Narrative:

ATC advised us of unknown traffic at 11-12 O'clock 500 ft above out altitude, unverified, crossing left to right at approximately 5 mi. We got a visual on that traffic just as we both were about to enter clouds. At that time the traffic was at 11:30-12 O'clock and our onboard TCASII system gave us an RA to descend. We descended approximately 300 ft and the TCASII announced that we were clear of conflict. We then advised center of our deviation and climbed back to our assigned altitude. Center advised us that they would try to identify this unknown aircraft.

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Original NASA ASRS Text

Title: ACR MLG ALT DEV EXCURSION FROM CLRNC ALT IN RESPONSE TO TCASII RA.

Narrative: ATC ADVISED US OF UNKNOWN TFC AT 11-12 O'CLOCK 500 FT ABOVE OUT ALT, UNVERIFIED, XING L TO R AT APPROX 5 MI. WE GOT A VISUAL ON THAT TFC JUST AS WE BOTH WERE ABOUT TO ENTER CLOUDS. AT THAT TIME THE TFC WAS AT 11:30-12 O'CLOCK AND OUR ONBOARD TCASII SYS GAVE US AN RA TO DSND. WE DSNDED APPROX 300 FT AND THE TCASII ANNOUNCED THAT WE WERE CLR OF CONFLICT. WE THEN ADVISED CENTER OF OUR DEV AND CLBED BACK TO OUR ASSIGNED ALT. CENTER ADVISED US THAT THEY WOULD TRY TO IDENT THIS UNKNOWN ACFT.

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.