AN AIRBUS 319 WAS DISPATCHED IN NON COMPLIANCE WITH THE L ENG OIL CAP NOT LOCKED AFTER SVCING. ACFT RETURNED TO THE FIELD DUE TO LOSS OF OIL.

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THIS IS A STATEMENT IN REGARDS TO ACFT X ON SEP/TUE/03 DURING THE SECOND SHIFT AT ZZZ INTL ARPT; HANGAR LINE MAINT OPS. I BEGAN A LINE CHK ON ACFT X AT XA30. MY FIRST TASK WAS TO SVC BOTH ENGS WITH OIL. I FIRST SVCED ENG #2 WITH 4.0 QUARTS USING THE PRESSURE FILL METHOD; NO DIFFICULTIES HAD OCCURRED DURING THE SVCING OF THIS ENG. I THEN BEGAN SVCING THE #1 ENG USING THE PRESSURE FILL METHOD. DURING THE SVCING THE OIL WAS FOAMING UP IN THE SIGHT GLASS; AND IT WAS DIFFICULT TO SEE WHERE THE OIL LEVEL WAS AS IT WAS RISING. TO DECREASE THE FOAM AS I WAS FILLING THE OIL I RELEASED THE LOCKING MECHANISM ON THE OIL CAP SO AS TO RELIEVE ANY PRESSURE. I COMPLETED THE SVCING WITH 3 QUARTS; REMOVED THE FILLER HOSE AND CLOSED THE COWL ACCESS PANEL. I DID NOT APPLY THE LOCK ON THE OIL CAP. PRIOR TO STARTING THE LINE CHK; THE #2 ENG HAD BEEN RUN TO OP'S CHK REPLACED PARTS; I NOTICED ON SHUTDOWN THE OIL QUANTITY. AT THE TIME I STARTED THE LINE CHK; I NOTICED THE QUANTITY ON THE #2 ENG HAD DECREASED BY ANOTHER QUART; TAKING THIS INTO CONSIDERATION AS WELL AS QUANTITY'S WRITTEN IN THE LOGBOOK I SVCED THE ENGS ACCORDINGLY. BOTH ENGS WERE SVCED IN CONSIDERATION WITH PREVIOUSLY KNOWN QUANTITY'S; NOT TO THE LEVEL MARK ON THE SIGHT GLASS. ON SEP/WED/03; A REPRESENTATIVE FROM ACFT STANDARDS HAD APCHED AT THE BEGINNING OF MY SHIFT AND NOTIFIED ME THAT ACFT X HAD TAKEN OFF AND RETURNED WITH AN OIL LOSS PROB. IT WAS FOUND THAT THE OIL CAP WAS OFF; THE ACFT WAS SVCED AND DEPARTED. WHEN I WAS INFORMED OF THIS I DENIED ANY ERROR ON MY PART; BUT SOON AFTERWARD; I RECALLED NOT LOCKING THE OIL CAP. I THEN NOTIFIED THE ACFT STANDARDS REPRESENTATIVE OF MY RECOLLECTION AND GAVE A STATEMENT TO MGMNT OF WHICH I RECEIVED A LETTER IN MY FILE AND 3 DAYS UNPAID OFF AND ALSO AWAITING AN FAA INFORMAL HEARING. THE MAINT MANUAL FOR AIRBUS A320 CALLS FOR AN OVERFLOW HOSE TO BE INSTALLED WHILE SVCING USING THE PRESSURE FILL METHOD. EVEN THOUGHT I WAS NOT SVCING THE TANK TO ITS FULL MARK THE OVERFLOW HOSE WOULD HAVE BEEN HELPFUL IN VENTING EXCESS PRESSURE AND WOULD NOT HAVE REQUIRED THE CAP TO BE USED AS A VENT. I ALREADY HAVE STARTED AND WILL CONTINUE TO USE THE OVERFLOW HOSE WHEN SVCING THIS TYPE OF ACFT. I FEEL A SMALL PART OF THE CONTRIBUTING FACTOR IS THE REPETITIVENESS OF THE LINE CHKS THAT ARE SO MUCH A LARGE PART OF ACFT MAINT. I BECAME COMPLACENT.