SA-227 AC Metro III
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Ground Event / Encounter Loss Of Aircraft Control; Ground Excursion Runway
Flight Crew Regained Aircraft Control
Human Factors
Near conclusion of landing rollout aircraft rolled into the grass parallel to the runway and then back onto the runway before exiting the runway and taxiing to the ramp. No damage to property or aircraft occurred.While slowing in the rollout the aircraft became uncontrollable with use of differential braking and power lever position settings in which the levers were even/parallel to each other. The aircraft began a turn to the left.After initial application of increased right engine reverse thrust; in an effort to further slow us down; failed to pull the plane back to the right I didn't think there was enough time for application of increased thrust on the left engine to prevent us from hitting the runway edge light we were aiming toward so in order to avoid damage to the aircraft; airport; and any person the plane continued to the left away from the light and into the grass where I thought the aircraft would be handled more safely and returned to the runway in a controllable manner.A series of events caused this event some of which were mechanical failures; mechanical disparities; leading to aircraft lack of control. Thank God that went pretty well; it could have been much worse. Happy I made a choice I thought was the safest and it had a safe conclusion. And disappointment that the airplane was unable to stay on the runway.Each pilot briefed the QRH individually and then again together before executing the operating procedures. I was proud of our preparation for a possible situation however we could have done better. Because I never felt like an emergency was occurring or was going to occur; I did not declare an emergency. In retrospect I should have advised tower of the aircraft's condition. Not having a mechanical malfunction of this sort would prevent this from happening. Better differential thrust control would help prevent this from occurring again.
SA-227 Captain reported a runway excursion when he lost directional control during the landing roll.