Narrative:

We were flying on the cetus 2 arrival into dtw, had just leveled at 12000 ft passing cetus. Captain had to use restroom. Left cockpit for a few mins. Approach issued clearance to descend 10000 ft and then reduce to 210 KTS. I read back the clearance and began to comply. The captain then returned to flight deck as I was passing through 11000 ft and slowing. I briefed the changes to the captain. When passing 10500 ft ATC told us to level at 11000 ft. Captain answered climbing to 11000 ft, and inquired about the clearance. ATC then said you're almost to 10000 ft, maintain 10000 ft. There did not appear to be any conflict. I told the captain that I believed 10000 ft and 210 KTS was what I read back. We both are not sure if there was ever a problem, however, are aware how easy it is on both parts (ATC/pilot) to make errors, especially in a busy area during a peak push/arrival time like dtw. This is why I am writing.

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Original NASA ASRS Text

Title: AN ACR DC9 FLC DSND BELOW THEIR ASSIGNED ALT DURING A SID. THE CAPT HAD LEFT THE FLT STATION AND THE FO WAS FLYING AND RESPONDED TO THE DSCNT CLRNC. NO CONFLICT.

Narrative: WE WERE FLYING ON THE CETUS 2 ARR INTO DTW, HAD JUST LEVELED AT 12000 FT PASSING CETUS. CAPT HAD TO USE RESTROOM. LEFT COCKPIT FOR A FEW MINS. APCH ISSUED CLRNC TO DSND 10000 FT AND THEN REDUCE TO 210 KTS. I READ BACK THE CLRNC AND BEGAN TO COMPLY. THE CAPT THEN RETURNED TO FLT DECK AS I WAS PASSING THROUGH 11000 FT AND SLOWING. I BRIEFED THE CHANGES TO THE CAPT. WHEN PASSING 10500 FT ATC TOLD US TO LEVEL AT 11000 FT. CAPT ANSWERED CLBING TO 11000 FT, AND INQUIRED ABOUT THE CLRNC. ATC THEN SAID YOU'RE ALMOST TO 10000 FT, MAINTAIN 10000 FT. THERE DID NOT APPEAR TO BE ANY CONFLICT. I TOLD THE CAPT THAT I BELIEVED 10000 FT AND 210 KTS WAS WHAT I READ BACK. WE BOTH ARE NOT SURE IF THERE WAS EVER A PROB, HOWEVER, ARE AWARE HOW EASY IT IS ON BOTH PARTS (ATC/PLT) TO MAKE ERRORS, ESPECIALLY IN A BUSY AREA DURING A PEAK PUSH/ARR TIME LIKE DTW. THIS IS WHY I AM WRITING.

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.