Narrative:

At 9000 ft, my leg, captain read back clearance to 7000 ft and frequency change. Flipped switch. I said I wasn't sure it was our number and to confirm. Captain repeated we were cleared to 7000 ft so I started down. Traffic ahead, TCASII alert, we stopped descent. Much radio congestion. Finally got a call in. He'd taken wrong clearance. I held 9000 ft and said at least twice 'let's check clearance' and did not start down until captain said very positively that we were cleared to 7000 ft. He never had any doubt abort clearance and I was only somewhat unsure. Had I been more convinced, I would have insisted we check prior to starting descent. I thought he probably heard it better than I did. Much radio congestion. Numerous similar sounding call signs. We called ATC on landing and learned we both read back clearance simultaneously and they didn't hear us take the other aircraft's.

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Original NASA ASRS Text

Title: ACR X SIMILAR NUMBERS CALL SIGN UNAUTH DSCNT FROM ASSIGNED ALT TCASII RA HAD LTSS FROM ACR Y.

Narrative: AT 9000 FT, MY LEG, CAPT READ BACK CLRNC TO 7000 FT AND FREQ CHANGE. FLIPPED SWITCH. I SAID I WASN'T SURE IT WAS OUR NUMBER AND TO CONFIRM. CAPT REPEATED WE WERE CLRED TO 7000 FT SO I STARTED DOWN. TFC AHEAD, TCASII ALERT, WE STOPPED DSCNT. MUCH RADIO CONGESTION. FINALLY GOT A CALL IN. HE'D TAKEN WRONG CLRNC. I HELD 9000 FT AND SAID AT LEAST TWICE 'LET'S CHK CLRNC' AND DID NOT START DOWN UNTIL CAPT SAID VERY POSITIVELY THAT WE WERE CLRED TO 7000 FT. HE NEVER HAD ANY DOUBT ABORT CLRNC AND I WAS ONLY SOMEWHAT UNSURE. HAD I BEEN MORE CONVINCED, I WOULD HAVE INSISTED WE CHK PRIOR TO STARTING DSCNT. I THOUGHT HE PROBABLY HEARD IT BETTER THAN I DID. MUCH RADIO CONGESTION. NUMEROUS SIMILAR SOUNDING CALL SIGNS. WE CALLED ATC ON LNDG AND LEARNED WE BOTH READ BACK CLRNC SIMULTANEOUSLY AND THEY DIDN'T HEAR US TAKE THE OTHER ACFT'S.

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.