Narrative:

We were cleared to descend from FL370 to FL350. Shortly after leveling off ATC cleared us to descend to FL310. While expediting the descent for turbulence passing FL330 I noticed traffic below us and I slowed our descent so as not to get an RA. Passing FL307 and the traffic changing to white on the TCASII, ATC said that we had descended below our clearance altitude of FL330. We immediately climbed back to FL330. ATC then advised us of a deviation and to call their office. We both recalled setting FL310 in the window and both verbally and visually confirming it. I was confident in the fact that we set it correctly. After landing, we called ZID at the number given and was told that after reviewing the tapes, we were indeed cleared to FL310 and it was a controller error. Thanks again to the TCASII for the alert.

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Original NASA ASRS Text

Title: B737 WITH ZID WAS ISSUED DSCNT TO OCCUPIED ALT RESULTING IN OPERROR.

Narrative: WE WERE CLRED TO DSND FROM FL370 TO FL350. SHORTLY AFTER LEVELING OFF ATC CLRED US TO DSND TO FL310. WHILE EXPEDITING THE DSCNT FOR TURB PASSING FL330 I NOTICED TFC BELOW US AND I SLOWED OUR DSCNT SO AS NOT TO GET AN RA. PASSING FL307 AND THE TFC CHANGING TO WHITE ON THE TCASII, ATC SAID THAT WE HAD DSNDED BELOW OUR CLRNC ALT OF FL330. WE IMMEDIATELY CLBED BACK TO FL330. ATC THEN ADVISED US OF A DEV AND TO CALL THEIR OFFICE. WE BOTH RECALLED SETTING FL310 IN THE WINDOW AND BOTH VERBALLY AND VISUALLY CONFIRMING IT. I WAS CONFIDENT IN THE FACT THAT WE SET IT CORRECTLY. AFTER LNDG, WE CALLED ZID AT THE NUMBER GIVEN AND WAS TOLD THAT AFTER REVIEWING THE TAPES, WE WERE INDEED CLRED TO FL310 AND IT WAS A CTLR ERROR. THANKS AGAIN TO THE TCASII FOR THE ALERT.

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.