Narrative:

Air carrier X from roa-bwi. While at FL210, we were issued a descent clearance to cross 15 NM west of casanova at FL200. Arriving at FL200 approximately 25 NM west of casanova TCASII alerted us to a TA. ATC asked our altitude and instructed us to make an immediate left turn to 360 degree and descend to FL190. The traffic passed 1000 ft above and approximately 6 NM off our right. We asked the controller what happened and he said that we mistook a callout for traffic as a clearance. Our readback was never challenged when we readback `cross 15 NM west of casanova at FL200'. Workload, most likely or intelligibility on the readback may have been the problem for the controller. We are normally accustomed to aircraft `traffic' callouts reference to the position of the clock (ie. `12 O'clock 15 NM') not in reference to a VOR. This is an excellent example of how TCASII can help avoid traffic when a miscom (human factor) takes place. Supplemental information from acn 268642: air carrier X descended without ATC clearance within separation limits of air carrier Y. Radar controller took measures to ensure safety.

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Original NASA ASRS Text

Title: ACR X NON ADHERENCE TO ATC CLRNC UNAUTH DSCNT HAD LTSS FROM ACR Y. PLTDEV.

Narrative: ACR X FROM ROA-BWI. WHILE AT FL210, WE WERE ISSUED A DSCNT CLRNC TO CROSS 15 NM W OF CASANOVA AT FL200. ARRIVING AT FL200 APPROX 25 NM W OF CASANOVA TCASII ALERTED US TO A TA. ATC ASKED OUR ALT AND INSTRUCTED US TO MAKE AN IMMEDIATE L TURN TO 360 DEG AND DSND TO FL190. THE TFC PASSED 1000 FT ABOVE AND APPROX 6 NM OFF OUR R. WE ASKED THE CTLR WHAT HAPPENED AND HE SAID THAT WE MISTOOK A CALLOUT FOR TFC AS A CLRNC. OUR READBACK WAS NEVER CHALLENGED WHEN WE READBACK `CROSS 15 NM W OF CASANOVA AT FL200'. WORKLOAD, MOST LIKELY OR INTELLIGIBILITY ON THE READBACK MAY HAVE BEEN THE PROB FOR THE CTLR. WE ARE NORMALLY ACCUSTOMED TO ACFT `TFC' CALLOUTS REF TO THE POS OF THE CLOCK (IE. `12 O'CLOCK 15 NM') NOT IN REF TO A VOR. THIS IS AN EXCELLENT EXAMPLE OF HOW TCASII CAN HELP AVOID TFC WHEN A MISCOM (HUMAN FACTOR) TAKES PLACE. SUPPLEMENTAL INFO FROM ACN 268642: ACR X DSNDED WITHOUT ATC CLRNC WITHIN SEPARATION LIMITS OF ACR Y. RADAR CTLR TOOK MEASURES TO ENSURE SAFETY.

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.