Narrative:

At FL350 150 NM south of yeg heading 340 degrees, ATC gave us a garbled command. The first officer was flying, so I answered requesting a repeat of the directive. ATC responded, but was still unreadable, however, I heard 'now' in the command. It was unclr whether ATC wanted a turn, a descent, or both. Hearing the 'now' in the voice message, I was immediately alerted and scanned the TCASII. I saw an aircraft converging from 90 degrees to our right at our altitude. I immediately took control of the aircraft and did a descending left turn to 320 degrees. In the descent, ATC was finally understood as wanting a descent to FL330. Passing FL333, I observed the other aircraft just passing ours, still reading FL350 on the TCASII. We never got a TCASII alert. I feel if we had remained at FL350, we would possibly have collided. Edmonton center apologized, acknowledging their error. I called center after landing and was informed the controller had been relieved pending an internal investigation. I was later called by a representative of the national canadian safety board. I gave him this same information and directed him to call our airlines for any further follow-up. There was little traffic and it was late at night. I feel the ATC controller was probably a bit lax and simply did not notice the traffic conflict until it was too late to provide normal separation in a timely manner. Without TCASII, I don't know if we would have understood the need to descend in time to avoid a possible collision, granting the 2 unreadable voice communications and the time-consuming need to request a repeat of the directive.

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Original NASA ASRS Text

Title: NMAC TURNED INTO A POTENTIAL CONFLICT LTSS BY USE OF TCASII. TCASII SAVES THE DAY AGAIN!

Narrative: AT FL350 150 NM S OF YEG HDG 340 DEGS, ATC GAVE US A GARBLED COMMAND. THE FO WAS FLYING, SO I ANSWERED REQUESTING A REPEAT OF THE DIRECTIVE. ATC RESPONDED, BUT WAS STILL UNREADABLE, HOWEVER, I HEARD 'NOW' IN THE COMMAND. IT WAS UNCLR WHETHER ATC WANTED A TURN, A DSCNT, OR BOTH. HEARING THE 'NOW' IN THE VOICE MESSAGE, I WAS IMMEDIATELY ALERTED AND SCANNED THE TCASII. I SAW AN ACFT CONVERGING FROM 90 DEGS TO OUR R AT OUR ALT. I IMMEDIATELY TOOK CTL OF THE ACFT AND DID A DSNDING L TURN TO 320 DEGS. IN THE DSCNT, ATC WAS FINALLY UNDERSTOOD AS WANTING A DSCNT TO FL330. PASSING FL333, I OBSERVED THE OTHER ACFT JUST PASSING OURS, STILL READING FL350 ON THE TCASII. WE NEVER GOT A TCASII ALERT. I FEEL IF WE HAD REMAINED AT FL350, WE WOULD POSSIBLY HAVE COLLIDED. EDMONTON CTR APOLOGIZED, ACKNOWLEDGING THEIR ERROR. I CALLED CTR AFTER LNDG AND WAS INFORMED THE CTLR HAD BEEN RELIEVED PENDING AN INTERNAL INVESTIGATION. I WAS LATER CALLED BY A REPRESENTATIVE OF THE NATIONAL CANADIAN SAFETY BOARD. I GAVE HIM THIS SAME INFO AND DIRECTED HIM TO CALL OUR AIRLINES FOR ANY FURTHER FOLLOW-UP. THERE WAS LITTLE TFC AND IT WAS LATE AT NIGHT. I FEEL THE ATC CTLR WAS PROBABLY A BIT LAX AND SIMPLY DID NOT NOTICE THE TFC CONFLICT UNTIL IT WAS TOO LATE TO PROVIDE NORMAL SEPARATION IN A TIMELY MANNER. WITHOUT TCASII, I DON'T KNOW IF WE WOULD HAVE UNDERSTOOD THE NEED TO DSND IN TIME TO AVOID A POSSIBLE COLLISION, GRANTING THE 2 UNREADABLE VOICE COMS AND THE TIME-CONSUMING NEED TO REQUEST A REPEAT OF THE DIRECTIVE.

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.