On March 31, 2007, approximately 1830 mountain daylight time, a Swearingen SA-226T, N46SA, departed the runway and impacted a snow bank during the landing roll at McCall Municipal Airport, McCall, Idaho. The airline transport pilot and his six passengers were not injured, but the aircraft, which is registered to Matheson-Talmage LLC, sustained substantial damage. The 14 CFR Part 91 business flight, which departed Paine Field, Everett, Washington, approximately two hours and fifteen minutes prior to the accident, was being operated in visual meteorological conditions. The aircraft had been on an IFR flight plan. According to the pilot, the touchdown and rollout were uneventful until he activated/engaged the nose wheel steering system to make a left turn off the runway. At that time, the aircraft suddenly veered to the right and departed the side of the runway. After the aircraft departed the runway, it encountered a four foot high snow bank, which resulted in substantial damage to the airframe. After the accident, the nose wheel steering system was examined and tested, and it was determined that the nose wheel was making uncommanded turns when the system was activated. The source of the system malfunction that lead to the uncommanded turns could not be positively identified.