LATE DSCNT ON ROBBINSVILLE ARR TO NEWARK RESULTS IN A TFC CONFLICT REQUIRING ATC INTERVENTION.

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FLYING RBV1 ARR TO EWR. CLRED OUT OF 11000 FT TO 8000 FT AT RBV. VNAV/LNAV ENGAGED. BECAME DISTRACTED DOING APCH CHKS/BRIEFINGS. DIDN'T NOTICE AUTOPLT FAILED TO INITIATE DSCNT TO 8000 FT MSL AT RBV. APCH CTLR QUERIED OUR ALT AND I BEGAN DSCNT TO 8000 FT. PASSING 10000 FT MSL; APCH ADVISED TFC AHEAD AT 9500 FT MSL AND TO HOLD DSCNT AT 10000 FT. I DISENGAGED AUTOPLT AND STOPPED DSCNT AT 9500 FT AND CORRECTED BACK TO 10000 FT. WE ADVISED WE HAD TFC IN SIGHT; NO FACTOR; AND FOLLOWED 30 DEG HDG CHANGE COMMAND FROM APCH CTL. CTLR ADVISED 'NO PROB.' I TRUSTED THE AUTOPLT; WHICH PREVIOUSLY WORKED THROUGHOUT THE FLT; BUT IN THIS CASE DIDN'T. MANUAL FLYING RESOLVED THE SIT. ONE MUST ALWAYS MONITOR THE SIT AND REDUCE/INCREASE THE LEVEL OF AUTOMATION AS REQUIRED FOR THE SAFE CONDUCT OF THE FLT. SUPPLEMENTAL INFO FROM ACN 480810: AS THE PNF; I SHOULD HAVE SEEN THE ERROR EARLY ENOUGH TO PREVENT IT. HOWEVER DUE TO FREQ CONGESTION ON COMPANY (OPS; MAINT; ETC); I WAS OUT OF THE LOOP FOR TOO LONG. THE PF WAS VERY BUSY DURING THIS PHASE OF FLT. I ALSO WAS A LITTLE TIRED DUE TO EARLY CHK IN AND POOR SLEEP (NEW BABY AT HOME). I ALSO BELIEVE COMPLACENCY WAS A FACTOR AS I'VE FLOWN THIS ARR HUNDREDS OF TIMES WITH FMC'S WORKING PERFECTLY.