ATR72 CREW HAD MAINT CLOSE A NACELLE ACCESS DOOR. IT WAS NOT DONE PROPERLY. HAVING THE DOOR OPEN IN FLT RESULTED IN AN ENG FIRE WARNING.

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PRIOR TO BOARDING PAX ON JAN/TUE/02 FOR FLT ABCD; MAINT HAD BEEN WORKING ON THE ACFT REGARDING A PROB WITH THE 'A' IGNITER ON THE #2 ENG. AFTER THE MECH SIGNED IT OFF; THE FO DID A WALKAROUND AND SAID EVERYTHING LOOKED NORMAL. UPON CLBOUT; PASSING 500 FT; THE FO AND I FELT A NOTICEABLE RHYTHMIC VIBRATION. AS THE SPD OF THE ACFT INCREASED; THE VIBRATIONS SUBSIDED. I NOTIFIED ATC OF THE PROB AND TOLD THEM TO STAND BY AS I CONTACTED MAINT. WITH MAINT ON THE RADIO; I TOLD THE FLT ATTENDANTS TO SEE IF THEY SAW ANYTHING IRREGULAR. THE FLT ATTENDANT SAID AN ACR Y PLT WHO WAS SITTING IN THE BACK RPTED A SMALL ACCESS DOOR WAS OPEN ON TOP OF THE #2 ENG. I DECIDED; WITH MY FO AND MAINT; TO RETURN TO ATL. AFTER CONTACTING OUR DISPATCHER; WE BEGAN THE TURN TOWARD ATL. IN THE TURN WE RECEIVED A FIRE WARNING ON THE #2 ENG. AT THAT TIME I TOOK THE CTLS AND WE RAN THE EMER PROCS FOR ENG FIRE INFLT. AS THE PROP FEATHERED THE FIRE WARNING WENT AWAY. I DECLARED AN EMER WITH ATC APCH AND TOLD THE FO TO KEEP MAINT AND DISPATCH UPDATED AS WELL AS THE FLT ATTENDANTS AND THE PEOPLE. WE LANDED WITHOUT INCIDENT AND ON THE POSTFLT; NOTICED THE R ENG NACELLE PANEL HAD COME OFF.