On July 5, 1998, approximately 1050 mountain daylight time, a North American AT-6G, N8399H, was substantially damaged when it departed the side of the runway while on landing roll at Vance Brand Municipal Airport, Longmont, Colorado. The airline transport rated pilot and his passenger were not injured. The airplane was being operated by the pilot under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the cross-country flight which originated from Centennial Airport, Englewood, Colorado, approximately 50 minutes before the accident. No flight plan had been filed. The pilot said he requested airport advisories on unicom and was told that the active runway was 29, and that the wind was "light and variable." The pilot stated that he flew over the field to position himself for traffic pattern entry, and observed that the "windsock's movement was hardly perceptible and the velocity of the indicator indicated less than 5 knots." The pilot reported that after touchdown, "the aircraft's rollout path took multiple, short, quick inputs from the rudder to keep properly aligned with the runway centerline as the aircraft proceeded down the runway." He further reported the rudder inputs became ineffective after approximately 400 to 500 feet of landing roll and the aircraft departed the right side of the runway. The aircraft subsequently impacted an airport taxiway sign, damaging the left main landing gear and the left wing. The aircraft came to rest on its nose. The pilot reported to the Investigator-In-Charge that tail wheel steering cables had recently been replaced. Postaccident evaluation of the tail wheel steering cables by maintenance personnel found a tension of 5 pounds on these cables; 20 pounds is considered normal (see attached letter).