On August 3, 2007, about 0900 mountain daylight time, a Luscombe 8E, N45486, impacted terrain while turning base to runway 36 at Polson Airport, Polson, Montana. The private pilot operated the airplane under the provisions of 14 CFR Part 91. The pilot was not injured, and the airplane was substantially damaged. Visual meteorological conditions prevailed, and no flight plan had been filed. The flight originated at Livingston, Montana, about 0630. The pilot reported to the National Transportation Safety Board investigator that as he was turning left base for final the controls jammed. He applied rudder and added power in an attempt to compensate but roll control was not regained. He reduced power and examined the cockpit area to see if anything was binding the controls. He did not identify anything in the cockpit that could have caused the binding. The airplane continued left thru two full turns (720 degrees) before the main landing gear contacted the ground, at which point the airplane cartwheeled and came to rest inverted. He egressed the airplane uninjured. The pilot stated to a Federal Aviation Administration (FAA) inspector that when he knew it was safe to approach the airplane, he checked the controls to see if they were still jammed and determined that they were no longer binding or jammed. They functioned normally. The on-scene FAA inspector examined the airplane's control system and could not identify any evidence of binding or jammed controls.