On August 19, 2002, approximately 0935 Pacific daylight time, a Piper PA-18 airplane, N1277A, registered to and being flown by a private pilot who had recently bought the airplane, ran off the right side of runway 25 during landing at Chehalem Airpark (17S), Newberg, Oregon, and collided with a fence. The airplane was substantially damaged. At the time, the pilot, who was qualified and current under 14 CFR 61 to act as pilot-in-command of the airplane, had a certificated flight instructor aboard in order to fulfill an insurance policy provision requiring him to accumulate 10 hours in the aircraft prior to flying without an instructor. Neither pilot was injured. Visual meteorological conditions, with calm winds, were reported at McMinnville, Oregon, at 0953, and no flight plan had been filed for the 14 CFR 91 instructional flight, which departed Stark's Twin Oaks Airpark (7S3), Hillsboro, Oregon, about 0915. Both pilots reported that during the landing under calm wind conditions, the aircraft began to pull to the right, and that application of left rudder was unsuccessful in controlling the aircraft's direction. Subsequent to the accident the aircraft's brake system was examined. The examination revealed that the right main gear brake disc was warped (refer to attached NTSB Forms 6120.1).