ATR72 EXPERIENCED ELEVATOR TRIM PROB DURING CLB. TRIM INDICATOR SHOWED FULL NOSE DOWN. RPTR CAPT TOOK OVER CTL OF ACFT; SLOWED AND RETURNED TO DEP ARPT FOR UNEVENTFUL LNDG.

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CONDITIONS PRIOR TO TKOF: LIGHT WET SNOW; OCCASIONAL LIGHT RAIN. PRECIP MELTING ON CONTACT WITH ACFT. TEMP +3 DEGS C. FO CHKED ACFT FOR CONTAMINATION ON WALKAROUND. I CHKED ACFT FOR CONTAMINATION AFTER SETTLING INTO COCKPIT. (WENT OUTSIDE FOR EXTERIOR CHK.) MONITORED OTHER ACFT AS THEY TAXIED BY FOR ANY POSSIBLE CONTAMINATION. ALL WERE CLEAN; OUR ACFT WAS CLEAN. CHKED ACFT FOR POSSIBLE CONTAMINATION PRIOR TO TKOF (BOTH PLTS) AGREED WAS CLEAN. BOTH PLTS AGREED DEICING WAS NOT NECESSARY. TKOF AND INITIAL CLBOUT NORMAL. AT INITIAL LEVELOFF ALT; 5000 FT; FO (PF) RPTED ACFT REQUIRED EXCESSIVE FORWARD CTL PRESSURE IN LEVELOFF. I TOOK ACFT CTL AND COULD NOT MAINTAIN ALT AS AIRSPD INCREASED (ACFT KEPT CLBING). ELEVATOR TRIM INDICATOR SHOWED FULL NOSE DOWN TRIM POS. I SLOWED ACFT BELOW 180 KTS AND WAS ABLE TO MAINTAIN ALT. NOTIFIED ATC AND RETURNED TO ORD; DID NOT DECLARE EMER. LNDG NORMAL WITH FULL FLAPS. NO ICE ON ACFT DURING FLT; ACFT CLEAN AT LNDG. FINAL NOTE: PRIOR TO TKOF; ALL ACFT CTLS OPERATED SMOOTHLY AND NORMALLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR CAPT VERIFIED ACFT AS ATR72A. HE HAS NO FEEDBACK ON CAUSE OF THE NOSE TRIM PROB AND HASN'T HEARD OF ANY SIMILAR PROBS WITH OTHER ACFT OF THE SAME MAKE/MODEL. APPARENTLY THE ATR72 HAS AN AERODYNAMIC GAP BTWN THE HORIZ STABILIZER AND THE ELEVATOR WHICH THEORETICALLY PREVENTS ICING PROBS ON THE HORIZ TAIL SURFACES. WITH SPD INCREASING IT TOOK MUCH MORE FORCE TO HOLD THE ELEVATOR; BUT AS SPD WAS REDUCED THE FORCES REDUCED MAKING ACFT CTLABLE FOR NORMAL LNDG. RPTR WAS ASKED IF IT WAS POSSIBLE THAT THE TAIL ICED UP WHEN FLYING AT THE HIGHER/COLDER ALTS AND THEN UPON DSCNT THE ICE MELTED OFF AIDING IN MORE NORMAL ELEVATOR MOVEMENT. RPTR THOUGHT THAT WAS POSSIBLE BUT STILL DIDN'T KNOW SPECIFICALLY THE CAUSE OF THE ELEVATOR TRIM POS.