Narrative:

While working a radar position (arrival, departure, overflt), I was vectoring an mdt X on a downwind leg for a visual approach. At the same time, I was also working a small transport Y swbound at 8000 ft. When I took the handoff from the center, the mdt was at 11000 ft. Upon transfer of control from center, I descended the mdt to 9000 ft. My data block indicated the mdt was TCASII equipped. WX was reported: 6000 scattered, 15000 broken, visibility 10. Both aircraft were on a converging course. The mdt was sebound, the small transport swbound. Traffic was exchanged between both aircraft when they were about 8 mi apart. About 15 seconds later the mdt pilot advised he had the 'traffic in sight.' I then instructed the pilot to 'maintain visual separation with the small transport and descend and maintain 7000 ft.' he acknowledged the clearance and I immediately went back to the small transport pilot and advised the mdt 'has you in sight, will maintain visual separation and will be descending through your altitude.' the small transport pilot acknowledged the transmission and advised that he was, 'in and out of the tops of this scattered layer.' meanwhile, the mdt was descending through 8700 ft still on a converging course with approximately 3 mi now between aircraft. I again went to the mdt pilot to confirm he still had his traffic in sight and he responded 'affirmative.' as soon as I unkeyed, the small transport pilot advised me he was IMC! I then made the decision to turn the small transport to a 270 degree heading away from the mdt. However, by that time, it was too late, separation was lost. Conflict alert alarmed and I'd say the aircraft passed with less than a mile lateral separation and 200-300 ft vertical. After both aircraft diverged, the mdt pilot came back and advised me that he was 'using TCASII!' wrong! TCASII must not and cannot be used to apply any type of separation let alone visual. The pilot must visually observe the aircraft he is instructed to maintain visual from and if he cannot, he should advise ATC immediately so the controller has time to revert to standard IFR separation. TCASII should only be used for what it was designed for -- traffic collision and avoidance - - not separation!

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Original NASA ASRS Text

Title: AIR CARRIER MDT IN AND OUT OF CLOUDS USING TCASII FOR VISUAL SEPARATION

Narrative: WHILE WORKING A RADAR POS (ARR, DEP, OVERFLT), I WAS VECTORING AN MDT X ON A DOWNWIND LEG FOR A VISUAL APCH. AT THE SAME TIME, I WAS ALSO WORKING A SMT Y SWBOUND AT 8000 FT. WHEN I TOOK THE HDOF FROM THE CTR, THE MDT WAS AT 11000 FT. UPON TRANSFER OF CTL FROM CTR, I DSNDED THE MDT TO 9000 FT. MY DATA BLOCK INDICATED THE MDT WAS TCASII EQUIPPED. WX WAS RPTED: 6000 SCATTERED, 15000 BROKEN, VISIBILITY 10. BOTH ACFT WERE ON A CONVERGING COURSE. THE MDT WAS SEBOUND, THE SMT SWBOUND. TFC WAS EXCHANGED BTWN BOTH ACFT WHEN THEY WERE ABOUT 8 MI APART. ABOUT 15 SECONDS LATER THE MDT PLT ADVISED HE HAD THE 'TFC IN SIGHT.' I THEN INSTRUCTED THE PLT TO 'MAINTAIN VISUAL SEPARATION WITH THE SMT AND DSND AND MAINTAIN 7000 FT.' HE ACKNOWLEDGED THE CLRNC AND I IMMEDIATELY WENT BACK TO THE SMT PLT AND ADVISED THE MDT 'HAS YOU IN SIGHT, WILL MAINTAIN VISUAL SEPARATION AND WILL BE DSNDING THROUGH YOUR ALT.' THE SMT PLT ACKNOWLEDGED THE XMISSION AND ADVISED THAT HE WAS, 'IN AND OUT OF THE TOPS OF THIS SCATTERED LAYER.' MEANWHILE, THE MDT WAS DSNDING THROUGH 8700 FT STILL ON A CONVERGING COURSE WITH APPROX 3 MI NOW BTWN ACFT. I AGAIN WENT TO THE MDT PLT TO CONFIRM HE STILL HAD HIS TFC IN SIGHT AND HE RESPONDED 'AFFIRMATIVE.' AS SOON AS I UNKEYED, THE SMT PLT ADVISED ME HE WAS IMC! I THEN MADE THE DECISION TO TURN THE SMT TO A 270 DEG HDG AWAY FROM THE MDT. HOWEVER, BY THAT TIME, IT WAS TOO LATE, SEPARATION WAS LOST. CONFLICT ALERT ALARMED AND I'D SAY THE ACFT PASSED WITH LESS THAN A MILE LATERAL SEPARATION AND 200-300 FT VERT. AFTER BOTH ACFT DIVERGED, THE MDT PLT CAME BACK AND ADVISED ME THAT HE WAS 'USING TCASII!' WRONG! TCASII MUST NOT AND CANNOT BE USED TO APPLY ANY TYPE OF SEPARATION LET ALONE VISUAL. THE PLT MUST VISUALLY OBSERVE THE ACFT HE IS INSTRUCTED TO MAINTAIN VISUAL FROM AND IF HE CANNOT, HE SHOULD ADVISE ATC IMMEDIATELY SO THE CTLR HAS TIME TO REVERT TO STANDARD IFR SEPARATION. TCASII SHOULD ONLY BE USED FOR WHAT IT WAS DESIGNED FOR -- TFC COLLISION AND AVOIDANCE - - NOT SEPARATION!

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.