Narrative:

Made evasive action according to TCASII (ie, followed TCASII RA) and descended approximately 400 ft below FL280. TCASII conflict then indicated clear of conflict; then climbed back to our assigned altitude of FL280. TCASII then showed conflicting aircraft to be 1300 ft above and 4 mi horizontal. ATC had called this traffic out to us as air carrier large transport descending to FL290 however, we were in clouds and didn't have him in sight. I discussed entire thing with the ATC controller, not only after, but while in process of descending out of FL280 and reason why. To my knowledge my TCASII was operating normally. However I requested it be checked out for any fault.

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Original NASA ASRS Text

Title: ACR MLG ALTDEV EXCURSION FROM CLRNC ALT IN RESPONSE TO TCASII RA AT FL280.

Narrative: MADE EVASIVE ACTION ACCORDING TO TCASII (IE, FOLLOWED TCASII RA) AND DSNDED APPROX 400 FT BELOW FL280. TCASII CONFLICT THEN INDICATED CLR OF CONFLICT; THEN CLBED BACK TO OUR ASSIGNED ALT OF FL280. TCASII THEN SHOWED CONFLICTING ACFT TO BE 1300 FT ABOVE AND 4 MI HORIZ. ATC HAD CALLED THIS TFC OUT TO US AS ACR LGT DSNDING TO FL290 HOWEVER, WE WERE IN CLOUDS AND DIDN'T HAVE HIM IN SIGHT. I DISCUSSED ENTIRE THING WITH THE ATC CTLR, NOT ONLY AFTER, BUT WHILE IN PROCESS OF DSNDING OUT OF FL280 AND REASON WHY. TO MY KNOWLEDGE MY TCASII WAS OPERATING NORMALLY. HOWEVER I REQUESTED IT BE CHKED OUT FOR ANY FAULT.

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.