ACFT EQUIPMENT PROBLEM; ALT DEVIATION.

Narrative

ON CLBOUT FROM 3000 TO 5000' AFTER DEP FROM DANBURY; CT; ON AN INSTRUMENT FLT PLAN TO CHARLESTON; WV; WHEN THE AUTOPLT PITCH TRIM WHEEL SWITCH WAS ROTATED TO LEVEL OFF AT 5000' THE ACFT CONTINUED TO TRIM UP AND CONTINUED UP TO APPROX 5300' BEFORE THE UNCONTROLLED TRIM UP CONDITION WAS CORRECTED BY DISABLING THE AUTOPLT WITH ITS MAIN SWITCH AND BY PULLING THE TRIM CB. THE ACFT WAS HAND FLOWN BACK TO THE CORRECT ALT WHILE MAINTAINING PROPER HDG. THE CTLR INQUIRED ABOUT THE ALT AND WAS INFORMED OF THE TRIM DIFFICULTIES. THE ACFT WAS IN VFR CONDITIONS ON TOP AND NOT IN CONFLICT WITH ANY OTHER TFC DURING THE MANEUVER. WHEN THE AUTOPLT WAS RE-ENGAGED IT WOULD HOLD COURSE BUT NOT ALT. AT DEST THE AUTOPLT WAS CHKED AND FOUND TO HAVE A QTY OF WATER IN A STATIC LINE CONNECTED TO THE ALT DIAPHRAGM TRIM UNIT IN THE TAIL. OUTSIDE AIR TEMP WAS APPROX -10 DEGS C; SO IT WOULD APPEAR THAT THE WATER CONDENSED IN THE STATIC LINE TO THE ALT TRIM; FROZE AND LOCKED THE TRIM FULL UP. WHEN IT MELTED THE TRIM WORKED OK. CONDENSATION IN THAT LINE HAS BEEN A PROB BEFORE AND DRAINING THE LINE IS A 50 HR MAINT ITEM WHICH I REQUIRE OF MY SVC FAC. IT JUST BUILT UP QUICKLY AND FROZE THIS TIME. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR STATES PROB OCCURS WHEN ACFT SITS ON GND IN NICE; WARM MOIST ENVIRONMENT AND THEN CLBS INTO COOLER AIR AND IT 'RAINS' INSIDE THE TUBE. THIS TUBE IS A LOOP (U TUBE) ARRANGEMENT. IN LIQUID FORM IT WOULD PROBABLY NOT CAUSE MUCH PROB; BUT WHEN IT FREEZES AND ALT HOLD IS ENGAGED; IT CAN CAUSE 200-300' ERRONEOUS READING. ALSO STATED MUST BE VERY CAREFUL WHEN BLOWING LINE AT SERVICING TO PROTECT THE DIAPHRAGM. RPTR IS CONSIDERING INSTALLING A DRAIN VALVE SO CAN CLEAR W/O REMOVING WHOLE BACK END OF BAGGAGE COMPARTMENT. HAS DISCUSSED PROB WITH MFR.