Narrative:

Flying RBV1 arrival to ewr. Cleared out of 11000 ft to 8000 ft at rbv. VNAV/LNAV engaged. Became distracted doing approach checks/briefings. Didn't notice autoplt failed to initiate descent to 8000 ft MSL at rbv. Approach controller queried our altitude and I began descent to 8000 ft. Passing 10000 ft MSL, approach advised traffic ahead at 9500 ft MSL and to hold descent at 10000 ft. I disengaged autoplt and stopped descent at 9500 ft and corrected back to 10000 ft. We advised we had traffic in sight, no factor, and followed 30 degree heading change command from approach control. Controller advised 'no problem.' I trusted the autoplt, which previously worked throughout the flight, but in this case didn't. Manual flying resolved the situation. One must always monitor the situation and reduce/increase the level of automation as required for the safe conduct of the flight. Supplemental information from acn 480810: as the PNF, I should have seen the error early enough to prevent it. However due to frequency congestion on company (operations, maintenance, etc), I was out of the loop for too long. The PF was very busy during this phase of flight. I also was a little tired due to early check in and poor sleep (new baby at home). I also believe complacency was a factor as I've flown this arrival hundreds of times with FMC's working perfectly.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.