Narrative:

Air carrier X en route to bwi level FL220 given clearance to cross 15 mi west of csn FL200. Crew acknowledged and began descent to FL200, TCASII traffic Y alert came on about 9 NM 12 O'clock. Concurrently ZDC issued immediate clearance to descend to FL190, turn left 360 degrees. We did as traffic passed off our right side at a distance greater than 5 NM. We asked the controller what had happened and he stated that we mistook a callout for traffic as clearance. This miscom is understandable, yet I'm puzzled as to why my clearance readback was not challenged. Most likely workload or intelligibility of my readback may have been a problem for the controller. Additionally, I have been accustomed to traffic callouts in reference to my position aircraft. If this was traffic callout, why was it given in reference to csn VOR.

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Original NASA ASRS Text

Title: ACR X NON ADHERENCE TO ATC CLRNC DSCNT TO OCCUPIED ALT WAS IN CONFLICT WITH Y. POSSIBLE LTSS.

Narrative: ACR X ENRTE TO BWI LEVEL FL220 GIVEN CLRNC TO CROSS 15 MI W OF CSN FL200. CREW ACKNOWLEDGED AND BEGAN DSCNT TO FL200, TCASII TFC Y ALERT CAME ON ABOUT 9 NM 12 O'CLOCK. CONCURRENTLY ZDC ISSUED IMMEDIATE CLRNC TO DSND TO FL190, TURN L 360 DEGS. WE DID AS TFC PASSED OFF OUR R SIDE AT A DISTANCE GREATER THAN 5 NM. WE ASKED THE CTLR WHAT HAD HAPPENED AND HE STATED THAT WE MISTOOK A CALLOUT FOR TFC AS CLRNC. THIS MISCOM IS UNDERSTANDABLE, YET I'M PUZZLED AS TO WHY MY CLRNC READBACK WAS NOT CHALLENGED. MOST LIKELY WORKLOAD OR INTELLIGIBILITY OF MY READBACK MAY HAVE BEEN A PROB FOR THE CTLR. ADDITIONALLY, I HAVE BEEN ACCUSTOMED TO TFC CALLOUTS IN REF TO MY POS ACFT. IF THIS WAS TFC CALLOUT, WHY WAS IT GIVEN IN REF TO CSN VOR.

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.