On March 4, 1997, at 1000 central standard time, a Swearingen Fairchild SA227-AC, N373PH, registered to Capital Associates of Lakewood, Colorado, and operated by Conquest Airlines of Austin, Texas, as Flight 502 under Title 14 CFR Part 135, departed the runway during the landing roll at the Abilene Regional Airport, Abilene, Texas. Visual meteorological conditions prevailed for the scheduled passenger flight and an instrument flight plan was filed. The flight crew of 2 and the 10 passengers were not injured and the airplane did not sustain damage. The flight originated from Austin, Texas, at 0920. The 0953 National Weather Observer (METAR) report gave the winds from 210 degrees at 12 knots with gusts to 24 knots. The active runways were 17L and 22. Air Traffic Approach Control personnel reported that the crew requested a visual approach to runway 35R. The tower controllers issued the landing clearance and issued the current winds as 200 degrees at 14 knots. Subsequently, the airplane landed on runway 35R and departed the runway approximately 200 yards south of taxiway Delta. Passengers were transported by ground vehicles to the terminal without further incident. The crew reported the approach speed was Vref plus 10 knots. The first officer stated that the airplane touched down in the first 800 to 1,000 feet of the runway, and he recalled the wind from 240 degrees at 12 knots. The airplane tracked the runway heading to a taxi speed of 30 to 40 knots at which time the first officer engaged the nose wheel steering. The airplane made an unexpected left turn and the first officer applied full reverse and braking. The captain took command and applied right rudder and right brake; however, the airplane stopped about 5 feet off the left side of the runway. The captain reported the winds from 230 degrees at 15 knots with less than a 10 knot tailwind component. The captain recalled that the aircraft touched down in the first third of the runway and was rolling out to the end taxiway at a ground speed of 15 to 20 mph when the first officer engaged the nose wheel steering button, and the aircraft veered left at a 45 degree angle to the runway. The planned flight schedule for the day was from Austin, to Abilene, San Angelo, Beaumont, Austin, and San Antonio, with the final stop at Laredo, Texas. Maintenance personnel examined the airplane and performed a taxi check during which they centered the nose wheel and then engaged the nose wheel steering (NWS). Each time (5) the NWS was engaged the "aircraft made a left turn that increased it's rate rapidly." The NWS was deferred per the operator MEL, and the aircraft flown to Austin, Texas, where maintenance personnel removed and replaced the #4 brake assembly, repaired a shorting wire at the nose wheel steering power lever switch, and replaced the power lever switch. The aircraft was returned to service on March 7, 1997, and the tendency for the aircraft to taxi left was not noted on subsequent flights. The director of operations computed a 13 knot tailwind for the landing which exceeded the company limit of 10 knots. He stated that skid marks found in the last 500 feet of the runway "seem to indicate a high rate of speed very late in the landing roll." According to the director of operations, the steering "may have disconnected when the pilot flying attempted to use it, but there is no evidence that the nosewheel moved 'hardover' to the left." When the nose wheel steering actuator was removed and tested, discrepancies were found. A search of incident records for N373PH showed no history of problems with this aircraft's steering. Company records for 1993 and 1994 indicated 5 incidents of SA227 aircraft veering to the left and departing the runway. Following a modification of the aircraft, per Service Bulletin 32-006 (copy enclosed) for increased reliability of the nose wheel steering (NWS) system and to provide a fault indication system to alert the pilot of caster mode failure, uncommanded nose wheel steering discrepancies were not found in the company records. The director of operations commented that the incident "would not have occurred had the aircraft been landed into the wind or if the captain had made the landing." He further stated that "possible inadvertent disconnection of the steering system was [a] contributing factor."