Narrative:

Flight was las to phx. While on karlo 7 arrival into phx, phx approach control issued descent to 9000 ft at plsnt intersection. We were still 3-4 mi from plsnt intersection. I heard 9000 ft, but was not sure. Copilot was sure he heard 8000 ft. We discussed it and since the controller was busy, we decided that we would report out of 12000 ft and clearly annunciate 'out of 12000 ft for 8000 ft' and that I would accomplish a slow descent to be sure. When we reported out of 12000 ft for 8000 ft, the controller asked for us to repeat it. We again said out of 12000 ft for 8000 ft. We descended toward 8000 ft. At about 8300 ft the controller told us to climb to 9000 ft and said that had been the altitude to which we were cleared. There did not seem to be a traffic conflict, TCASII or otherwise, and the controller did not seem overly excited. I rapidly climbed back to 9000 ft. Nothing was ever said, from that point on, about a possible altitude violation. Supplemental information from acn 432014: the captain questioned me as to the altitude. He thought it might be 9000 ft. He wasn't sure. I confirmed the 8000 ft altitude by calling approach control and reporting, 'out of 12000 ft for 8000 ft.' approach control asked us to repeat, which we did. At 8300 ft approach control called us with an altitude, 'call sign, 9000 ft.' I called approach control and said, 'we confirmed 8000 ft out of 12000 ft.' his response was 'maintain 9000 ft.'

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Original NASA ASRS Text

Title: A DSNDING B737-300 OVERSHOOTS ITS ASSIGNED ALT WHEN THE FLC FAILS TO ASK THE CTLR TO REPEAT THE CLRED TO ALT.

Narrative: FLT WAS LAS TO PHX. WHILE ON KARLO 7 ARR INTO PHX, PHX APCH CTL ISSUED DSCNT TO 9000 FT AT PLSNT INTXN. WE WERE STILL 3-4 MI FROM PLSNT INTXN. I HEARD 9000 FT, BUT WAS NOT SURE. COPLT WAS SURE HE HEARD 8000 FT. WE DISCUSSED IT AND SINCE THE CTLR WAS BUSY, WE DECIDED THAT WE WOULD RPT OUT OF 12000 FT AND CLRLY ANNUNCIATE 'OUT OF 12000 FT FOR 8000 FT' AND THAT I WOULD ACCOMPLISH A SLOW DSCNT TO BE SURE. WHEN WE RPTED OUT OF 12000 FT FOR 8000 FT, THE CTLR ASKED FOR US TO REPEAT IT. WE AGAIN SAID OUT OF 12000 FT FOR 8000 FT. WE DSNDED TOWARD 8000 FT. AT ABOUT 8300 FT THE CTLR TOLD US TO CLB TO 9000 FT AND SAID THAT HAD BEEN THE ALT TO WHICH WE WERE CLRED. THERE DID NOT SEEM TO BE A TFC CONFLICT, TCASII OR OTHERWISE, AND THE CTLR DID NOT SEEM OVERLY EXCITED. I RAPIDLY CLBED BACK TO 9000 FT. NOTHING WAS EVER SAID, FROM THAT POINT ON, ABOUT A POSSIBLE ALT VIOLATION. SUPPLEMENTAL INFO FROM ACN 432014: THE CAPT QUESTIONED ME AS TO THE ALT. HE THOUGHT IT MIGHT BE 9000 FT. HE WASN'T SURE. I CONFIRMED THE 8000 FT ALT BY CALLING APCH CTL AND RPTING, 'OUT OF 12000 FT FOR 8000 FT.' APCH CTL ASKED US TO REPEAT, WHICH WE DID. AT 8300 FT APCH CTL CALLED US WITH AN ALT, 'CALL SIGN, 9000 FT.' I CALLED APCH CTL AND SAID, 'WE CONFIRMED 8000 FT OUT OF 12000 FT.' HIS RESPONSE WAS 'MAINTAIN 9000 FT.'

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.