A CANADAIR CL65 WAS DISPATCHED IN NON COMPLIANCE WITH THE L ENG REVERSER COWLING ACCESS DOOR OPEN.

Narrative

I WAS DOING OVERNIGHT MAINT ON A CANADAIR REGIONAL JET. OUR HANGAR WAS FULL; THEREFORE; I WAS WORKING ON THE RAMP IN FRONT OF THE HANGAR WHERE THERE WAS POOR LIGHTING. I NOTICED THE SEAL MISSING FROM THE L-HAND ENG PYLON PANEL UNDER THE UPPER TRANSLATING COWL STANG FAIRING. IN ORDER TO INSTALL A NEW SEAL; THE UPPER TRANSLATING COWL DOOR HAD TO BE OPENED AND THE STANG FAIRING REMOVED TO GET THE PYLON PANEL OFF. I ELECTED TO LEAVE THE LOWER TRANSLATING COWL DOOR LATCHED TO MINIMIZE THE CHANCES OF JET OR PROP BLAST DAMAGE. I INSTALLED A NEW SEAL; REINSTALLED THE PANEL AND STANG FAIRING; AND CLOSED THE UPPER COWL DOOR. LATER; AFTER THE ACFT HAD BEEN REPOSITIONED TO THE GATE; I WAS REVIEWING ON NIGHT'S WORK IN MY MIND AS I CLEANED UP IN THE HANGAR. I COULD NOT RECALL WHETHER I HAD RELATCHED THE UPPER COWL DOOR. I IMMEDIATELY CALLED THE LINE SUPVR AND ASKED IF THERE HAD BEEN A CREW CALL-IN FOR FINDING THE DOOR UNLATCHED ON THEIR WALKAROUND. HE SAID NO. I THEN ASKED THAT HE IMMEDIATELY SEND A MECH OUT TO CHK THE DOOR AS THIS WAS RIGHT AT SCHEDULED DEP TIME. HE DID SO. 20 MINS PASSED AND I DIDN'T HEAR ANYTHING BACK; SO I CALLED AGAIN. THE SUPVR TOLD ME THE ACFT HAD DEPARTED WHEN THE MECH ARRIVED AT THE GATE. SOON THEREAFTER; THE ACFT RETURNED TO THE ARPT DUE TO A RUMBLING SOUND THAT CONCERNED THE CREW. THEY DECLARED AN EMER AS A PRECAUTION. WHEN THEY ARRIVED AT THE GATE THE L-HAND UPPER TRANSLATING COWL DOOR WAS FOUND OPEN.