ACR MDT ALTDEV EXCURSION FROM CLRNC ALT IN RESPONSE TO TCASII RA.

Narrative

I WAS WORKING THE E ARR POS WHEN AN MDT COMMUTER CHK IN ON ARR AT 10000 FT. HE MDT WAS TOLD TO EXPECT RWY 23; THE NEXT ARR WAS AN MLG CHKING IN LEAVING 12700 FT FOR 11000. I INSTRUCTED MLG TO EXPECT RWY 23 AND ISSUED THE MDT AS TFC AT 12 O'CLOCK 2.5 MI LEVEL AT 10000. THE PLT QUESTIONED THE ALT AND I INFORMED THE PLT 'HE'S AT 10000.' THE PLT MISUNDERSTOOD THE ADVISORY AS A DSCNT CLRNC AND CONTINUED PAST 11000 FT TO 10000 FT. DURING THIS PROCESS I ISSUED A DSCNT CLRNC TO THE MDT TO 9000 FT. I LATER NOTICED THE MLG DSNDING THROUGH 10500 FT AS THE MDT PASSED THROUGH 9600. I ASKED THE MLG TO VERIFY HIS ALT. WHEN THE PLT DID NOT RESPOND; I INSTRUCTED THE MLG TO MAINTAIN 11000 AND ISSUED THE MDT TFC; THE MLG RESPONDED WITH 'YOU GAVE US 10000 FT AND WE'RE ALREADY THERE. WE HAVE THE TFC IN SIGHT.' AT THIS POINT THE MDT INFORMED ME THAT HE HAD A TCASII RA AND WAS CLBING. I INFORMED THE MDT OF THE POS OF THE TRAILING ACFT. THE MDT CLBED FROM BELOW THE ALT THE TRAILING ACFT WAS DSNDING TO; CLBED ABOVE THE MLG THEN DSNDED BACK BELOW THE MLG (SEE EXAMPLE BELOW). IT IS MY OPINION THAT SEPARATION WOULD HAVE BEEN MAINTAINED IF TCASII RA HAD NOT OCCURRED; IE; TCASII MADE A SAFE OP VERY HAZARDOUS OP FOR THE ACFT INVOLVED.