A Swearingen SA.227AC Metro III, OY-NPB, sustained substantial damage in a runway excursion accident at Oslo-Gardermoen (OSL), Norway. None of the eleven occupants were injured. The airplane operated on scheduled flight M3990 from OLA (?rland) to OSL. During roll-out following a normal landing, when the aircraft’s speed had decreased to less than 60 kt, the aircraft started an uncommanded turn to the right. The nose wheel steering system was activated. The course could not be corrected, and the aircraft veered off the side of the runway where it became stuck in hard-packed snow one meter deep. The investigation has shown that the loss of control was most likely caused by a fault in the nose wheel steering. The same problem had occurred six days earlier, but the commander then managed to disconnect the nose wheel steering in time and regained control. The fault could not be reproduced, and the aircraft was released to service after inspection and testing without remarks. The nose wheel steering system on this aircraft type is relatively complex, and over time several improvements have been made to improve its reliability. The Flight Manual has a caution related to activation of nose wheel steering at speeds higher than normal taxi speed. There was some freezing rain/freezing fog at the time of the accident. Comprehensive technical examination of the nose wheel steering on OY-NPB uncovered no single causal factor, but some indications of unsatisfactory maintenance. Irregularities that alone or in combination could have caused a temporary fault with the steering were present. The Accident Investigation Board believes that a temporary fault caused the nose wheel to unintentionally lock itself in a position towards the right. No other defects or irregularities that could explain why the aircraft veered off the runway were found.