A B737-300 INCURRED DAMAGE TO THE R ENG FAN THRUST REVERSER COWLING WHEN THE FLAPS AND SLATS WERE EXTENDED DURING MAINT PROC.

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#2 ENG HAD BOTH THRUST REVERSER COWLS OPENED. I WAS TRYING TO HELP ANOTHER TECHNICIAN FINISH UP. HE NEEDED AN ACO (ACFT CHANGE ORDER) COMPLIED WITH THAT REQUIRED THE FLAPS DOWN TO REMOVE THE WHEEL WELL PANELS. I TOLD HIM IF HE HELPED ME CLOSE THE INBOARD C-DUCT; I WOULD DROP THE FLAPS AND HELP HIM FINISH UP. WE ASKED THE TECHNICIAN UNDER THE OUTBOARD SIDE TO HELP US CLOSE THE INBOARD SIDE. AFTER THAT THEY RETURNED TO WHAT THEY WERE DOING. I BEGAN TO REMOVE THE PANELS TO THE POINT I NEEDED THE FLAPS DOWN. AT THAT POINT I WALKED THE ACFT AND FOUND NO PERSONNEL AROUND THE ACFT AND THOUGHT THE ACFT WAS CONFIGURED CORRECTLY. THINKING IT WAS THE B-700 NOT THE B-300; B-500 THAT ALLOWED YOU TO DROP THE FLAPS WITH THE OUTBOARD C-DUCT OPEN. AT THAT TIME I BEGAN TO DROP THE FLAPS UNTIL I WAS INFORMED BY THE TECHNICIAN WHO RETURNED TO THE #2 ENG THAT THE #5 SLAT HIT THE C-DUCT; I RETURNED THE FLAPS UP AND INSPECTED THE DAMAGE. AFTER THAT I INFORMED THE LEAD OF THE INCIDENT. THE ACFT WAS FOUND TO ONLY HAVE DAMAGE TO THE OUTBOARD C-DUCT. IT WAS REPLACED AND RETURNED TO SVC THE FOLLOWING DAY.