Narrative:

Air carrier X (B757) was at 11000 ft. Traffic was issued to air carrier X. Traffic was an air carrier Y FK28 departure. Traffic was issued at 11 O'clock and 5 mi sebound out of 8200 ft for 10000 ft. Air carrier X said he finally got the traffic in sight, but also stated that he had a TCASII resolution. Air carrier X descended to 10600 ft. At the time he stated this, targets merged and 600 ft vertical separation existed. If the pilot of air carrier X did not get visual on his traffic, there most likely could have been a midair collision. Also what saved this situation was, at the time of this incident, the aircraft were in VMC conditions. Supplemental information from acn 262998: my aircraft was level at 11000 ft being vectored for approach. ATC advised climbing traffic at 12 O'clock. The only aircraft I saw at the time appeared to be above us. The TCASII then commanded a descent now. Not seeing any aircraft in conflict, I told the first officer to descend now. At 10600 ft, I saw an air carrier DC9 or fokker at 12:30 position and approximately 500-600 ft below me. I told the first officer to level off and I told ATC I had a TCASII RA to descend. I was at 10600 ft and returning to 11000 ft. ATC questioned that TCASII called for descent and I confirmed it had. My guess is that my TCASII judged his rapid climb to 10000 ft and commanded my descent not knowing the other aircraft was going to level at 10000 ft.

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Original NASA ASRS Text

Title: APCH CTLR CALLED TFC TO 757 ON VECTOR AT 11000 FT. 757 GOT TCASII RA ON FK28 CLBING TO 10000 FT AND FOLLOWED RA TO DSND. RESULT WAS LTSS WHICH WAS ALMOST AN NMAC.

Narrative: ACR X (B757) WAS AT 11000 FT. TFC WAS ISSUED TO ACR X. TFC WAS AN ACR Y FK28 DEP. TFC WAS ISSUED AT 11 O'CLOCK AND 5 MI SEBOUND OUT OF 8200 FT FOR 10000 FT. ACR X SAID HE FINALLY GOT THE TFC IN SIGHT, BUT ALSO STATED THAT HE HAD A TCASII RESOLUTION. ACR X DSNDED TO 10600 FT. AT THE TIME HE STATED THIS, TARGETS MERGED AND 600 FT VERT SEPARATION EXISTED. IF THE PLT OF ACR X DID NOT GET VISUAL ON HIS TFC, THERE MOST LIKELY COULD HAVE BEEN A MIDAIR COLLISION. ALSO WHAT SAVED THIS SIT WAS, AT THE TIME OF THIS INCIDENT, THE ACFT WERE IN VMC CONDITIONS. SUPPLEMENTAL INFO FROM ACN 262998: MY ACFT WAS LEVEL AT 11000 FT BEING VECTORED FOR APCH. ATC ADVISED CLBING TFC AT 12 O'CLOCK. THE ONLY ACFT I SAW AT THE TIME APPEARED TO BE ABOVE US. THE TCASII THEN COMMANDED A DSCNT NOW. NOT SEEING ANY ACFT IN CONFLICT, I TOLD THE FO TO DSND NOW. AT 10600 FT, I SAW AN ACR DC9 OR FOKKER AT 12:30 POS AND APPROX 500-600 FT BELOW ME. I TOLD THE FO TO LEVEL OFF AND I TOLD ATC I HAD A TCASII RA TO DSND. I WAS AT 10600 FT AND RETURNING TO 11000 FT. ATC QUESTIONED THAT TCASII CALLED FOR DSCNT AND I CONFIRMED IT HAD. MY GUESS IS THAT MY TCASII JUDGED HIS RAPID CLB TO 10000 FT AND COMMANDED MY DSCNT NOT KNOWING THE OTHER ACFT WAS GOING TO LEVEL AT 10000 FT.

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.