Narrative:

Air carrier X was descending in visual conditions towards a landing on runway 27 at mci. We had been cleared to 7000 and were on radar vectors. We received what we heard was a clearance to 4000 but the controller did not wait for an acknowledgement, but continued in the same breath to issue someone else an instruction. (Obviously nonstandard. He was very busy, almost overloaded.) the captain and I confirmed the altitude and put it in the alerter. We were at about 9000 ft when we received this. Passing through about 6700 ft we received a TCASII RA telling us to climb. At the same time, the controller told us to maintain 7000. There was an small transport Y below us at 6000 ft. We climbed back up to 7000. The controller told us he never cleared us below 7000. The captain and I were both positive he cleared our flight to 4000, but over the telephone they said it was another flight they had cleared to 4000, not similar call signs, however. We were air carrier X, the other flight was same company. He never did or would play back the tape, but said there was no conflict. Obviously, after giving an aircraft a new altitude clearance, a controller should wait for a readback before continuing on. We should have confirmed the altitude eventually, but we were very busy also and the frequency also very busy. We forgot to question him, but proceeded to descend anyway. In that situation there is definitely chance for errors to be made. I will definitely continue to read back (at least attempt to) all new altitude clrncs.

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Original NASA ASRS Text

Title: ACR X NON ADHERENCE TO ATC CLRNC (DSCNT TO WRONG ALT) HAD LTSS FROM SMT Y. PLTDEV. TCASII RA EVASIVE ACTION TAKEN.

Narrative: ACR X WAS DSNDING IN VISUAL CONDITIONS TOWARDS A LNDG ON RWY 27 AT MCI. WE HAD BEEN CLRED TO 7000 AND WERE ON RADAR VECTORS. WE RECEIVED WHAT WE HEARD WAS A CLRNC TO 4000 BUT THE CTLR DID NOT WAIT FOR AN ACKNOWLEDGEMENT, BUT CONTINUED IN THE SAME BREATH TO ISSUE SOMEONE ELSE AN INSTRUCTION. (OBVIOUSLY NONSTANDARD. HE WAS VERY BUSY, ALMOST OVERLOADED.) THE CAPT AND I CONFIRMED THE ALT AND PUT IT IN THE ALERTER. WE WERE AT ABOUT 9000 FT WHEN WE RECEIVED THIS. PASSING THROUGH ABOUT 6700 FT WE RECEIVED A TCASII RA TELLING US TO CLB. AT THE SAME TIME, THE CTLR TOLD US TO MAINTAIN 7000. THERE WAS AN SMT Y BELOW US AT 6000 FT. WE CLBED BACK UP TO 7000. THE CTLR TOLD US HE NEVER CLRED US BELOW 7000. THE CAPT AND I WERE BOTH POSITIVE HE CLRED OUR FLT TO 4000, BUT OVER THE TELEPHONE THEY SAID IT WAS ANOTHER FLT THEY HAD CLRED TO 4000, NOT SIMILAR CALL SIGNS, HOWEVER. WE WERE ACR X, THE OTHER FLT WAS SAME COMPANY. HE NEVER DID OR WOULD PLAY BACK THE TAPE, BUT SAID THERE WAS NO CONFLICT. OBVIOUSLY, AFTER GIVING AN ACFT A NEW ALT CLRNC, A CTLR SHOULD WAIT FOR A READBACK BEFORE CONTINUING ON. WE SHOULD HAVE CONFIRMED THE ALT EVENTUALLY, BUT WE WERE VERY BUSY ALSO AND THE FREQ ALSO VERY BUSY. WE FORGOT TO QUESTION HIM, BUT PROCEEDED TO DSND ANYWAY. IN THAT SITUATION THERE IS DEFINITELY CHANCE FOR ERRORS TO BE MADE. I WILL DEFINITELY CONTINUE TO READ BACK (AT LEAST ATTEMPT TO) ALL NEW ALT 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.