Narrative:

Level at assigned altitude 10000'. Received traffic advisory from TCAS and noticed intruder aircraft on display at 1-2 O'clock, 500' above our aircraft. No visibility contact was established. TCAS issued RA (descent). We complied with the advisory and departed 10000'. Shortly after descent was established, ATC advised us of traffic. We descended to approximately 9600' during the avoidance maneuver. TCAS then issued a 'clear of conflict' advisory, at which time we promptly returned to our assigned altitude. During the entire event we maintained our assigned heading of 345 degrees, which had been assigned at center's request. I do not know if the intruder was in contact with ATC. Departure did advise us of the traffic after the avoidance maneuver was initiated. I do not recall the specifics of the ATC traffic advisory. Given the 500' sep (vertical) at the beginning of the event, I had not expected an RA. I assumed that the intruder must have begun a descent, though I did not have the opportunity to confirm or refute this assumption. If the intruder remained level throughout the event, the system needs to be fine-tuned. RA's resulting from traffic that is level at 500' above or below our aircraft, such as VFR traffic in a TRSA or arsa, could be detrimental to safety.

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Original NASA ASRS Text

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

Narrative: LEVEL AT ASSIGNED ALT 10000'. RECEIVED TFC ADVISORY FROM TCAS AND NOTICED INTRUDER ACFT ON DISPLAY AT 1-2 O'CLOCK, 500' ABOVE OUR ACFT. NO VIS CONTACT WAS ESTABLISHED. TCAS ISSUED RA (DSNT). WE COMPLIED WITH THE ADVISORY AND DEPARTED 10000'. SHORTLY AFTER DSNT WAS ESTABLISHED, ATC ADVISED US OF TFC. WE DSNDED TO APPROX 9600' DURING THE AVOIDANCE MANEUVER. TCAS THEN ISSUED A 'CLR OF CONFLICT' ADVISORY, AT WHICH TIME WE PROMPTLY RETURNED TO OUR ASSIGNED ALT. DURING THE ENTIRE EVENT WE MAINTAINED OUR ASSIGNED HDG OF 345 DEGS, WHICH HAD BEEN ASSIGNED AT CENTER'S REQUEST. I DO NOT KNOW IF THE INTRUDER WAS IN CONTACT WITH ATC. DEP DID ADVISE US OF THE TFC AFTER THE AVOIDANCE MANEUVER WAS INITIATED. I DO NOT RECALL THE SPECIFICS OF THE ATC TFC ADVISORY. GIVEN THE 500' SEP (VERT) AT THE BEGINNING OF THE EVENT, I HAD NOT EXPECTED AN RA. I ASSUMED THAT THE INTRUDER MUST HAVE BEGUN A DSNT, THOUGH I DID NOT HAVE THE OPPORTUNITY TO CONFIRM OR REFUTE THIS ASSUMPTION. IF THE INTRUDER REMAINED LEVEL THROUGHOUT THE EVENT, THE SYS NEEDS TO BE FINE-TUNED. RA'S RESULTING FROM TFC THAT IS LEVEL AT 500' ABOVE OR BELOW OUR ACFT, SUCH AS VFR TFC IN A TRSA OR ARSA, COULD BE DETRIMENTAL TO 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.