D10 CTLR IS ADVISED BY MD80 FLT CREW OF NMAC WITH HELI AT 5000 FT WHILE ON VECTORS FOR APCH TO DFW.

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AN MD82 INBOUND TO DFW DSNDING TO 5000 FT RPTED TFC HAD PASSED UNDERNEATH HIM AND CLBED 600-700 FT TO AVOID. TFC WAS OBSERVED BY A DIFFERENT ACFT TO BE CLBING OUT OF 5000 FT -- 'A HELI.' I DID NOT OBSERVE THE TFC ON THE RADAR AND NO TA WAS GIVEN TO ACR X. ACR X INDICATED THAT THE TFC DID NOT APPEAR ON THE TCASII. SUPPLEMENTAL INFO FROM ACN 641032: THE FO WAS FLYING THE ACFT AT THE TIME OF THE INCIDENT. I ASSUMED CTL OF THE ACFT TO EXECUTE THE EVASIVE ACTION MANEUVER. THIS WAS DONE AS I WAS NOT SURE THE FO HAD THE TFC I HAD CALLED TO HIM IN SIGHT. THERE WAS NO OTHER REASON FOR MY ASSUMING CTL. WHILE ON THE CEDAR CREEK ARR TO DFW; WE WERE CLRED TO MAINTAIN 5000 FT. WE HAD JUST BEEN CLRED TO INTERCEPT AND TRACK THE RWY 31R LOC TO EXPECT THE STADIUM VISUAL TO RWY 31R. AT OR ABOUT THE 15 DME POINT ON THE LOC; I NOTICE TFC AT MY 11 O'CLOCK POS TRAVELING W TO E ACROSS OUR FLT PATH. I QUERIED ATC ABOUT THE TFC BUT RECEIVED NO RESPONSE. WITHIN 5-10 SECONDS; I REALIZED THE TFC WAS AT MY ALT. IMMEDIATE EVASIVE ACTION WAS TAKEN; CLBING TO ABOUT 5500-5700 FT. ATC CAME BACK TO INDICATE THAT THEY ONLY HAD A BASIC TARGET AT THAT POS. WE SAFELY COMPLETED THE EVASION AND RETURNED TO OUR ASSIGNED ALT. ATC THEN RPTED THAT THE TFC WAS RPTING A XPONDER WITH ALT OF 4700 FT. WE COMPLETED THE STADIUM VISUAL TO A NORMAL LNDG. THE TFC DID NOT PRESENT ON OUR TCASII IN ANY MANNER; IE; THERE WAS NO TARGET AT ALL ON TCASII ASSOCIATED WITH THIS TFC. AFTER THE TFC PASSED US IT BEGAN TO PRESENT WITH A YELLOW TARGET WITH ALT DIFFERENTIAL READOUT. THE TFC WAS A LATE MODEL BOEING MD-5XX HELI. THE ACFT WAS CLOSE ENOUGH TO SEE THE REGISTRATION NUMBER; BUT I WAS NOT ABLE TO RECORD IT DUE TO THE CIRCUMSTANCES.