MECH FAILS TO SECURE WINGTIP LIGHT ASSEMBLY.

Narrative

ON ACFT R WING TAIL POS LIGHT ASSEMBLY; I REMOVED LIGHT ASSEMBLY TO CHANGE THE TOP POS BULB. THE ASSEMBLY MUST BE REMOVED FOR THIS JOB AS THE BULB IS 'HARD WIRED.' I TOOK THE UNIT INTO THE SHOP TO CHANGE THE BULB AND UPON RETURNING TO THE ACFT; HOOKED UP CANNON PLUG AND SLID THE UNIT INTO THE WINGTIP. NOTE: ON REMOVAL OF THE UNIT; I HAD LAID THE 8 SECURING SCREWS ON TOP OF THE WING. BEFORE I SECURED THE UNIT INTO THE WINGTIP; I WANTED TO BE SURE IT WORKED. I WENT INTO THE COCKPIT AND ACTIVATED THE LIGHTS. I WENT OUT TO THE WINGTIP TO FIND THEM WORKING PROPERLY AND RETURNED TO COCKPIT TO SHUT THEM OFF AS THE LIGHTS WOULD BE BLINDING WHILE SECURING THE UNIT. AFTER SHUTTING LIGHTS OFF FROM COCKPIT; I STOPPED FOR 3-4 MINS TO TALK TO A MECH WHO WAS DOING ACFT INTERIOR WORK. AFTER LEAVING INTERIOR OF ACFT; I WAS THINKING I WANTED TO FINISH ALL EXTERIOR WORK QUICKLY; AS IT WAS 18 DEGS F WITH THE WIND CHILL FACTOR. A LADDER I HAD OUT ON THE L ENG CAUGHT MY EYE AS I WAS COMING DOWN THE STAIRS. I WAS RUNNING THROUGH MY MIND ITEMS I HAD TO COMPLETE TO GET INSIDE OUT OF THE WX. WITH THE WINGTIP LIGHT FIXED; ALL I HAD TO DO WAS PUT LADDERS AWAY. I PUT THE LADDER AWAY ON THE L ENG THAT HAD CAUGHT MY EYE; THEN PUT AWAY THE LADDER ON THE R WINGTIP. I NEVER HAD INSTALLED THE 8 SCREWS TO SECURE THE LIGHT ASSEMBLY. THE ACFT DEPARTED THE FIELD AND UPON ARR AT THE NEXT STATION; A PREFLT WALKAROUND BY SOMEONE REVEALED THE R WING TAIL POS LIGHT ASSEMBLY MISSING.