Narrative:

ZID issued a descent clearance to our flight. We were instructed to descend to what we thought was FL210 from FL350. We entered the altitude in the altitude alerter, read back 'FL210' to the controller and started a 1000 FPM rate of descent. The controller questioned us saying 'what do you believe your assigned altitude to be?' as we passed through FL290. We responded 'FL210.' he then said to 'maintain FL280. Your assigned altitude was FL310, no problem, though, there was no traffic.' when we received the descent clearance initially, the FMC showed a top of descent profile to begin in 80 NM. I wondered why we were being assigned such a low altitude (FL210) so far away from our planned top of descent point. However, I did not question the controller, and assumed we were descended so low for traffic. The first officer thought, initially, that he heard FL310, but saw FL210 in the altitude alert window after I had placed FL210 in the alerter. However, the first officer read back 'FL210' to the controller. The ZID controller did not correct or respond to our readback of 'FL210.' I should have questioned the controller and had the first officer confirm and verify the assigned altitude. When in doubt, clarify, confirm, and verify assigned altitudes. Don't trust 'no response' to a readback.

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Original NASA ASRS Text

Title: AN ACR FLC FLYING A B737-300 NEAR CMH FAILS TO LEVEL AT ASSIGNED ALT AS INSTRUCTED BY ATC.

Narrative: ZID ISSUED A DSCNT CLRNC TO OUR FLT. WE WERE INSTRUCTED TO DSND TO WHAT WE THOUGHT WAS FL210 FROM FL350. WE ENTERED THE ALT IN THE ALT ALERTER, READ BACK 'FL210' TO THE CTLR AND STARTED A 1000 FPM RATE OF DSCNT. THE CTLR QUESTIONED US SAYING 'WHAT DO YOU BELIEVE YOUR ASSIGNED ALT TO BE?' AS WE PASSED THROUGH FL290. WE RESPONDED 'FL210.' HE THEN SAID TO 'MAINTAIN FL280. YOUR ASSIGNED ALT WAS FL310, NO PROB, THOUGH, THERE WAS NO TFC.' WHEN WE RECEIVED THE DSCNT CLRNC INITIALLY, THE FMC SHOWED A TOP OF DSCNT PROFILE TO BEGIN IN 80 NM. I WONDERED WHY WE WERE BEING ASSIGNED SUCH A LOW ALT (FL210) SO FAR AWAY FROM OUR PLANNED TOP OF DSCNT POINT. HOWEVER, I DID NOT QUESTION THE CTLR, AND ASSUMED WE WERE DSNDED SO LOW FOR TFC. THE FO THOUGHT, INITIALLY, THAT HE HEARD FL310, BUT SAW FL210 IN THE ALT ALERT WINDOW AFTER I HAD PLACED FL210 IN THE ALERTER. HOWEVER, THE FO READ BACK 'FL210' TO THE CTLR. THE ZID CTLR DID NOT CORRECT OR RESPOND TO OUR READBACK OF 'FL210.' I SHOULD HAVE QUESTIONED THE CTLR AND HAD THE FO CONFIRM AND VERIFY THE ASSIGNED ALT. WHEN IN DOUBT, CLARIFY, CONFIRM, AND VERIFY ASSIGNED ALTS. DON'T TRUST 'NO RESPONSE' TO A READBACK.

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.