Narrative:

We were cleared by ZTL to descend from FL280 to cross the dehan intersection at 8000 ft on the awson arrival. During the descent; we were handed off to atl approach and given a clearance to descend to 4000 ft. Passing 9000 ft; approach control issued a TA for unidented VFR traffic and gave us a heading to avoid the traffic. As we were passing 6500 ft; approach control instructed to climb back up to 7000 ft to avoid the unknown traffic. About this time were received a TCAS RA to climb. An immediate climb was initiated. I spotted traffic about the 10 O'clock position and above us as we started the climb. At no time did this traffic ever appear on TCAS. The only traffic ever displayed was below us and climbing toward us. The captain leveled off at 7500 ft to avoid the traffic above us although the TCAS was still commanding a climb. The RA target showed 100 ft directly below us. The RA then disappeared from the screen. The traffic above was never pointed out by ATC and never showed up on TCAS. We never saw traffic below us at any time. The unusual thing is that as we climbed to avoid the unknown aircraft below us; the closure rate increased on TCAS. At the same time we were closing in on the aircraft above us that never appeared on TCAS. It almost seems as if the aircraft above was displaying erroneous information regarding his altitude and our TCAS reacted to that information. I seriously doubt that there was ever an aircraft below us. You would think that if an aircraft passed 100 ft below you; that you would acquire it visually and we never did. It is fortunate that this maneuver occurred during day VFR conditions. Approach control was notified of the situation. ZTL has also been notified and is investigating.

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Original NASA ASRS Text

Title: F2000 WITH A80 TRACON EXPERIENCED UNEXPLAINED TCAS RA AT 6500 FT WITH ISSUED TFC CONFLICTING TFC MODE C WAS LIKELY ERRONEOUS.

Narrative: WE WERE CLRED BY ZTL TO DSND FROM FL280 TO CROSS THE DEHAN INTXN AT 8000 FT ON THE AWSON ARR. DURING THE DSCNT; WE WERE HANDED OFF TO ATL APCH AND GIVEN A CLRNC TO DSND TO 4000 FT. PASSING 9000 FT; APCH CTL ISSUED A TA FOR UNIDENTED VFR TFC AND GAVE US A HDG TO AVOID THE TFC. AS WE WERE PASSING 6500 FT; APCH CTL INSTRUCTED TO CLB BACK UP TO 7000 FT TO AVOID THE UNKNOWN TFC. ABOUT THIS TIME WERE RECEIVED A TCAS RA TO CLB. AN IMMEDIATE CLB WAS INITIATED. I SPOTTED TFC ABOUT THE 10 O'CLOCK POS AND ABOVE US AS WE STARTED THE CLB. AT NO TIME DID THIS TFC EVER APPEAR ON TCAS. THE ONLY TFC EVER DISPLAYED WAS BELOW US AND CLBING TOWARD US. THE CAPT LEVELED OFF AT 7500 FT TO AVOID THE TFC ABOVE US ALTHOUGH THE TCAS WAS STILL COMMANDING A CLB. THE RA TARGET SHOWED 100 FT DIRECTLY BELOW US. THE RA THEN DISAPPEARED FROM THE SCREEN. THE TFC ABOVE WAS NEVER POINTED OUT BY ATC AND NEVER SHOWED UP ON TCAS. WE NEVER SAW TFC BELOW US AT ANY TIME. THE UNUSUAL THING IS THAT AS WE CLBED TO AVOID THE UNKNOWN ACFT BELOW US; THE CLOSURE RATE INCREASED ON TCAS. AT THE SAME TIME WE WERE CLOSING IN ON THE ACFT ABOVE US THAT NEVER APPEARED ON TCAS. IT ALMOST SEEMS AS IF THE ACFT ABOVE WAS DISPLAYING ERRONEOUS INFO REGARDING HIS ALT AND OUR TCAS REACTED TO THAT INFO. I SERIOUSLY DOUBT THAT THERE WAS EVER AN ACFT BELOW US. YOU WOULD THINK THAT IF AN ACFT PASSED 100 FT BELOW YOU; THAT YOU WOULD ACQUIRE IT VISUALLY AND WE NEVER DID. IT IS FORTUNATE THAT THIS MANEUVER OCCURRED DURING DAY VFR CONDITIONS. APCH CTL WAS NOTIFIED OF THE SITUATION. ZTL HAS ALSO BEEN NOTIFIED AND IS INVESTIGATING.

Data retrieved from NASA's ASRS site as of January 2009 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.