AFTER CHANGING A LH OVERWING SLIDE ON A B757-200; A MECHANIC DESCRIBES THE EVENTS THAT LED TO THE LH TRAILING EDGE FLAP BEING DAMAGED BY CONTACT WITH A CARGO BELT LOADER.

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THE NIGHT BEGAN WITH MY ASSIGNMENT TO ACFT 1SV; LOG 1 (LAVATORY WILL NOT HOLD WATER). BORESCOPE INSPECTION #1 ENG. L-HAND OVERWING SLIDE REPLACEMENT FLT ARRIVED AT XA20. I ACCOMPLISHED THE 1SV; LOG 1; AND HAD THE BORESCOPE 95% ACCOMPLISHED. ONLY THE COWLINGS TO CLOSE. AFTER THE L-HAND SLIDE REPLACEMENT; DURING RETRACTING OF THE FLAPS; THE TRAILING EDGE L-HAND FLAP WAS DAMAGED BY THE BELT LOADER. THE MAINT MANUAL 25-65-01 HAD NO REF ON HOW TO GET THE SLIDE IN PLACE. I HAD A LOT OF WORK AT THAT TIME OF NIGHT AND TOO MANY THINGS GOING ON AT ONCE. THE LAST AND ONLY STEP I MISSED WAS TO FIND A GUIDE MAN TO CLR THE FLAPS. 20 YRS WITH AIRLINE AND IT ONLY TOOK 1 SECOND TO MISS A MAJOR STEP. ALWAYS THINK OF ANY THINGS THAT ARE IN PLACE; AND ALWAYS GET CLRNC FROM A SECOND SET OF EYES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED AFTER REACHING IN AND PULLING OUT A BIG DIAPER FROM THE LAV DUMP CHUTE AND ACCOMPLISHING A #1 ENGINE BORESCOPE INSPECTION; HE COMPLETED THE LH OVERWING SLIDE REPLACEMENT BY USING A CARGO BELT LOADER AND PROTECTIVE PAD. THE BELT LOADER HELPED HIM MOVE THE SLIDE TO THE UPPER INBOARD WING SURFACE AND ALSO POSITION THE SLIDE PACK ONTO THE SWING-OUT SLIDE SUPPORT TRAY; MOUNTED INSIDE THE EXTERNAL FUSELAGE. REPORTER STATED HE DID MOVE THE BELT LOADER AWAY FROM THE ACFT; ALTHOUGH THERE WAS ANOTHER TRUCK DIRECTLY BEHIND THE BELT LOADER; LIMITING HIS BACKUP AREA. HE LEFT THE BELT LOADER PLATFORM ITSELF IN A RAISED SLANTED POSITION; BELIEVING HE PROBABLY HAD ENOUGH CLEARANCE TO RAISE THE FLAPS. BECAUSE HE DIDN'T GET A GUIDE MAN TO VERIFY FLAP CLEARANCE AS THEY WERE RETRACTED; THE UPPER SURFACE OF THE FLAP STRUCK THE LOWER SURFACE OF THE RAISED BELT LOADER PLATFORM.