Narrative:

I was working approach with 8-10 aircraft on my frequency. Traffic was moderate to heavy. My job is to sequence aircraft on downwind and base legs so as the final controller may vector aircraft to the final. I turned air carrier X heavy 'left to 265 degrees, descend to 7000 ft.' air carrier Y, same company, read back the instructions as 'Y heading 260 degrees, down to 7.' the transmission was quick and not clear. I then observed air carrier Y altitude at 10700 ft. I transmitted to air carrier Y 'your altitude should be 12000 ft.' the aircraft responded by saying 'I read back 7500 ft.' no other aircraft were involved. My airspace lower limits stop at 11000 ft. No evasive action was required. The factors that contributed to this were controller and lack of pilot awareness. The situation occurred because of pilot awareness and improper radio communication by the aircraft. My suggestion is for pilot as well as controllers speak slower and clrer so as less readback errors occur. Supplemental information from acn 225541: in the approach phase, downwind for landing on runway 9R at atl, was given a turn to 265 degrees and descent from 12000 ft to 7000 ft. I read back 260 degrees and 7000 ft. The controller came back, turn to 265 degrees. I 'rogered' it. Captain was on radio #2 talking to company at this time. At 10300 ft, the controller said 'air carrier Y, your assigned altitude was 12000 ft, climb to 12000 ft.' I immediately climbed to 11000 ft as the controller told us to level at 11000 ft. TCASII never 'issued' an alert and at no time did I feel a conflict was imminent. Contributing further was 'clutter' on the radio from other aircraft and me flying and talking at the same time, even though the autoplt was on.

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Original NASA ASRS Text

Title: ACR Y NON ADHERENCE TO ATC CLRNC UNAUTHORIZED DSCNT FROM ASSIGNED ALT. PLTDEV. CLRNC REPLY TO WRONG CALL SIGN SAME COMPANY.

Narrative: I WAS WORKING APCH WITH 8-10 ACFT ON MY FREQ. TFC WAS MODERATE TO HVY. MY JOB IS TO SEQUENCE ACFT ON DOWNWIND AND BASE LEGS SO AS THE FINAL CTLR MAY VECTOR ACFT TO THE FINAL. I TURNED ACR X HVY 'LEFT TO 265 DEGS, DSND TO 7000 FT.' ACR Y, SAME COMPANY, READ BACK THE INSTRUCTIONS AS 'Y HDG 260 DEGS, DOWN TO 7.' THE XMISSION WAS QUICK AND NOT CLR. I THEN OBSERVED ACR Y ALT AT 10700 FT. I XMITTED TO ACR Y 'YOUR ALT SHOULD BE 12000 FT.' THE ACFT RESPONDED BY SAYING 'I READ BACK 7500 FT.' NO OTHER ACFT WERE INVOLVED. MY AIRSPACE LOWER LIMITS STOP AT 11000 FT. NO EVASIVE ACTION WAS REQUIRED. THE FACTORS THAT CONTRIBUTED TO THIS WERE CTLR AND LACK OF PLT AWARENESS. THE SITUATION OCCURRED BECAUSE OF PLT AWARENESS AND IMPROPER RADIO COM BY THE ACFT. MY SUGGESTION IS FOR PLT AS WELL AS CTLRS SPEAK SLOWER AND CLRER SO AS LESS READBACK ERRORS OCCUR. SUPPLEMENTAL INFO FROM ACN 225541: IN THE APCH PHASE, DOWNWIND FOR LNDG ON RWY 9R AT ATL, WAS GIVEN A TURN TO 265 DEGS AND DSCNT FROM 12000 FT TO 7000 FT. I READ BACK 260 DEGS AND 7000 FT. THE CTLR CAME BACK, TURN TO 265 DEGS. I 'ROGERED' IT. CAPT WAS ON RADIO #2 TALKING TO COMPANY AT THIS TIME. AT 10300 FT, THE CTLR SAID 'ACR Y, YOUR ASSIGNED ALT WAS 12000 FT, CLB TO 12000 FT.' I IMMEDIATELY CLBED TO 11000 FT AS THE CTLR TOLD US TO LEVEL AT 11000 FT. TCASII NEVER 'ISSUED' AN ALERT AND AT NO TIME DID I FEEL A CONFLICT WAS IMMINENT. CONTRIBUTING FURTHER WAS 'CLUTTER' ON THE RADIO FROM OTHER ACFT AND ME FLYING AND TALKING AT THE SAME TIME, EVEN THOUGH THE AUTOPLT WAS ON.

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.