Narrative:

I was working approach control at clt. I descended air carrier X from 11000 ft to 9000 ft and issued traffic to him on an air carrier Y climbing to 8000 ft. Air carrier X reported the air carrier Y 'in sight.' when the 2 aircraft's targets unmerged, I noticed air carrier X mode C at 8600 ft and I asked his altitude. Air carrier X advised me that he had a TCASII advisory and descended. Later by phone he indicated that he had lost sight of the air carrier Y in a cloud and had to respond to the TCASII advisory. Air carrier X descended into the air carrier Y and they passed within 600 ft vertically and 1 1/2 mi horizontally. Although I favor using TCASII, the problem was the equipment. When TA's were issued, the pilots were made aware of each other's intentions and there was more than 1000 ft and 3 mi separation. In this case the TCASII alert should have been ignored. Supplemental information from acn 250375: approaching charlotte, descending out of 11000 ft for 9000 ft (somewhere between 9500 ft and 9000 ft) we got a TCASII TA and at roughly the same time an advisory by ATC of traffic to our left. We then got a TCASII descend advisory and I (PNF) called traffic in sight. At first, I thought the traffic was above our altitude. It was difficult to determine target altitude given our different and changing attitudes. It was also difficult to maintain visual with the atmospheric conditions. The first officer (PF) began following TCASII RA and at the same time began to help acquire target visually. At about 8800 ft to 8700 ft as we were given clear of conflict by TCASII, we were asked our altitude by ATC and I stated we were following a TCASII RA. The controller acknowledged and stated our assigned altitude of 9000 ft and that conflicting traffic was at 8000 ft. Although we as users of the TCASII system must at the very least take seriously any TCASII advisory information given to us, we also must not blindly follow an advisory with 100 percent confidence, especially when correlating the information with intermittent visual cues and deceiving atmospheric conditions. There were seconds during this incident where I questioned in my mind if I did in fact have the traffic called by ATC and also the possibility that TCASII was interacting with another aircraft of which I was not aware of. The busy screen in the approach environment was made less user friendly since I was also in the WX mode and painting cloud formations. I cannot help but think that in this particular case I may have either been given a less than desirable resolution (at least in terms of how ATC or a pilot with accurate visual cues would look at it) or a bad advisory! It seems that in this particular incident, TCASII was an unwelcome distraction in a very high workload environment, since the incident would have been avoided altogether if TCASII was not installed.

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Original NASA ASRS Text

Title: TCASII LOGIC. ACR X TCASII RA HAD LTSS FROM ACR Y. PLTDEV. EVASIVE ACTION TAKEN.

Narrative: I WAS WORKING APCH CTL AT CLT. I DSNDED ACR X FROM 11000 FT TO 9000 FT AND ISSUED TFC TO HIM ON AN ACR Y CLBING TO 8000 FT. ACR X RPTED THE ACR Y 'IN SIGHT.' WHEN THE 2 ACFT'S TARGETS UNMERGED, I NOTICED ACR X MODE C AT 8600 FT AND I ASKED HIS ALT. ACR X ADVISED ME THAT HE HAD A TCASII ADVISORY AND DSNDED. LATER BY PHONE HE INDICATED THAT HE HAD LOST SIGHT OF THE ACR Y IN A CLOUD AND HAD TO RESPOND TO THE TCASII ADVISORY. ACR X DSNDED INTO THE ACR Y AND THEY PASSED WITHIN 600 FT VERTICALLY AND 1 1/2 MI HORIZLY. ALTHOUGH I FAVOR USING TCASII, THE PROB WAS THE EQUIP. WHEN TA'S WERE ISSUED, THE PLTS WERE MADE AWARE OF EACH OTHER'S INTENTIONS AND THERE WAS MORE THAN 1000 FT AND 3 MI SEPARATION. IN THIS CASE THE TCASII ALERT SHOULD HAVE BEEN IGNORED. SUPPLEMENTAL INFO FROM ACN 250375: APCHING CHARLOTTE, DSNDING OUT OF 11000 FT FOR 9000 FT (SOMEWHERE BTWN 9500 FT AND 9000 FT) WE GOT A TCASII TA AND AT ROUGHLY THE SAME TIME AN ADVISORY BY ATC OF TFC TO OUR L. WE THEN GOT A TCASII DSND ADVISORY AND I (PNF) CALLED TFC IN SIGHT. AT FIRST, I THOUGHT THE TFC WAS ABOVE OUR ALT. IT WAS DIFFICULT TO DETERMINE TARGET ALT GIVEN OUR DIFFERENT AND CHANGING ATTITUDES. IT WAS ALSO DIFFICULT TO MAINTAIN VISUAL WITH THE ATMOSPHERIC CONDITIONS. THE FO (PF) BEGAN FOLLOWING TCASII RA AND AT THE SAME TIME BEGAN TO HELP ACQUIRE TARGET VISUALLY. AT ABOUT 8800 FT TO 8700 FT AS WE WERE GIVEN CLR OF CONFLICT BY TCASII, WE WERE ASKED OUR ALT BY ATC AND I STATED WE WERE FOLLOWING A TCASII RA. THE CTLR ACKNOWLEDGED AND STATED OUR ASSIGNED ALT OF 9000 FT AND THAT CONFLICTING TFC WAS AT 8000 FT. ALTHOUGH WE AS USERS OF THE TCASII SYS MUST AT THE VERY LEAST TAKE SERIOUSLY ANY TCASII ADVISORY INFO GIVEN TO US, WE ALSO MUST NOT BLINDLY FOLLOW AN ADVISORY WITH 100 PERCENT CONFIDENCE, ESPECIALLY WHEN CORRELATING THE INFO WITH INTERMITTENT VISUAL CUES AND DECEIVING ATMOSPHERIC CONDITIONS. THERE WERE SECONDS DURING THIS INCIDENT WHERE I QUESTIONED IN MY MIND IF I DID IN FACT HAVE THE TFC CALLED BY ATC AND ALSO THE POSSIBILITY THAT TCASII WAS INTERACTING WITH ANOTHER ACFT OF WHICH I WAS NOT AWARE OF. THE BUSY SCREEN IN THE APCH ENVIRONMENT WAS MADE LESS USER FRIENDLY SINCE I WAS ALSO IN THE WX MODE AND PAINTING CLOUD FORMATIONS. I CANNOT HELP BUT THINK THAT IN THIS PARTICULAR CASE I MAY HAVE EITHER BEEN GIVEN A LESS THAN DESIRABLE RESOLUTION (AT LEAST IN TERMS OF HOW ATC OR A PLT WITH ACCURATE VISUAL CUES WOULD LOOK AT IT) OR A BAD ADVISORY! IT SEEMS THAT IN THIS PARTICULAR INCIDENT, TCASII WAS AN UNWELCOME DISTR IN A VERY HIGH WORKLOAD ENVIRONMENT, SINCE THE INCIDENT WOULD HAVE BEEN AVOIDED ALTOGETHER IF TCASII WAS NOT INSTALLED.

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.