Narrative:

While flying the arlin 3 arrival into phoenix (the captain was flying), we were cleared to cross arlin at 11000 ft, descend to 9000 ft. We received the clearance just a couple of mi before payne (the fix just before arlin), at an altitude of approximately 12000 ft while in a descent. The airplane was being flown with the autoplt engaged. After receiving the clearance, I asked the captain if he wanted 9000 ft or 11000 ft set in the MCP altitude window. He elected 9000 ft. I set 9000 ft. This is where I believe our breakdown in crew coordination occurred. Our normal procedure is to set the altitude clearance limit in the MCP. In this situation, it should have raised a red flag in my mind since we deviated from the normal procedure. But, I missed this cue to clarify that we both had the same understanding of the clearance. I went on to preset the next communication and navigation frequencys. The next time I looked at the altimeter, we were descending past 10600 ft and I told the captain we were only cleared to 11000 ft until past arlin. During the ensuing correction, we descended to approximately 10400 ft. I told approach control what we had done and we were correcting back to 11000 ft. He came back and said '...descend to 9000 ft.' we descended to 9000 ft and rest of the flight was uneventful. The controller made no other mention of it and no traffic was affected. There are several lessons to emphasize from this event dealing with crew coordination and crew responsibilities. Any one of these things, done differently, may have prevented this altitude deviation: 1) careful altitude monitoring by both pilots is essential, especially with 1000 ft of assigned altitude. 2) stick to established/normal procedure. Deviation from any expectation should be questioned. When in doubt, speak up. When he said set 9000 ft, I could have said, 'we're only cleared to 11000 ft until we pass arlin.' 3) perhaps some degree of complacency was at play here. We had flown together for 6 days and we had flown together before. We had developed a good working relationship and our flts had all gone without a hitch. As the PNF, my job is to back up the PF.

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Original NASA ASRS Text

Title: B737 FLT MISCOM P50 ALT DSCNT CLRNC, OVERSHOOTING ARLIN STAR XING RESTR.

Narrative: WHILE FLYING THE ARLIN 3 ARR INTO PHOENIX (THE CAPT WAS FLYING), WE WERE CLRED TO CROSS ARLIN AT 11000 FT, DSND TO 9000 FT. WE RECEIVED THE CLRNC JUST A COUPLE OF MI BEFORE PAYNE (THE FIX JUST BEFORE ARLIN), AT AN ALT OF APPROX 12000 FT WHILE IN A DSCNT. THE AIRPLANE WAS BEING FLOWN WITH THE AUTOPLT ENGAGED. AFTER RECEIVING THE CLRNC, I ASKED THE CAPT IF HE WANTED 9000 FT OR 11000 FT SET IN THE MCP ALT WINDOW. HE ELECTED 9000 FT. I SET 9000 FT. THIS IS WHERE I BELIEVE OUR BREAKDOWN IN CREW COORD OCCURRED. OUR NORMAL PROC IS TO SET THE ALT CLRNC LIMIT IN THE MCP. IN THIS SIT, IT SHOULD HAVE RAISED A RED FLAG IN MY MIND SINCE WE DEVIATED FROM THE NORMAL PROC. BUT, I MISSED THIS CUE TO CLARIFY THAT WE BOTH HAD THE SAME UNDERSTANDING OF THE CLRNC. I WENT ON TO PRESET THE NEXT COM AND NAV FREQS. THE NEXT TIME I LOOKED AT THE ALTIMETER, WE WERE DSNDING PAST 10600 FT AND I TOLD THE CAPT WE WERE ONLY CLRED TO 11000 FT UNTIL PAST ARLIN. DURING THE ENSUING CORRECTION, WE DSNDED TO APPROX 10400 FT. I TOLD APCH CTL WHAT WE HAD DONE AND WE WERE CORRECTING BACK TO 11000 FT. HE CAME BACK AND SAID '...DSND TO 9000 FT.' WE DSNDED TO 9000 FT AND REST OF THE FLT WAS UNEVENTFUL. THE CTLR MADE NO OTHER MENTION OF IT AND NO TFC WAS AFFECTED. THERE ARE SEVERAL LESSONS TO EMPHASIZE FROM THIS EVENT DEALING WITH CREW COORD AND CREW RESPONSIBILITIES. ANY ONE OF THESE THINGS, DONE DIFFERENTLY, MAY HAVE PREVENTED THIS ALTDEV: 1) CAREFUL ALT MONITORING BY BOTH PLTS IS ESSENTIAL, ESPECIALLY WITH 1000 FT OF ASSIGNED ALT. 2) STICK TO ESTABLISHED/NORMAL PROC. DEV FROM ANY EXPECTATION SHOULD BE QUESTIONED. WHEN IN DOUBT, SPEAK UP. WHEN HE SAID SET 9000 FT, I COULD HAVE SAID, 'WE'RE ONLY CLRED TO 11000 FT UNTIL WE PASS ARLIN.' 3) PERHAPS SOME DEGREE OF COMPLACENCY WAS AT PLAY HERE. WE HAD FLOWN TOGETHER FOR 6 DAYS AND WE HAD FLOWN TOGETHER BEFORE. WE HAD DEVELOPED A GOOD WORKING RELATIONSHIP AND OUR FLTS HAD ALL GONE WITHOUT A HITCH. AS THE PNF, MY JOB IS TO BACK UP THE PF.

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.