Narrative:

We had difficulty confirming an altitude clearance; and descended through the assigned altitude. We had been flying the minee 3 arrival to orlando and; as I recall; had been vectored off while in the vicinity of the lakeland VOR. The ATC controller had assigned a descent to 12000 ft; and asked for a 'good rate down due to traffic.' I read back the clearance to 12000 ft; acknowledging the instruction for the descent rate. As we descended; the first officer asked 'if the clearance was to one-two or one-three thousand.' I replied that I thought I'd heard 12000 ft; but would confirm the clearance. Additional factors were the published 13000 ft restr at the andro fix (we were off the arrival; but thought that might still be a controller's consideration); as well as a TCAS target ahead that appeared to be converging. I thought that target may have been the reason for the 'good rate' instruction. It indicated a lower altitude -- as I recall; it was roughly 1000 ft below us initially. As we descended; we were unable to ask for confirmation as the frequency was congested. The controller was extremely busy; and each time I tried to key the microphone; another call interrupted. As we approached 13000 ft; we wanted very much to confirm the assignment; but the radio was still congested. Normally; one would pause briefly before continuing the descent; but we were still concerned with: 1) the possibility that 12000 ft was correct; and that I had read it back without correction by the controller; 2) the 'good rate due to traffic' instruction; 3) the TCAS target was still ahead; and now co-altitude. We continued the descent and passing roughly 12600 ft the controller asked that we verify we were maintaining 13000 ft. I replied 'negative; descending to 12000 ft.' he asked that we maintain 13000 ft; we replied that we would; and climbed back up. There was no possibility of further discussion; as the frequency remained quite busy; and we were handed off shortly thereafter. The difficulty in this case was not a new one -- voice communications works well; but a heavy workload makes it difficult. I thought I heard one altitude assignment; and read it back. I believe the first officer and I were both focused on the job at hand; and listening attentively; but apparently the error in this case was mine; since the controller must have said '13000 ft' when he assigned our altitude. He did not notice my error when I read it back; I believe due to the heavy workload. We all work to make our air traffic system successful. I believe the two of us in the cockpit were paying attention. Despite that; I still made an error. Frequency congestion thwarted my attempts to confirm the clearance once a doubt surfaced. I don't know if there is a better way to manage inbound traffic (to prevent controller overload); perhaps in future yrs; a data link system could reduce the possibility of these events.

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Original NASA ASRS Text

Title: B737-500 FLT CREW DSNDS THROUGH THEIR ASSIGNED ALT ON APCH TO MCO.

Narrative: WE HAD DIFFICULTY CONFIRMING AN ALT CLRNC; AND DSNDED THROUGH THE ASSIGNED ALT. WE HAD BEEN FLYING THE MINEE 3 ARR TO ORLANDO AND; AS I RECALL; HAD BEEN VECTORED OFF WHILE IN THE VICINITY OF THE LAKELAND VOR. THE ATC CTLR HAD ASSIGNED A DSCNT TO 12000 FT; AND ASKED FOR A 'GOOD RATE DOWN DUE TO TFC.' I READ BACK THE CLRNC TO 12000 FT; ACKNOWLEDGING THE INSTRUCTION FOR THE DSCNT RATE. AS WE DSNDED; THE FO ASKED 'IF THE CLRNC WAS TO ONE-TWO OR ONE-THREE THOUSAND.' I REPLIED THAT I THOUGHT I'D HEARD 12000 FT; BUT WOULD CONFIRM THE CLRNC. ADDITIONAL FACTORS WERE THE PUBLISHED 13000 FT RESTR AT THE ANDRO FIX (WE WERE OFF THE ARR; BUT THOUGHT THAT MIGHT STILL BE A CTLR'S CONSIDERATION); AS WELL AS A TCAS TARGET AHEAD THAT APPEARED TO BE CONVERGING. I THOUGHT THAT TARGET MAY HAVE BEEN THE REASON FOR THE 'GOOD RATE' INSTRUCTION. IT INDICATED A LOWER ALT -- AS I RECALL; IT WAS ROUGHLY 1000 FT BELOW US INITIALLY. AS WE DSNDED; WE WERE UNABLE TO ASK FOR CONFIRMATION AS THE FREQ WAS CONGESTED. THE CTLR WAS EXTREMELY BUSY; AND EACH TIME I TRIED TO KEY THE MIKE; ANOTHER CALL INTERRUPTED. AS WE APCHED 13000 FT; WE WANTED VERY MUCH TO CONFIRM THE ASSIGNMENT; BUT THE RADIO WAS STILL CONGESTED. NORMALLY; ONE WOULD PAUSE BRIEFLY BEFORE CONTINUING THE DSCNT; BUT WE WERE STILL CONCERNED WITH: 1) THE POSSIBILITY THAT 12000 FT WAS CORRECT; AND THAT I HAD READ IT BACK WITHOUT CORRECTION BY THE CTLR; 2) THE 'GOOD RATE DUE TO TFC' INSTRUCTION; 3) THE TCAS TARGET WAS STILL AHEAD; AND NOW CO-ALT. WE CONTINUED THE DSCNT AND PASSING ROUGHLY 12600 FT THE CTLR ASKED THAT WE VERIFY WE WERE MAINTAINING 13000 FT. I REPLIED 'NEGATIVE; DSNDING TO 12000 FT.' HE ASKED THAT WE MAINTAIN 13000 FT; WE REPLIED THAT WE WOULD; AND CLBED BACK UP. THERE WAS NO POSSIBILITY OF FURTHER DISCUSSION; AS THE FREQ REMAINED QUITE BUSY; AND WE WERE HANDED OFF SHORTLY THEREAFTER. THE DIFFICULTY IN THIS CASE WAS NOT A NEW ONE -- VOICE COMS WORKS WELL; BUT A HVY WORKLOAD MAKES IT DIFFICULT. I THOUGHT I HEARD ONE ALT ASSIGNMENT; AND READ IT BACK. I BELIEVE THE FO AND I WERE BOTH FOCUSED ON THE JOB AT HAND; AND LISTENING ATTENTIVELY; BUT APPARENTLY THE ERROR IN THIS CASE WAS MINE; SINCE THE CTLR MUST HAVE SAID '13000 FT' WHEN HE ASSIGNED OUR ALT. HE DID NOT NOTICE MY ERROR WHEN I READ IT BACK; I BELIEVE DUE TO THE HVY WORKLOAD. WE ALL WORK TO MAKE OUR AIR TFC SYS SUCCESSFUL. I BELIEVE THE TWO OF US IN THE COCKPIT WERE PAYING ATTN. DESPITE THAT; I STILL MADE AN ERROR. FREQ CONGESTION THWARTED MY ATTEMPTS TO CONFIRM THE CLRNC ONCE A DOUBT SURFACED. I DON'T KNOW IF THERE IS A BETTER WAY TO MANAGE INBOUND TFC (TO PREVENT CTLR OVERLOAD); PERHAPS IN FUTURE YRS; A DATA LINK SYS COULD REDUCE THE POSSIBILITY OF THESE EVENTS.

Data retrieved from NASA's ASRS site as of January 2009 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.