AIR CARRIER MECH AND MAINT CTLR BOTH UNAWARE THAT DAMAGED LAVATORY FILL NIPPLE COMPROMISED THE INTEGRITY OF THE MLG'S PRESSURE HULL; AND THEY ALLOWED ACFT TO BE RELEASED WITH LAVATORY SECURED AS A DEFERRED MAINT ITEM.

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LCL RAMP PERSONNEL ADVISED MAINT OF A PROBLEM ON THE SAID ACFT'S R HAND LAVATORY EXTERNAL FILL PORT. I PROCEEDED TO INSPECT THE SAID FILL PORT AND NOTED THAT THE FILL PORT NIPPLE ITSELF HAD BEEN PULLED AWAY FROM THE FUSELAGE APPROX 1 INCH; SHEARING 8 RIVETS. NO OTHER DISCREPANCIES WITH THE FITTING OR SURROUNDING STRUCTURE WAS NOTED. THE FILL PORT FLANGE WAS THE ONLY THING SEPARATED FROM THE ACFT FUSELAGE. AT THE TIME; I DID NOT REALIZE THAT THE AREA THE LAVATORY FILL PORT FLANGE SECURED TO WAS INSIDE THE PRESSURIZED SECTION OF THE ACFT; SO I PROCEEDED TO ADVISE MAINT CTL OF THE PROBLEM AND REQUEST THE ITEM BE DEFERRED AS A PAX CONVENIENCE ITEM; PERCEIVING IT AS AFFECTING SOLELY THE LAVATORY ITSELF. THE LAVATORY WAS; IN TURN; DEFERRED; DRAINED AND BLOCKED OFF. LATER; IT HAD BEEN BROUGHT TO MY ATTN THAT THE SAID AREA WAS PRESSURIZED AND THAT THE ITEM SHOULD NOT HAVE BEEN DEFERRED DUE TO THE FACT THAT A PRESSURIZATION LEAK WOULD EXIST AND THE INTEGRITY OF THE STRUCTURE COULD HAVE BEEN COMPROMISED WITHOUT SUPPORT OF THE RIVETS SECURING THE FITTING. SUPPLEMENTAL INFO FROM ACN 210366: BEING THE MAINT CTLR ON DUTY; PROCEEDED TO SEND A MECH OUT TO EVALUATE THE SITUATION. AT THE TIME; I DID NOT REALIZE THAT THE AREA THE LAVATORY FILL PORT FLANGE SECURED TO WAS WITHIN THE PRESSURIZED SECTION OF THE ACFT; SO I AUTHORIZED A DEFERRED MAINT ITEM PERCEIVING IT AS AFFECTING SOLELY THE LAVATORY ITSELF. I THEN INSTRUCTED THE SAID MECH TO DRAIN THE LAVATORY AND BLOCK IT OFF.