PC-12
Flight Crew Air Transport Pilot (ATP)
Troubleshooting; Time Pressure; Distraction; Workload
Aircraft Equipment Problem Critical
Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft; Manuals; Procedure
Level at 15;000; autopilot(AP) on then the AP and yaw dampers (YD) disconnected. I noticed rudder trim was pegged max right. No 'tone' warning through headset to indicate runaway trim. No CAS annunciation (ie: 'YD Hold Nose Left or Right'). I grabbed the control wheel and countered the right rudder with left aileron. I was also holding left rudder; but could not put in enough input (could not push hard enough) to offset the full right rudder trim. So we were flying in a left wing down; forward slip in order to maintain heading. I slowed to reduce control pressures; and was able to maintain altitude at about 170 knots. We did not notice the right rudder trim travel until it was pegged because we were in and out of LT to MOD turbulence; which apparently masked the trim travel. My First Officer pulled out the QRH; but there was nothing listed for our particular fault (I looked later on and the closest thing I found to our situation was 'Abnormal Disconnect'). While I was flying; my First Officer attempted to center the trim with the rudder trim switch; and was successful. We completed the balance of the flight with the YD and AP off (and we did not touch the rudder trim switch again until we landed). After landing; I 'exercised' the rudder trim switch; and found that if I held right trim on the rocker switch; then released the switch at 'center' (green AC symbol); the trim continued to travel to the right until it stopped at full right. During the time that the above was happening; we were given a descent to 11;000 by ATC; which was accomplished without incident. We did not declare an emergency.
PC-12 Captain reported the autopilot and yaw dampener disconnected without other alerts. The rudder trim displaced full right until the First Officer centered the trim after a suspected trim switch malfunctioned.