AN ATR42 ON CLBOUT AT 10000 FT WITH AUTOPLT ENGAGED LEVELED OFF AT 13000 FT THEN DISPLAYED 'PITCH MISTRIM.' FLT CREW DISCONNECTED AUTOPLT AND RETURNED TO DEP ARPT CITING NEED FOR CONSIDERABLE FORWARD ELEVATOR PRESSURE.

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WE DEPARTED ZZZ AT XA45Z. WEATHER AT DEP WAS LIGHT RAIN AND +3 DEGS CENTIGRADE. UPON ENTERING CLOUDS WE ACTIVATED ALL DEICE AND ANTI-ICE SYSTEMS ON THE ACFT. WE CLBED OUT ON THE INSTRUMENT DEP. I HAND FLEW THE PLANE UP TO 10000 FT MSL AND THEN ACTIVATED THE AUTOPLT. THE AUTOPLT FLEW THE PLANE FROM 10000 FT UP TO 13000 FT AND LEVELED OUT FOR CRUISE. DURING THE CLB OUT WE ENCOUNTERED LIGHT TO MODERATE MIXED ICING. AFTER LEVEL OFF THE AUTOPLT DISPLAYED THE CAPT 'PITCH MISTRIM.' AT THIS POINT I DISCONNECTED THE AUTOPLT AND HAD TO HOLD CONSIDERABLE FORWARD PRESSURE ON THE YOKE TO KEEP THE ACFT FROM CLBING. I ALSO NOTICED THAT THE AUTOPLT HAD RUN THE PITCH TRIM FULL FORWARD. AT THIS POINT THE FO AND I DECIDED TO RETURN TO ZZZ. SO WE ASKED FOR VECTORS FOR THE ILS APCH. AS WE DESCENDED AND CONFIGURED FOR APCH; THE PITCH CONTROLS BEGAN TO ACT NORMALLY. WE NO LONGER HAD TO APPLY FORWARD PRESSURE ON THE CONTROLS TO MAINTAIN LEVEL FLT. WE LANDED NORMALLY AT ZZZ AND NOTIFIED DISPATCH AND MAINT CONTROL OF OUR PROBS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AIRPLANE WAS CHKED FOR ICE OR FROST ON THE PREFLT CHK AND NONE WAS NOTED. THE AIRPLANE WAS DISPATCHED IN A LIGHT RAIN AND +3 DEGS CENTIGRADE AND THE FLT CONTROLS WERE FREE AND OPERATED NORMALLY. THE ANTI-ICE SYSTEMS WERE ACTIVATED WHEN ENTERING CLOUDS AND REMAINED ON DURING THE ENTIRE FLT. THE AUTOPLT WAS ENGAGED AT 10000 FT AND CLBED THE AIRPLANE UP TO 13000 FT WHERE IT LEVELED THE AIRPLANE AND THEN DISPLAYED 'PITCH MISTRIM.' AUTOPLT WAS DISCONNECTED AND HAND FLOWN WHERE IT WAS NOTED FORWARD PRESSURE WAS REQUIRED TO MAINTAIN LEVEL FLT. RETURNED TO THE DEP ARPT WHERE MAINT DISCOVERED WATER FROZEN IN THE TAIL AREA.