ACFT EQUIP PROB. IMPROPERLY SECURED COWLING CAME OFF INFLT. THE ACCESS COWLING WAS OPENED FOR OIL SVC JUST PRIOR TO FLT. PAX SAW THE UNLATCHED COWLING; BUT THOUGHT THE MECH WAS STILL WORKING ON IT.

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I PERFORMED AS AN FO ON AN ATR42. THE CREW HAD COMPLETED A 'QUICK-TURN.' I HAD PERFORMED A PREFLT INSPECTION; AND FOUND NO ABNORMALITIES. LATER; APPROX 5 MINS PRIOR TO DEP; A CERTIFIED MECH ENTERED THE COCKPIT AND MADE AN ENTRY IN THE MAINT LOG. I WAS BUSY (CLRNC; ETC); THE CAPT CHKED THE MAINT SIGN-OFF; AND MENTIONED NOTHING TO ME. AFTER TKOF; APPROX 500 FT AGL; WE HEARD AND FELT A 'THUD' ON THE L SIDE. WE DISCUSSED POSSIBLE BIRD STRIKE AND POSSIBLE COMPRESSOR STALL. NEITHER A VISUAL INSPECTION BY THE CAPT (ON L ENG) NOR COCKPIT INST READINGS REVEALED ANY ABNORMALITIES. THE FLT CONTINUED NORMALLY. HALFWAY TO DEST (HOU); OUR COMPANY CONTACTED US; AND REQUESTED WE INSPECT THE ACFT FOR MISSING PANELS. A MECH AT DFW HAD SEEN SOMETHING FALL OFF AN ATR AT OUR DEP TIME. NOW THE CAPT COULD SEE TORN METAL ON TOP OF THE L ENG. (THE PANEL DOOR OPENED FOR OIL CHKS IS THE L ONE; WHICH IS OUTBOARD ON THE L ENG; AND NOT VISIBLE FROM THE COCKPIT.) I INQUIRED ABOUT THE MAINT ENTRY JUST BEFORE DEP; AND LEARNED IT WAS A ROUTINE OIL CHK; SIGNED OFF AS COMPLETE BY THE MECH. THE FLT WAS COMPLETED WITHOUT FURTHER IRREGULARITIES. A PAX NOTED THE UNLATCHED ENG COWLING AS SHE BOARDED; BUT ELECTED NOT TO BRING ATTN TO IT; AS SHE SAW THE MECH AROUND THE ACFT. ONCE IN HOUSTON; WHEN WE ALL SAW THE WHOLE DOOR WAS MISSING; SHE MENTIONED SEEING IT UNLATCHED IN DFW.