AN F100 WITH ZBW CTLR HAS A TCASII RA DURING DSCNT AND TAKES EVASIVE ACTION. THE INTRUDER TFC WAS VFR WITH AN ADJACENT SECTOR WITH ALT SUPPRESSED.

Narrative

THE RADAR CTLR WAS TALKING TO ACR X ON A PAX FLT FROM ORD TO HPN. THE ACFT WAS DSNDING ON THE VALRE 2 ARR. AS THE ACFT APCHED THE WEARD INTXN THE PLT ADVISED HE WAS RESPONDING TO AN RA FROM HIS TCASII. THE ONLY TARGET ON THE SCOPE HAD NO ALT RPTING. WE SOON FOUND THE #2 ACFT IN THIS INCIDENT WAS BEING SUPPRESSED IN THE CONFLICT ALERT LIST FROM AN ALERT WITH ANOTHER ACFT. SUPPRESSING THIS ALERT HAD STOPPED A MODE C READOUT FROM THE VFR ACFT ON BOTH OF THE SCOPES I WAS WORKING WITH AS WELL AS THE SCOPE OF THE CTLR THAT HAS GIVEN US THE ACR X ACFT. WHEN THE CONFLICT ALERT SUPPRESSION WAS DEACTIVATED; THE MODE C OF THE VFR ACFT; (ACFT Y); CAME ON TO BOTH SCOPES WITH THE ALT SHOWING WITHIN 200 FT OF ACR X AND WELL WITHIN 1 MI. BY THIS TIME THE PLT OF ACR X HAD ALREADY TAKEN ACTION TO AVOID THE COLLISION. THIS CONFLICT ALERT PROB SEEMS TO BE A KNOWN PROB WITH THE NEWLY INSTALLED DSR EQUIP. THIS HAS HAPPENED TO ME BEFORE; BUT THIS IS THE CLOSEST I HAVE HAD 2 ACFT COME TOGETHER TO DATE. SUPPLEMENTAL INFO FROM ACN 454762: I WAS WORKING THE RADAR ASSOCIATE POS AT A SECTOR WHEN WE HAD XFERRED CTL AND COM OF ACR X. THE ADJOINING SECTOR HAD SUPPRESSED CONFLICT ALERT ON ACFT Y FOR ANOTHER ACFT. THIS CAUSED THE MODE C OF ACFT Y TO DISAPPEAR FROM OUR RADAR SCOPE (A KNOWN PROB WITH THE NEW DSR EQUIP). NOT KNOWING THE ALT OF ACFT Y WE DID NOT NOTIFY THE NEXT SECTOR OF A POTENTIAL CONFLICT WITH ACR X. A TCASII RA WAS INITIATED BY ACR X; WHICH PREVENTED A POSSIBLE MIDAIR COLLISION.