Narrative:

Air carrier X was instructed to turn right heading 240 degree to intercept localizer for runway 27R. He was at 6000. He read back the heading, but also included maintain 4000. This was not heard by the controller. Air carrier Y was on final for runway 26R at 5000. Air carrier X left 6000 for 4000 and conflicted with air carrier Y. The closest I observed the aircraft was air carrier Y at 5400 and air carrier X at 5000. Traffic was given to air carrier Y when air carrier X was at 6000. However air carrier Y did not report traffic in sight. Supplemental information from acn 116956: while being vectored for the 426 26R ILS at atl we were given a descent to 5000'. As we were leaving 6300' the approach controller gave us the following clearance, air carrier X descend to 6000' expect ILS 27R, localizer frequency 111.3. The first officer was flying and he leveled the aircraft at 6000'. Several minutes later we were given clearance 'turn right to 240 degree intercept the localizer, descend to 4000' until established.' I read back the clearance as given, nothing further was said by the controller until we were descending through 5000'. At that time the controller told us to maintain 6000' and stated 'you were told to maintain 6000'.' I advised him that wasn't the case, that we were given 4000' until established and inquired if he wanted us to climb back to 6000'. He answered in the negative.

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Original NASA ASRS Text

Title: ACR DESCENDED BELOW ASSIGNED ALT RESULTING IN LESS THAN STANDARD SEPARATION WITH ANOTHER ACFT ON APCH.

Narrative: ACR X WAS INSTRUCTED TO TURN RIGHT HDG 240 DEG TO INTERCEPT LOC FOR RWY 27R. HE WAS AT 6000. HE READ BACK THE HDG, BUT ALSO INCLUDED MAINTAIN 4000. THIS WAS NOT HEARD BY THE CTLR. ACR Y WAS ON FINAL FOR RWY 26R AT 5000. ACR X LEFT 6000 FOR 4000 AND CONFLICTED WITH ACR Y. THE CLOSEST I OBSERVED THE ACFT WAS ACR Y AT 5400 AND ACR X AT 5000. TFC WAS GIVEN TO ACR Y WHEN ACR X WAS AT 6000. HOWEVER ACR Y DID NOT REPORT TFC IN SIGHT. SUPPLEMENTAL INFORMATION FROM ACN 116956: WHILE BEING VECTORED FOR THE 426 26R ILS AT ATL WE WERE GIVEN A DSCNT TO 5000'. AS WE WERE LEAVING 6300' THE APCH CTLR GAVE US THE FOLLOWING CLRNC, ACR X DSND TO 6000' EXPECT ILS 27R, LOC FREQ 111.3. THE F/O WAS FLYING AND HE LEVELED THE ACFT AT 6000'. SEVERAL MINUTES LATER WE WERE GIVEN CLRNC 'TURN RIGHT TO 240 DEG INTERCEPT THE LOC, DSND TO 4000' UNTIL ESTABLISHED.' I READ BACK THE CLRNC AS GIVEN, NOTHING FURTHER WAS SAID BY THE CTLR UNTIL WE WERE DESCENDING THROUGH 5000'. AT THAT TIME THE CTLR TOLD US TO MAINTAIN 6000' AND STATED 'YOU WERE TOLD TO MAINTAIN 6000'.' I ADVISED HIM THAT WASN'T THE CASE, THAT WE WERE GIVEN 4000' UNTIL ESTABLISHED AND INQUIRED IF HE WANTED US TO CLIMB BACK TO 6000'. HE ANSWERED IN THE NEGATIVE.

Data retrieved from NASA's ASRS site as of August 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.