Narrative:

While cruising at FL290 and working with toronto center, we were approaching yxu when toronto ATC advised us that we had traffic at 12 O'clock at FL280 (opp direction medium large transport +/- 5 mi). We saw the traffic on TCAS as stated. No visibility sightings. We were checking waypoint coords for next wp and TCAS traffic. TCAS traffic remained 1000' below us, the TCAS voice said something too soft to be understood. I looked at the vertical speed and saw lights on from 0-4000' descent. As best I recall the TCAS voice said 'descend, descend, descend.' I immediately moved the V/south wheel to start a descent. We no more than started our descent (300-500 FPM) when the TCAS said 'monitor vertical speed.' vertical speed remained 300-500 FPM. A glance at the TCAS showed our traffic at 600' below us. It then disappeared. I called ATC to advise them that I had descended due to a TCAS advisory and asked if a conflict still existed. Also stated that we were at FL286 and leveling at FL284. They said they had no conflict at that time and I was cleared back to FL290. We returned to FL290. The controller later came back and said that the descent would be reported to transport canada. (I asked him previously to let me know that) the controller also stated at that time, that what might have triggered my TCAS was the fact that at the time I began my descent he had a heavy widebody transport at 12 mi, opp direction, climbing to FL280. In his words the widebody transport was 'climbing rather well.' he said that was information I might need when checking the TCAS (or inferred such). Remainder of trip uneventful. I don't know why this occurred. Possibly faulty ATC transponder on my aircraft or medium large transport below me or widebody transport ahead climbing. Voice on this TCAS set too low. (Volume).

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Original NASA ASRS Text

Title: ALT DEVIATION ALT EXCURSION FROM ASSIGNED BY PIC OF WDB AS HE RECEIVES A TCASII RA TO DESCEND AFTER HAVING RECEIVED A TRAFFIC ADVISORY WITH TRAFFIC SIGHTED.

Narrative: WHILE CRUISING AT FL290 AND WORKING WITH TORONTO CTR, WE WERE APCHING YXU WHEN TORONTO ATC ADVISED US THAT WE HAD TFC AT 12 O'CLOCK AT FL280 (OPP DIRECTION MLG +/- 5 MI). WE SAW THE TFC ON TCAS AS STATED. NO VIS SIGHTINGS. WE WERE CHKING WAYPOINT COORDS FOR NEXT WP AND TCAS TFC. TCAS TFC REMAINED 1000' BELOW US, THE TCAS VOICE SAID SOMETHING TOO SOFT TO BE UNDERSTOOD. I LOOKED AT THE VERT SPD AND SAW LIGHTS ON FROM 0-4000' DSNT. AS BEST I RECALL THE TCAS VOICE SAID 'DSND, DSND, DSND.' I IMMEDIATELY MOVED THE V/S WHEEL TO START A DSNT. WE NO MORE THAN STARTED OUR DSNT (300-500 FPM) WHEN THE TCAS SAID 'MONITOR VERT SPD.' VERT SPD REMAINED 300-500 FPM. A GLANCE AT THE TCAS SHOWED OUR TFC AT 600' BELOW US. IT THEN DISAPPEARED. I CALLED ATC TO ADVISE THEM THAT I HAD DSNDED DUE TO A TCAS ADVISORY AND ASKED IF A CONFLICT STILL EXISTED. ALSO STATED THAT WE WERE AT FL286 AND LEVELING AT FL284. THEY SAID THEY HAD NO CONFLICT AT THAT TIME AND I WAS CLRED BACK TO FL290. WE RETURNED TO FL290. THE CTLR LATER CAME BACK AND SAID THAT THE DSNT WOULD BE RPTED TO TRANSPORT CANADA. (I ASKED HIM PREVIOUSLY TO LET ME KNOW THAT) THE CTLR ALSO STATED AT THAT TIME, THAT WHAT MIGHT HAVE TRIGGERED MY TCAS WAS THE FACT THAT AT THE TIME I BEGAN MY DSNT HE HAD A HVY WDB AT 12 MI, OPP DIRECTION, CLBING TO FL280. IN HIS WORDS THE WDB WAS 'CLBING RATHER WELL.' HE SAID THAT WAS INFO I MIGHT NEED WHEN CHKING THE TCAS (OR INFERRED SUCH). REMAINDER OF TRIP UNEVENTFUL. I DON'T KNOW WHY THIS OCCURRED. POSSIBLY FAULTY ATC XPONDER ON MY ACFT OR MLG BELOW ME OR WDB AHEAD CLBING. VOICE ON THIS TCAS SET TOO LOW. (VOLUME).

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.