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!