Narrative:

We were instructed to 'contact mkc center on 133.47' and then told to 'descend to FL310.' we were at FL330. I acknowledged, 'descend to FL310 and switch to 133.47.' I then contacted ATC on 133.47 and said, 'out of FL330 for FL310.' controller said, 'verify at FL330.' I said 'negative, we were at FL325 for FL310.' controller again said 'verify FL330.' I then responded, 'climbing back to FL330.' I told controller that we were instructed to go to FL310 by previous controller. Nothing more was said, upon landing stl I contacted mkc center and asked about what happened. They said they were reviewing the tapes. We called from ord and were told that after clearance to FL310 and 133.47 the controller (although slower than normally) kept talking and said, 'the correct altitude to maintain is FL330.' the tapes show this to be true. The tapes only recorded '133.47' as my response. Apparently what happened was after the controller gave the first clearance we began our readback and cut out his correction during his pause. They only heard '133.47' and we had begun our descent and frequency change. We never had any doubts about acknowledging a correct ATC clearance. ATC manager feels that this is what happened after a review of the tapes and the timing, etc, but had to file a report anyway. I do not know of any way to correct this from happening again. I did think it odd that we were given a frequency change first and then a descent clearance. I also learned that ATC tapes only record ATC xmissions and do not record a stepped on situation. Only third parties would have heard us step on each other. ATC tapes show only their xmissions, even if stepped on. We can't pull our tapes to verify what we did. I feel like this is a system weakness and we've been had.

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Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION EXCURSION FROM CLRNC ALT.

Narrative: WE WERE INSTRUCTED TO 'CONTACT MKC CENTER ON 133.47' AND THEN TOLD TO 'DSND TO FL310.' WE WERE AT FL330. I ACKNOWLEDGED, 'DSND TO FL310 AND SWITCH TO 133.47.' I THEN CONTACTED ATC ON 133.47 AND SAID, 'OUT OF FL330 FOR FL310.' CTLR SAID, 'VERIFY AT FL330.' I SAID 'NEGATIVE, WE WERE AT FL325 FOR FL310.' CTLR AGAIN SAID 'VERIFY FL330.' I THEN RESPONDED, 'CLBING BACK TO FL330.' I TOLD CTLR THAT WE WERE INSTRUCTED TO GO TO FL310 BY PREVIOUS CTLR. NOTHING MORE WAS SAID, UPON LNDG STL I CONTACTED MKC CENTER AND ASKED ABOUT WHAT HAPPENED. THEY SAID THEY WERE REVIEWING THE TAPES. WE CALLED FROM ORD AND WERE TOLD THAT AFTER CLRNC TO FL310 AND 133.47 THE CTLR (ALTHOUGH SLOWER THAN NORMALLY) KEPT TALKING AND SAID, 'THE CORRECT ALT TO MAINTAIN IS FL330.' THE TAPES SHOW THIS TO BE TRUE. THE TAPES ONLY RECORDED '133.47' AS MY RESPONSE. APPARENTLY WHAT HAPPENED WAS AFTER THE CTLR GAVE THE FIRST CLRNC WE BEGAN OUR READBACK AND CUT OUT HIS CORRECTION DURING HIS PAUSE. THEY ONLY HEARD '133.47' AND WE HAD BEGUN OUR DSCNT AND FREQ CHANGE. WE NEVER HAD ANY DOUBTS ABOUT ACKNOWLEDGING A CORRECT ATC CLRNC. ATC MGR FEELS THAT THIS IS WHAT HAPPENED AFTER A REVIEW OF THE TAPES AND THE TIMING, ETC, BUT HAD TO FILE A RPT ANYWAY. I DO NOT KNOW OF ANY WAY TO CORRECT THIS FROM HAPPENING AGAIN. I DID THINK IT ODD THAT WE WERE GIVEN A FREQ CHANGE FIRST AND THEN A DSCNT CLRNC. I ALSO LEARNED THAT ATC TAPES ONLY RECORD ATC XMISSIONS AND DO NOT RECORD A STEPPED ON SITUATION. ONLY THIRD PARTIES WOULD HAVE HEARD US STEP ON EACH OTHER. ATC TAPES SHOW ONLY THEIR XMISSIONS, EVEN IF STEPPED ON. WE CAN'T PULL OUR TAPES TO VERIFY WHAT WE DID. I FEEL LIKE THIS IS A SYS WEAKNESS AND WE'VE BEEN HAD.

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.