AFTER CHANGING THE LEFT WING SCAN ILLUMINATION LIGHT; MECHANIC TRIES TO AVOID THE GATE AIR DUCT WHILE BACKING THE BELT LOADER; BUT GRAZES #1 ENG INLET COWL; CAUSING A NINE INCH LONG; FOUR INCH WIDE AND .325 INCH DEEP GOUGING OF INLET SKIN ON A B737.

Narrative

THE L-HAND WING SCAN LIGHT WAS FOUND TO BE NOT WORKING. BEING UNDER THE ASSUMPTION THAT OUR MANLIFT WAS STILL TAGGED OUT; I OBTAINED A BELT LOADER FROM THE NEXT GATE TO CHANGE THE LIGHT. I CAREFULLY APCHED THE SIDE OF THE FUSELAGE; RAISED THE HAND RAIL; AND CHANGED THE LIGHT. DURING THIS TIME THE GATE AIR CONDITIONING DUCT HAD BEEN TURNED ON (WHEN THIS IS TURNED ON IT EXPANDS AND HAS A TENDENCY TO ALSO EXTEND). I LOWERED THE HAND RAIL AND RESTARTED THE BELT LOADER TO BACK IT UP. BEING AWARE OF THE ACFT DUCT; I CUT THE WHEEL TO AVOID CRUSHING IT. I PROCEEDED TO BACK UP AND HAD TURNED THE WHEEL TOO MUCH BECAUSE I GRAZED THE #1 ENG INLET COWL AT THE 7 O'CLOCK POS. THE DAMAGE WAS MEASURED AT 9 INCHES LONG; 4 INCHES WIDE; AND .325 INCHES DEEP. I IMMEDIATELY INFORMED MY LEAD AND SUPVR OF THE EVENT. AT THAT TIME I FOUND OUT THAT THE MANLIFT HAD BEEN FIXED THE DAY BEFORE. I REALIZE THAT I SHOULD HAVE CHKED WITH MY SUPVR AS TO THE STATE OF THE MANLIFT BEFORE PROCEEDING. MY ASSUMPTION AND GREATER CONCERN FOR GATE AIR LED TO ACFT DAMAGE.