Narrative:

As we arrived into atl we were given a descent to 11000. Soon after, ATC called traffic at 12 O'clock for which we could not see due to IMC. Without any kind of TA the TCASII system told us to 'climb, climb.' the autoplt was disconnected and a climb was established. The TCASII issued another advisory to 'monitor vertical speed' and our climb was decreased. At about this time lights from an aircraft directly below us could be seen through the clouds. Approach control radar was very busy and a call to tell them we had encountered could not be worked in quite yet. They soon saw a deviation and contacted us to say that 11000 ft was assigned and that we had traffic 12 O'clock 5 mi. The conflict aircraft was reported by ATC to be assigned 10000 ft. At about this time a 'clear of conflict' message was received and we returned to a descent down to 11000. (Estimate that a climb was topped at 11700-11800 ft). We returned to 11000 ft, broke into VMC and received a 'TA' from the TCASII now for the 12000 ft traffic which we picked up visually. What probably happened here is that we were descending the other aircraft was climbing and the TCASII alerted on the potential conflict not knowing of the assigned level offs. It might help to know that our traffic is climbing to level at 10000 or descending to level at 11000. This incident reinforced my belief that TCASII is a good system, it must be believed and action taken.

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Original NASA ASRS Text

Title: ALTDEV ALT EXCURSION ON TCASII RA. POTENTIAL CONFLICT LTSS.

Narrative: AS WE ARRIVED INTO ATL WE WERE GIVEN A DSCNT TO 11000. SOON AFTER, ATC CALLED TFC AT 12 O'CLOCK FOR WHICH WE COULD NOT SEE DUE TO IMC. WITHOUT ANY KIND OF TA THE TCASII SYS TOLD US TO 'CLB, CLB.' THE AUTOPLT WAS DISCONNECTED AND A CLB WAS ESTABLISHED. THE TCASII ISSUED ANOTHER ADVISORY TO 'MONITOR VERT SPD' AND OUR CLB WAS DECREASED. AT ABOUT THIS TIME LIGHTS FROM AN ACFT DIRECTLY BELOW US COULD BE SEEN THROUGH THE CLOUDS. APCH CTL RADAR WAS VERY BUSY AND A CALL TO TELL THEM WE HAD ENCOUNTERED COULD NOT BE WORKED IN QUITE YET. THEY SOON SAW A DEV AND CONTACTED US TO SAY THAT 11000 FT WAS ASSIGNED AND THAT WE HAD TFC 12 O'CLOCK 5 MI. THE CONFLICT ACFT WAS RPTED BY ATC TO BE ASSIGNED 10000 FT. AT ABOUT THIS TIME A 'CLR OF CONFLICT' MESSAGE WAS RECEIVED AND WE RETURNED TO A DSCNT DOWN TO 11000. (ESTIMATE THAT A CLB WAS TOPPED AT 11700-11800 FT). WE RETURNED TO 11000 FT, BROKE INTO VMC AND RECEIVED A 'TA' FROM THE TCASII NOW FOR THE 12000 FT TFC WHICH WE PICKED UP VISUALLY. WHAT PROBABLY HAPPENED HERE IS THAT WE WERE DSNDING THE OTHER ACFT WAS CLBING AND THE TCASII ALERTED ON THE POTENTIAL CONFLICT NOT KNOWING OF THE ASSIGNED LEVEL OFFS. IT MIGHT HELP TO KNOW THAT OUR TFC IS CLBING TO LEVEL AT 10000 OR DSNDING TO LEVEL AT 11000. THIS INCIDENT REINFORCED MY BELIEF THAT TCASII IS A GOOD SYS, IT MUST BE BELIEVED AND ACTION TAKEN.

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.