A C650 departed the runway after landing when the nose wheel steering made a hard right turn after landing without pilot input or ability of pilot to control travel.

Narrative

Upon landing rollout; the C650 made an immediate uncommanded turn to the right departing the runway surface off onto the grass/clay surface and coming to rest some 90 FT from the runway. The conditions were dry runway; good VFR with northwest winds of 7 KTS gusting to 15 KTS. A visual left pattern was commenced along with the 'Before Landing' checklist completed. Following a normal landing and initial rollout through about 60 KTS; the C650 made the uncommanded turn to the right. The Pilot in Command flying (I was Second in Command in right seat) immediately gave maximum left rudder input and left braking. In fact; left main dual tires both left distinctive rubber marks on runway. This is very evident from picture taken at scene. Within about 3 seconds; the medium size jet exited the runway. No one was injured with 2 crew and 2 passengers on board. A Cessna technician found NO damage; however; the Citation did run over 1 runway light with right main wheel which showed no damage to gear/wheel. The runway light was replaced immediately. The following day Cessna technicians found no fault with nosewheel steering system; however; Cessna engineers did find; through situational analysis; that this exact situation could occur and recommended replacing the nosewheel steering unit. This was accomplished and signed off by Cessna. Plane was then placed back in service. I know the system well and even with say the nosewheel steering switch off; directional control would easily be maintained through differential braking with a free turning nosewheel. We did have the steering system armed before landing. Maintaining directional control should not be an issue unless this steering unit had some intermittent fault. The happening was so immediate and sudden that we the crew had just seconds to do anything if possible before exiting the runway edge.