Narrative:

Flight abcd, cos-den, 18 mins en route, altitude 12000 ft, heading 320 degrees. It was the first officer's leg. The captain had just gotten the ATIS and was coming back on communication #1 for ATC as the first officer took a handoff to den approach. Approach control was an extremely busy frequency at the time with IFR conditions, moderate turbulence, and snow. A call came for flight abce to descend to 11000 ft. After no response and some hesitation, the captain answered 'understand flight abcd cleared to 11000 ft.' with no response to this call except the continuous chatter between approach control and other aircraft, we accepted this clearance as being meant for us, and started to descend. Within a few seconds, another clearance came for flight abce to turn left to 290 degrees. Again, no response. We hesitated, then the captain again queried the controller that the clearance was for flight abcd, as we began to suspect something was wrong and leveled off at 11500 ft. We were eventually informed that the clearance was for flight abce. We maintained heading of 320 degrees, decided that we had not been cleared to 11000 ft, and proceeded to climb back to 12000 ft. The flight continued to den without further incident.

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Original NASA ASRS Text

Title: SIMILAR CALL SIGNS, FREQ CONGESTION. FLC RESPONDED AND INITIATED OTHER ACFT'S CLRNC.

Narrative: FLT ABCD, COS-DEN, 18 MINS ENRTE, ALT 12000 FT, HDG 320 DEGS. IT WAS THE FO'S LEG. THE CAPT HAD JUST GOTTEN THE ATIS AND WAS COMING BACK ON COM #1 FOR ATC AS THE FO TOOK A HDOF TO DEN APCH. APCH CTL WAS AN EXTREMELY BUSY FREQ AT THE TIME WITH IFR CONDITIONS, MODERATE TURB, AND SNOW. A CALL CAME FOR FLT ABCE TO DSND TO 11000 FT. AFTER NO RESPONSE AND SOME HESITATION, THE CAPT ANSWERED 'UNDERSTAND FLT ABCD CLRED TO 11000 FT.' WITH NO RESPONSE TO THIS CALL EXCEPT THE CONTINUOUS CHATTER BTWN APCH CTL AND OTHER ACFT, WE ACCEPTED THIS CLRNC AS BEING MEANT FOR US, AND STARTED TO DSND. WITHIN A FEW SECONDS, ANOTHER CLRNC CAME FOR FLT ABCE TO TURN L TO 290 DEGS. AGAIN, NO RESPONSE. WE HESITATED, THEN THE CAPT AGAIN QUERIED THE CTLR THAT THE CLRNC WAS FOR FLT ABCD, AS WE BEGAN TO SUSPECT SOMETHING WAS WRONG AND LEVELED OFF AT 11500 FT. WE WERE EVENTUALLY INFORMED THAT THE CLRNC WAS FOR FLT ABCE. WE MAINTAINED HDG OF 320 DEGS, DECIDED THAT WE HAD NOT BEEN CLRED TO 11000 FT, AND PROCEEDED TO CLB BACK TO 12000 FT. THE FLT CONTINUED TO DEN WITHOUT FURTHER INCIDENT.

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.