AN EMBRAER 120 DECLARED AN EMER AND DIVERTED DUE TO THE #2 ENG COWLING OPENING IN FLT INCURRING DAMAGE TO THE COWLING.

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PER STANDARD PRACTICE; I COMPLETED WALKAROUND INSPECTION 20 MINS PRIOR TO DEP WITH NO DISCREPANCIES NOTED. SOMETIME LATER; AND BEFORE ENG START; IT WAS NOTED THAT INBOARD COWL LATCHES ON #1 ENG WERE OPEN AND DOOR WAS HANGING WITH VISIBLE GAP. CAPT CALLED MAINT PERSONNEL TO CORRECT THE SIT AND AT THAT TIME ASKED TO INSURE THAT I FOUND NO DISCREPANCIES IN WALKAROUND INSPECTION. HE ALSO CHKED THE ONBOARD MAINT AND INSPECTION LOG TO VERIFY THAT THE SHIP HAD NOT UNDERGONE MAINT DURING OUR BREAK BTWN FLTS. IT SHOWED NO EVIDENCE TO THAT EFFECT. MAINT PERSONNEL CAME OUT TO ACFT; THEN CAME UP TO COCKPIT AND SAID THEY HAD CORRECTED THE PROB. THE CAPT AND I VERIFIED THAT LATCHES VISIBLE FROM COCKPIT ON BOTH ENGS WERE CLOSED; THEN WE STARTED ENGS. TAXIED AND TOOK OFF AS NORMAL. AT 1000 FT MSL OFF DEP END OF RWY 13L; ACFT YAWED AND LOUD NOISE WAS HEARD. CAPT AND I CHKED OUR RESPECTIVE WING AND ENG AND I FOUND THE #2 ENG OUTBOARD COWLING HAD OPENED AND BEEN DAMAGED IN SLIPSTREAM. CAPT DECLARED EMER AND WE RETURNED FOR VISUAL APCH AND LNDG ON RWY 17L WITHOUT INCIDENT; BEING CAREFUL NOT TO ALLOW AIRSPD TO REACH EXCESSIVE VALUE TO AVOID ADDITIONAL DAMAGE. IT SHOULD BE NOTED THAT ANOTHER COMPANY DEP ON W SIDE OF FIELD SIMULTANEOUSLY EXPERIENCED AND DECLARED THE SAME EMER; ALTHOUGH THEIR COWL DID NOT SUSTAIN DAMAGE AND ACFT WAS RETURNED TO SVC THE SAME EVENING. ADDITIONALLY; I WAS ADVISED BY ANOTHER COMPANY MECH THAT AFTER THESE EVENTS TOOK PLACE; ALL COWL DOORS WERE CHKED BY MAINT ON ACFT THAT HAD NOT YET DEPARTED; AND 2 MORE DOORS WERE FOUND UNSECURED. WE LATER WERE ADVISED THAT ONE OF THE MECHS WAS CHKING ENG SERIAL NUMBERS ON ALL ACFT DURING TIME BTWN PUSHES. I BELIEVE THE COWLINGS ON OUR ACFT WERE OPENED AFTER THE WALKAROUND INSPECTION. I ALSO FEEL THAT THE FLC SHOULD HAVE BEEN ADVISED OF MAINT INTERVENTION AND REASON SO THAT WE WOULD HAVE LOGICALLY SURMISED THAT A CAREFUL RE-INSPECTION OF BOTH ENGS; INBOARD AND OUTBOARD DOORS WAS IN ORDER.