SA-227 AC Metro III
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical; Ground Event / Encounter Loss Of Aircraft Control; Ground Excursion Runway
General Maintenance Action
Aircraft
I was the Pilot Flying. Upon touchdown and rollout all the correct callouts were made and upon hearing my First Officer (FO) (the Pilot Monitoring (PM)) call '40kts'; I commanded 'speeds low' and I placed the speed levers to the low position which in turn will activate the Nose Wheel Steering (NWS) system allowing me to control the aircraft. Right as the speed levers were selected 'low' the aircraft made a sudden uncontrolled and un-commanded turn to the right. I depressed the NWS power-lever button in an attempt to regain control with no effect. At this time I recognized the loss of directional control and with my left hand turned the NWS switch to the OFF position; deactivating the NWS; and also commanded full reverse on the left engine in an attempt to regain directional control. At this point it was too late to regain control so I made the decision to shut down both engines by pulling both 'engine stop & feather' knobs. At this time the aircraft had come to a stop off of and to the right of the runway. At this point I; requested assistance and informed the tower that I would need to contact my company before further action could be taken. After securing the aircraft and determining the situation was safe I immediately contacted our chief pilot and informed him of the situation. We coordinated with Operations and Maintenance to formulate a recovery plan as well as working with Airport Ops and security. I did not discover any visible damage to the aircraft nor to any ground equipment or airport signs. No injuries.
SA-227AT Captain reported loss of directional control after engaging the nose wheel steering resulted in a hard over turn to the right.