PREFLT INSPECTION -- ACR EXPERIENCED ROLL TO THE L AFTER TKOF AND THE FLC DECLARED AN EMER AND RETURNED TO LAND. IN THE PATTERN THEY DISCOVERED THAT THE AILERON TRIM WAS MIS-SET.

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AFTER TKOF ON RWY 18R; FO FLYING; STATED THAT THE ACFT WANTED TO ROLL HARD L. WE BOTH GOT ON THE CTLS TO MAINTAIN LEVEL FLT. WE TRIED TO EVALUATE THE SIT. COULD NOT FIND THE PROB. WE DECLARED AN EMER AND RETURNED AND LANDED WITH NO PROB. WE PERFORMED ALL CHKLIST AND COMPANY PROCS. ON DOWNWIND WE FOUND THE AILERON TRIM TO BE AT 10 UNITS L; IT SHOULD BE AT 1 OR ZERO. CONTRIBUTING FACTORS. 8.5 HRS ON DUTY. 3RD ACFT CHANGE WITH MINIMUM TURN TIME. FO CHKED AILERON TRIM AND IT APPEARED TO BE NORMAL 1 OR ZERO. CORRECTIVE ACTION. MARK TRIM WITH GREEN BAN OR REMOVE 10 UNIT POS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE FLC OF THIS EMB-120 BRASILIA SIMPLY MISSED THE AILERON TRIM HAVING BEEN MIS- SET. WHEN THE FO LOOKED AT THE TRIM INDICES AT THE REAR OF THE AISLE STAND HE THOUGHT THAT THE INDICATOR WAS POINTING AT 1 OR 0 INSTEAD OF 10 UNITS. AFTER THE FLC HAD DECLARED THE EMER AND ENTERED THE PATTERN FOR A LNDG THEY DISCOVERED THE SOURCE OF THE ROLL INPUT AND ZEROED THE TRIM. THEY LANDED ANYWAY AND HAD MAINT CONFIRM THEIR FINDINGS. THE FAA HAS INVESTIGATED THE INCIDENT AND DEFERRED DISCIPLINE TO THE COMPANY.