ACR Y TCASII TA. HAD NMAC LTSS FROM ACR Y; TCASII RA TO PARALLEL RWY. SYS ERROR. EVASIVE ACTION TAKEN.

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I WAS WORKING FINAL APCH AT DFW. ACR X WAS ON A R BASE DSNDING TO 3000 FT FOR RWY 18R. ACR Y WAS NE OF DFW DSNDING OUT OF 6000 FT ON A VISUAL APCH FOR RWY 17L BEING WORKED BY ANOTHER CTLR. I HAD 4 ACFT ON FREQ AND HAD JUST CLRED AN LTT FOR A VISUAL APCH TO FOLLOW ACR X. I SLOWED ACR X TO 170 KTS TO THE MARKER. FOR SOME REASON; UNKNOWN TO ME; I NEVER ISSUED AN APCH CLRNC TO ACR X. I OBSERVED ACR X OVERSHOOT FINAL. I ASKED HIM IF HE WAS TURNING TOWARD THE ARPT. HE SAID THAT HE HAD INITIATED A R TURN BECAUSE HE SAW ACR Y OFF TO HIS L AND ABOVE HIM ON FINAL FOR WRY 17L. I TOLD HIM TO TURN R TO JOIN FINAL AND DSNDED HIM. I CLRED HIM FOR A VISUAL (THEIR PATHS NEVER CROSSED ON FINAL). IN THE MEANTIME; THE CTLR WORKING ACR Y HAD ISSUED TFC ON ACR X. ACR Y STATED THAT HE SAW ACR X AND WAS 'WELL CLR OF THE TFC.' THIS SIT COULD HAVE BEEN AVOIDED IF: 1) I HAD CLRED ACR X FOR A VISUAL APCH; 2) ACR X HAD QUESTIONED ME ABOUT AN APCH CLRNC AS HE APCHED FINAL (SIMULTANEOUS VISUAL APCHS WERE ADVERTISED ON THE ATIS). SUPPLEMENTAL INFO FROM ACN 242766: ACR X ON APCH VECTORS FOR RWY 18R AT DFW. WE WERE ADVISED THAT WE COULD EXPECT A SHORT APCH. WE WERE ON DOWNWIND AT 5000 FT/210 KIAS; SLATS EXTENDED. APCH GAVE US A DSCNT TO 3000 FT AND A TURN TO 080 DEGS FROM A CURRENT 350 DEG HDG. WHILE ROLLING OUT ON 080 DEGS AND DSNDING THROUGH 3500 FT; APCH TOLD US TO SLOW TO 170 KIAS TO THE MARKER; BUT FAILED TO GIVE ANOTHER TURN OR AN APCH CLRNC. THE COPLT WAS FLYING THE AIRPLANE AND STATED THAT HE WAS BECOMING UNCOMFORTABLE CONTINUING ON THE 080 DEG HDG; STRAIGHT TOWARD PARALLEL TFC LNDG ON RWY 17L. WE HAD VISUAL CONTACT AND A TCASII TA ON AN ACFT COMING AT US FROM THE E TO LAND ON RWY 17L; AND A TFC CONFLICT SEEMED POSSIBLE. AS WE RAPIDLY APCHED THE RWY 18R LOC; I TOLD THE COPLT THAT APCH HAD BETTER TURN US 'REAL SOON.' ALMOST SIMULTANEOUSLY AS I WAS TELLING HIM TO DO SO; HE INITIATED A STEEP TURN AND DSCNT TO RETURN TO RWY 18R FINAL COURSE. I ESTIMATED ABOUT 1000 FT VERT AND LESS THAN A MI SEPARATION FROM THE RWY 17L ACFT; ALTHOUGH I DID LOSE SIGHT OF HIM IN THE TURN. AT THIS POINT APCH CALLED AND WONDERED WHERE WE WERE GOING; AND THAT TFC FROM RWY 17L HAD US IN SIGHT. WE WERE ALSO TOLD TO TURN R TO 220 DEG HDG AND INTERCEPT RWY 18R COURSE. I XMITTED THEN THAT WE HAD NEVER BEEN ISSUED; RECEIVED; OR ACKNOWLEDGED AN APCH CLRNC IN THE FIRST PLACE. SHORTLY THEREAFTER WE SWITCHED TO TWR FOR RWY 18R AND MADE A SUBSEQUENTLY UNEVENTFUL LNDG. SEVERAL PHONE CALLS AND CONVERSATIONS WITH DFW TRACON SUPVR ULTIMATELY CONFIRMED WHAT WE ALREADY KNEW. DFW SUPVR LISTENED TO THE TAPES AND STATED THAT AN APCH CLRNC HAD IN FACT NOT BEEN ISSUED BY THE CTLR; PRECIPITATING THE ENTIRE INCIDENT. SUPPLEMENTAL INFO FROM ACN 242972: AS WE APCHED THE RWY 18R LOC ON OUR BASE LEG (STILL DSNDING) THE CAPT STATED THAT THEY HAD BETTER TURN US REAL SOON AND I AGREED. THE APCH FREQ AT THIS POINT WAS VERY BUSY AND WE COULD NOT GET A CALL IN TO REQUEST THE TURN. I BEGAN LOOKING FOR TFC APCHING THE PARALLEL (RWY 17L) AND SAW AN ACR Y JET STARTING HIS TURN FROM BASE TO FINAL. I POINTED OUT THE TFC TO THE CAPT AND STATED THAT I WAS UNCOMFORTABLE WITH OUR SIT. TCASII GAVE US A TA. I DISCONNECTED THE AUTOPLT; LOWERED THE NOSE AND INITIATED A STEEP TURN BACK TOWARD RWY 18R FINAL APCH COURSE. SUPPLEMENTAL INFO FROM ACN 242777: ACR Y WHILE ON APCH TO LAND ON RWY 17L AT DFW AND AFTER BEING CLRED FOR A VISUAL APCH; WE ENCOUNTERED A NEAR MISS WITH ACFT WHICH WAS ON APCH TO RWY 18R AT DFW. WE WERE HDG APPROX 200 DEGS TO INTERCEPT FINAL AND ON THE GS. WE WERE DSNDING THROUGH 4000 FT WHEN OUR TCASII GAVE US A TA FOLLOWED BY AN RA AND COMMAND TO MONITOR VERT SPD. THE CTLR THEN ADVISED US THAT ACR X HAD OVERSHOT HIS RWY AND FOR US TO STOP DSCNT. WE ACQUIRED ACR X VISUALLY COMING AT US FROM THE W AT ABOUT A 90 DEG ANGLE; THEN HE FLEW UNDER US AT 500 FT BELOW US. WE LEVELED OFF UNTIL ACQUIRING HIM VISUALLY AGAIN; THEN CONTINUED OUR APCH AND UNEVENTFUL LNDG.