Narrative:

We were arriving at dtw via the mizar arrival. We had passed the nakes intersection and turned toward carlton VOR. We had been given several clearance changes in the interim. First we were cleared to descend to 11000 ft. Then we were told to turn left 20 degrees for spacing to dtw. Then we were reclred to intercept the mizar arrival and apparently at this time the controller restated the descent clearance to 11000 ft. Not expecting to be reclred to an altitude we had already been cleared to, I read back, 'descend to 7000 ft.' even now, I'm still not sure this isn't what the controller said. Regardless, even if the controller did reclr us to 11000 ft he never corrected my readback to descend to 7000 ft. I read back 7000 ft, reset the altitude alerter and my copilot verbally reconfirmed the altitude and touched the altitude alert window with his finger. This elaborate clearance altitude xchk is what leads me to believe that either the center controller said 7000 ft in error or we both misheard the 11000 ft for 7000 ft incorrectly. We both believed we were cleared for a descent to 7000 ft and were descending through 9800 ft when center came back on and restated that our clearance limit was still 11000 ft and to climb back to it. We said that we would return to 11000 ft but that we thought that we had been given further clearance to 7000 ft. I was not under the impression that there was any traffic conflict and there was no one within 20 mi of our position displayed on our TCASII. I believe the problem arose from several sources. First, as previously stated, I was not anticipating being reclred to an altitude we were already cleared and descending to. Secondly, I believe we fell victim to the fact that the phrases '7000' and '11000' sound very similar. The controller did not say 'descend to eleven, one one thousand' to denote the similarity and make it clear he meant 11000 ft. In that event I suspect I would have heard 11000 ft and not 7000 ft. Thirdly, I feel the controller was lax in not listening to and correcting my clearance readback to descend to 7000 ft. Lastly, in the confusion of multiple clearance changes we were also distraction by the arrival of the flight attendant on the flight deck with soft drinks. I do believe, however, that this distraction was not a primary source of the problem.

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Original NASA ASRS Text

Title: FLC OF AN EMB 120 DSNDED FROM ASSIGNED ALT BECAUSE THEY BELIEVED THAT THEY WERE CLRED TO A LOWER ALT. THEY EVEN ACKNOWLEDGED THE NEW ALT TO ATC DURING READBACK, BUT IT WAS NOT CORRECTED BY ATC.

Narrative: WE WERE ARRIVING AT DTW VIA THE MIZAR ARR. WE HAD PASSED THE NAKES INTXN AND TURNED TOWARD CARLTON VOR. WE HAD BEEN GIVEN SEVERAL CLRNC CHANGES IN THE INTERIM. FIRST WE WERE CLRED TO DSND TO 11000 FT. THEN WE WERE TOLD TO TURN L 20 DEGS FOR SPACING TO DTW. THEN WE WERE RECLRED TO INTERCEPT THE MIZAR ARR AND APPARENTLY AT THIS TIME THE CTLR RESTATED THE DSCNT CLRNC TO 11000 FT. NOT EXPECTING TO BE RECLRED TO AN ALT WE HAD ALREADY BEEN CLRED TO, I READ BACK, 'DSND TO 7000 FT.' EVEN NOW, I'M STILL NOT SURE THIS ISN'T WHAT THE CTLR SAID. REGARDLESS, EVEN IF THE CTLR DID RECLR US TO 11000 FT HE NEVER CORRECTED MY READBACK TO DSND TO 7000 FT. I READ BACK 7000 FT, RESET THE ALT ALERTER AND MY COPLT VERBALLY RECONFIRMED THE ALT AND TOUCHED THE ALT ALERT WINDOW WITH HIS FINGER. THIS ELABORATE CLRNC ALT XCHK IS WHAT LEADS ME TO BELIEVE THAT EITHER THE CTR CTLR SAID 7000 FT IN ERROR OR WE BOTH MISHEARD THE 11000 FT FOR 7000 FT INCORRECTLY. WE BOTH BELIEVED WE WERE CLRED FOR A DSCNT TO 7000 FT AND WERE DSNDING THROUGH 9800 FT WHEN CTR CAME BACK ON AND RESTATED THAT OUR CLRNC LIMIT WAS STILL 11000 FT AND TO CLB BACK TO IT. WE SAID THAT WE WOULD RETURN TO 11000 FT BUT THAT WE THOUGHT THAT WE HAD BEEN GIVEN FURTHER CLRNC TO 7000 FT. I WAS NOT UNDER THE IMPRESSION THAT THERE WAS ANY TFC CONFLICT AND THERE WAS NO ONE WITHIN 20 MI OF OUR POS DISPLAYED ON OUR TCASII. I BELIEVE THE PROB AROSE FROM SEVERAL SOURCES. FIRST, AS PREVIOUSLY STATED, I WAS NOT ANTICIPATING BEING RECLRED TO AN ALT WE WERE ALREADY CLRED AND DSNDING TO. SECONDLY, I BELIEVE WE FELL VICTIM TO THE FACT THAT THE PHRASES '7000' AND '11000' SOUND VERY SIMILAR. THE CTLR DID NOT SAY 'DSND TO ELEVEN, ONE ONE THOUSAND' TO DENOTE THE SIMILARITY AND MAKE IT CLR HE MEANT 11000 FT. IN THAT EVENT I SUSPECT I WOULD HAVE HEARD 11000 FT AND NOT 7000 FT. THIRDLY, I FEEL THE CTLR WAS LAX IN NOT LISTENING TO AND CORRECTING MY CLRNC READBACK TO DSND TO 7000 FT. LASTLY, IN THE CONFUSION OF MULTIPLE CLRNC CHANGES WE WERE ALSO DISTR BY THE ARR OF THE FLT ATTENDANT ON THE FLT DECK WITH SOFT DRINKS. I DO BELIEVE, HOWEVER, THAT THIS DISTR WAS NOT A PRIMARY SOURCE OF THE PROB.

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.