What was found
AFTER LEVEL OFF AT FL290 WITH AN OAT OF -30 C, RECEIVED THE AP HOLDING ND CAS MESSAGE ALONG WITH MISTRIM ON BOTH PFDS. FOLLOWED THE CHECKLIST AND WHEN DISCONNECTING THE AUTOPILOT CONFIRMED THAT THE TRIM WAS FROZEN AND THE AIRPLANE WAS IN FACT OUT OF TRIM. TRIM NORMAL AT LOWER ALTITUDE. MAINTENANCE INSPECTED THE PITCH TRIM ACTUATORS FOR WATER INTRUSION. CHECKED CABLE TENSION. LUBED SYSTEM IAW NTO 940-2017-00487. A SUCCESSFUL OPS CHECK FLIGHT WAS PERFORMED. AIRCRAFT WAS RETURNED TO SERVICE.