Narrative:

While on the lagrange arrival into atl, ZTL gave us several 1000 ft step-down altitude assignments. The last altitude assignment was 12000 ft and we were told about traffic at 11000 ft (a beech king air). We reported the traffic in sight (it was passing behind us and diverging) and we were handed off to atlanta approach. Atlanta approach assigned us 11000 ft (we thought) and I read back the descent clearance as, 'down to one one, eleven thousand, air carrier X.' I then set 11000 ft in the altitude alerter and confirmed the altitude assignment with my first officer (he was PF and I was PNF). I then switched to communication #2 to call our company and get a gate assignment. While I was talking with our company, the approach controller queried the first officer on our altitude, which was 11500 ft. The controller stated that he was only pointing out traffic at 11000 ft and not giving us a descent clearance, he then cleared us to 8000 ft. There was no traffic conflict. The remainder of the flight was uneventful. This misunderstanding could have been easily avoided had the controller listened to and corrected my incorrect readback of what was obviously an altitude clearance and not an acknowledgement of traffic. It's unfortunate that the FAA feels they are not responsible for listening to and correcting incorrect readbacks from pilots, it removes a vital safety link. The controller was not unusually busy, nor were we. I'm not sure what else I could have done to prevent this. Both of us were certain we were given a descent clearance and I read it back clearly ('one, one, eleven thousand'). We also audibly confirmed the altitude assignment by stating 'eleven thousand' to each other after the altitude alerter was set. Fortunately there were no traffic conflicts. Fortunately most controllers listen to readbacks and correct incorrect clrncs.

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Original NASA ASRS Text

Title: ALT EXCURSION OCCURS WHEN AN ACR FLT CREW INTERPRETS A TA FOR TFC AT A LOWER ALT AS AN ALT ASSIGNMENT, 25 MI W OF ATL, GA.

Narrative: WHILE ON THE LAGRANGE ARR INTO ATL, ZTL GAVE US SEVERAL 1000 FT STEP-DOWN ALT ASSIGNMENTS. THE LAST ALT ASSIGNMENT WAS 12000 FT AND WE WERE TOLD ABOUT TFC AT 11000 FT (A BEECH KING AIR). WE RPTED THE TFC IN SIGHT (IT WAS PASSING BEHIND US AND DIVERGING) AND WE WERE HANDED OFF TO ATLANTA APCH. ATLANTA APCH ASSIGNED US 11000 FT (WE THOUGHT) AND I READ BACK THE DSCNT CLRNC AS, 'DOWN TO ONE ONE, ELEVEN THOUSAND, ACR X.' I THEN SET 11000 FT IN THE ALT ALERTER AND CONFIRMED THE ALT ASSIGNMENT WITH MY FO (HE WAS PF AND I WAS PNF). I THEN SWITCHED TO COM #2 TO CALL OUR COMPANY AND GET A GATE ASSIGNMENT. WHILE I WAS TALKING WITH OUR COMPANY, THE APCH CTLR QUERIED THE FO ON OUR ALT, WHICH WAS 11500 FT. THE CTLR STATED THAT HE WAS ONLY POINTING OUT TFC AT 11000 FT AND NOT GIVING US A DSCNT CLRNC, HE THEN CLRED US TO 8000 FT. THERE WAS NO TFC CONFLICT. THE REMAINDER OF THE FLT WAS UNEVENTFUL. THIS MISUNDERSTANDING COULD HAVE BEEN EASILY AVOIDED HAD THE CTLR LISTENED TO AND CORRECTED MY INCORRECT READBACK OF WHAT WAS OBVIOUSLY AN ALT CLRNC AND NOT AN ACKNOWLEDGEMENT OF TFC. IT'S UNFORTUNATE THAT THE FAA FEELS THEY ARE NOT RESPONSIBLE FOR LISTENING TO AND CORRECTING INCORRECT READBACKS FROM PLTS, IT REMOVES A VITAL SAFETY LINK. THE CTLR WAS NOT UNUSUALLY BUSY, NOR WERE WE. I'M NOT SURE WHAT ELSE I COULD HAVE DONE TO PREVENT THIS. BOTH OF US WERE CERTAIN WE WERE GIVEN A DSCNT CLRNC AND I READ IT BACK CLRLY ('ONE, ONE, ELEVEN THOUSAND'). WE ALSO AUDIBLY CONFIRMED THE ALT ASSIGNMENT BY STATING 'ELEVEN THOUSAND' TO EACH OTHER AFTER THE ALT ALERTER WAS SET. FORTUNATELY THERE WERE NO TFC CONFLICTS. FORTUNATELY MOST CTLRS LISTEN TO READBACKS AND CORRECT INCORRECT CLRNCS.

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.