clear sky.