As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SEA - DURING APPROACH RUDDER TRIM WENT FROM NEUTRAL TO FULL RIGHT WITHOUT ANY PILOT INPUT TO RUDDER TRIM KNOB. RUNAWAY RUDDER TRIM CHECKLIST COMPLETED AND RUD TRIM ACT CB PULLED. LANDING UNEVENTFUL. REMOVED AND REPLACED RUDDER TRIM SWITCH IAW Q400 AMM 27-22-11 PGS 401-407. OPS CHECKS GOOD, NO FURTHER DEFECTS NOTED. RETURNED TO SERVICE.