What was found
(CAN) AFTER REACHING CRUISE ALTITUDE ELEVATOR TRIM FAILED TO MOVE ANY FURTHER. NOSE DOWN AIRCRAFT RETURNED TO BASE AND AN ELEVATOR TRIM SYSTEM INSPECTION WAS CARRIED OUT. NO DISCREPANCIES COULD BE FOUND IN THE SYSTEM, BUT WATER WAS FOUND ON THE TAB ACTUATOR SCREW. IT IS BELIEVED THAT THIS WATER FROZE AT ALTITUDE ALLOWING NOSEUP TRIM BUT NOT NOSE DOWN. THE WATER WAS REMOVED, ACTUATORS GREASED AND A TEST FLIGHT CARRIED OUT. THE PROBLEM COULD NOT BE DUPLICATED IN THE HANGAR OR ON THE TEST FLIGHT. THE AIRCRAFT WAS RETURNED TO SERVICE.