ATR72 ON APCH RWY 17R WAS UNABLE TO TRIM AILERON. CLRED TO FLY A PATTERN TO DETERMINE FLT CHARACTERISTICS OF THE ACFT AT LOWER SPD AND RUDDER INPUTS. FOUND ACFT FLEW OK AND MADE A SUCCESSFUL LNDG. MAINT FOUND TRIM SWITCH BAD.

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WE HAD BEEN CRUISING AT FL210 ON OUR FLT FROM FLO TO HRL. WE ACCUMULATED SOME RIME ICE ON OUR DSCNT APPROX 80 NM N OF HRL. WE WERE CLRED FOR THE ILS RWY 17R INTO HRL APPROX 12 NM OUT. MY FO DISCONNECTED THE AUTOPLT AFTER THE ADU GAVE US A TRIM INDICATION. HE STARTED COMPLAINING ABOUT A HVY L WING. WE TRIED TO TRIM THE L WING UP; BUT NO RESPONSE. THE AILERON TRIM HAD FROZEN AND TRIM SWITCH WAS INOP. WE REQUESTED TO FLY A PATTERN; SO WE COULD CHK OUT AND OBSERVE THE ACFT'S RESPONSES TO LOWER SPDS AND DIFFERENT RUDDER TRIM INPUTS. AFTER WE CONFIRMED OUR CTL OF THE ACFT; WE LANDED SAFELY. MAINT WAS CALLED AND THEY REPLACED THE AILERON TRIM SWITCH. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISED HAS BEEN ON THE ACFT 1 1/2 YRS AND HAS NEVER HAD THIS PROB BEFORE. IS NOT AWARE IT HAS BEEN A PROB. COMPANY HAS TAKEN NO SPECIAL ACTION ON THE EVENT. DOES NOT BELIEVE ICING HAD ANYTHING TO DO WITH THE FAILURE OF THE AILERON TRIM SYS. AFTER A LNDG AND CHKING THE SYS; WAS ABLE TO GET THE TRIM TO MOVE ONE TIME AND THEN IT FAILED AGAIN. MAINT REPLACED THE FAULTY SWITCH. ADVISED THE ACFT MUST BE FLOWN USING THE TRIM CONSTANTLY.