C-GNQZ, a Pilatus PC-12/47E aircraft operated by North Star Air, was conducting flight BF711 from Pickle Lake, ON (CYPL) to Webequie, ON (CYWP). When the aircraft was in a climbing right turn after the departure from CYPL with the autopilot engaged, an un-commanded disconnect of the autopilot and yaw damper occurred. The flight crew attempted to re-trim the rudder to the left prior to re-engaging the autopilot and yaw damper, however the rudder trim remained stuck at approximately 80% nose-right and difficulty in maintaining straight flight was encountered. The flight crew elected to divert to Thunder Bay, ON (CYQT), and attempts to troubleshoot and rectify the problem by following the QRH and resetting the rudder trim circuit breaker were unsuccessful. Attempts to re-engage the autopilot and yaw damper generated a “YD (Yaw Damper) Hold Nose Left” Cockpit Alerting System message. The autopilot and yaw damper were left disengaged for the remainder of the flight. When the flight was approximately 120 nm North of CYQT, the flight crew declared an emergency. Since turning to the left was difficult, a right hand approach to Runway 25 was elected. The possibility of a compromised nose-wheel steering system was discussed, so it was decided to hold the nose-wheel off the ground as long as possible.
The subsequent landing was uneventful and the nose-wheel steering operated normally.
The operator’s maintenance replaced the autopilot trim adapter.