A B737 WAS DISPATCHED IN NON COMPLIANCE WITH A L WING UPPER PANEL NOT SECURED. PANEL FELL OFF AIRPLANE ON TAXI OUT.

Narrative

ON FEB/SUN/04; I WAS WORKING ON ACFT X; A B737 AT ACR X HANGAR AT ZZZ ARPT. I HAD BEEN WORKING WITH A CO-WORKER ON A SPOILER LUBE AND INSPECTION WORK CARD. WE HAD SPLIT THE WORK PACKAGE UP WHERE I TOOK THE L-HAND SIDE OF THE ACFT AND THE REMOVAL/INSTALLATION OF THE UPPER PANELS ON BOTH SIDES. WE HAD FINISHED OUR LUBE WORK AND STARTED TO CLOSE UP OUR ACCESS PANELS. I FIRST INSTALLED THE UPPER R-HAND WING PANEL. THEN I MOVED OVER TO INSTALL THE L-HAND LOWER 2 PANELS. I HAD SEVERAL STRIPPED SCREW HEADS ON ONE OF THE LOWER L-HAND PANELS SO I HAD TO HUNT DOWN FOR NEW FASTENERS FOR IT. THEN I CONTINUED TO FINISH UP THE REST OF THE JOB. 3 DAYS LATER (MAR/WED/04) MY CO-WORKER FOR THE PREVIOUS NIGHT CALLED ME AT HOME TO TELL ME THAT ON TAXI OUT; THE MORNING AFTER OUR WORK; ACFT X TURNED BACK TO THE GATE BECAUSE A PAX NOTICED THE ACCESS PANEL ON THE UPPER L-HAND WING FALL OFF OF THE AIRPLANE. THIS IS AN OVERSIGHT ON MY PART. I SHOULD HAVE DOUBLE-CHKED MY WORK OR HAD ASKED SOMEONE ELSE TO CHK IT. I THINK SOME OF THE CONTRIBUTING FACTORS MAY HAVE BEEN THAT ACFT X IS A NEW COMPANY FOR ME; I HAVE ONLY BEEN THERE 1 MONTH AND AM STILL GETTING USED TO THE ENVIRONMENT AND LOCATIONS OF EQUIP; FASTENERS; PARTS; ETC. THIS WAS ALSO ONLY THE 2ND TIME I PERFORMED THAT WORK CARD AND NOT EVERY ASPECT OF THAT JOB. IT WAS ALSO LATE AT NIGHT AND I WAS TIRED.