Narrative:

We were being radar vectored for a visual approach to runway 30L at sjc. We were level at 6000 ft MSL when we were told to turn to a heading of 040 degrees and descend to 5100 ft. During the turn and descend, approach control pointed out traffic (another commercial carrier also going to san jose). About this time, a couple things happened almost simultaneously -- first, the SELCAL chime went off, which momentarily distracted us -- second, approach control made a transmission that was then blocked by another aircraft's transmission and third, approach control made a transmission that both the captain and myself heard that cleared us to 4000 ft, which I, the PNF, acknowledged. We started to slow descent. At approximately 4700 ft, approach control (but with another person speaking), asked us if we were at 5100 ft. I replied that we were cleared to 4000 ft. However, due to the confusion, we stopped the descent at 4600 ft, and then slowly started to climb. Shortly, approach control asked us if we had the airport in sight, which we said we did, and were cleared for a visual approach. I feel the confusion was caused by the SELCAL distraction, the blocked ATC transmission, and the changing of the controllers with the new controller not knowing what the old controller had cleared our airplane to do.

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Original NASA ASRS Text

Title: FLC OF AN MD83 DSNDED BELOW ASSIGNED ALT DUE TO DISTRACTIONS WHEN ATC WAS XMITTING CLRNC INSTRUCTIONS DURING VECTORS FOR A VISUAL APCH.

Narrative: WE WERE BEING RADAR VECTORED FOR A VISUAL APCH TO RWY 30L AT SJC. WE WERE LEVEL AT 6000 FT MSL WHEN WE WERE TOLD TO TURN TO A HDG OF 040 DEGS AND DSND TO 5100 FT. DURING THE TURN AND DSND, APCH CTL POINTED OUT TFC (ANOTHER COMMERCIAL CARRIER ALSO GOING TO SAN JOSE). ABOUT THIS TIME, A COUPLE THINGS HAPPENED ALMOST SIMULTANEOUSLY -- FIRST, THE SELCAL CHIME WENT OFF, WHICH MOMENTARILY DISTRACTED US -- SECOND, APCH CTL MADE A XMISSION THAT WAS THEN BLOCKED BY ANOTHER ACFT'S XMISSION AND THIRD, APCH CTL MADE A XMISSION THAT BOTH THE CAPT AND MYSELF HEARD THAT CLRED US TO 4000 FT, WHICH I, THE PNF, ACKNOWLEDGED. WE STARTED TO SLOW DSCNT. AT APPROX 4700 FT, APCH CTL (BUT WITH ANOTHER PERSON SPEAKING), ASKED US IF WE WERE AT 5100 FT. I REPLIED THAT WE WERE CLRED TO 4000 FT. HOWEVER, DUE TO THE CONFUSION, WE STOPPED THE DSCNT AT 4600 FT, AND THEN SLOWLY STARTED TO CLB. SHORTLY, APCH CTL ASKED US IF WE HAD THE ARPT IN SIGHT, WHICH WE SAID WE DID, AND WERE CLRED FOR A VISUAL APCH. I FEEL THE CONFUSION WAS CAUSED BY THE SELCAL DISTR, THE BLOCKED ATC XMISSION, AND THE CHANGING OF THE CTLRS WITH THE NEW CTLR NOT KNOWING WHAT THE OLD CTLR HAD CLRED OUR AIRPLANE TO DO.

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.