AN MD10 AND A MEDIUM TWIN TURBOPROP IN A DAYLIGHT VMC CLOSE PROX ON FINAL APCH WITH A TCASII RA.

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EXECUTING A SID FROM THE NW; ATIS ADVERTISING CHARTED VISUAL APCHS. APCH INITIALLY TOLD US TO EXPECT RWY XX. WE ASKED FOR RWY XY-R INSTEAD; BEING CLOSER TO OUR RAMP. GRANTED. GIVEN INITIAL HDG TOWARD RWY YX-R. WE HAD PREVIOUSLY STRUNG THE LOC/BACK COURSE APCH; SINCE THIS IS ONE OF THE FEW BACK COURSE APCHS IN OUR ENTIRE SYS. GOOD TRAINING/REFRESHING PROCS; ETC. WE ASKED FOR DIRECT (OWN NAV) IAF; PERMITTING US A 12 MI FINAL APCH. GRANTED. WE NAVED TOWARD IAF. TURNED OVER TO ANOTHER APCH CTLR. HE GAVE US A VECTOR CLOSER TO THE RWY. WE AGAIN ASKED IF WE COULD PROCEED DIRECT TO IAF. NOPE. HE HAD TFC HE SAID. OK. WE'D STILL PLAN ON A VISUAL; BACKED UP BY OUR MAP ON THE NAV DISPLAY AND THE LOC/BACK COURSE SIGNAL; AND PLANNED ON FOLLOWING THE VERT STEPDOWNS AS DEPICTED ON THE APCH PLATE; WITH THE AUTOPLT ENGAGED; AND USING VERT SPD. HE GAVE US A 220 DEG HDG. THIS IS A 40 DEG INTERCEPT TO THE EXTENDED RWY CTRLINE. THE COURSE WOULD PUT US INSIDE THE IAF; BUT OUTSIDE THE FAF FOR THE APCH. HE ASKED US TO RPT THE RWY IN SIGHT. WE WERE DSNDING TO 3000 FT AND SAW THE RWY. WE RPTED AS REQUESTED. HE TURNED US OVER TO THE TWR; WITH ONLY A 'PARTING SHOT' STATEMENT; 'THERE'S TFC OUT THERE FOR RWY YY-L; A LIFE-FLT ACFT; HE HAS YOU IN SIGHT.' NO PROX OR ALT INFO WAS GIVEN US. WE WERE CONCENTRATING ON THE RWY; NAV WAS ARMED TO INTERCEPT THE EXTENDED CTRLINE. WE WERE STILL WELL N OF THE LOC; AND DSNDING SLOWLY TOWARD 3000 FT MSL. TCASII CAME ALIVE; WITH A YELLOW BALL TARGET AND AURAL 'TFC; TFC.' I SPIED A TWIN ENG TURBOPROP PARALLELING THE FINAL APCH COURSE; AND AT OUR ALT. I DISENGAGED THE AUTOPLT; BANKING SHARPLY TO THE R; STILL N OF THE LOC TO AVOID THE OTHER ACFT. DURING MY ROLLING MANEUVER; TCASII SOUNDED 'CLB; CLB.' I COULD STILL SEE THE TFC OFF OUR L WING. ALMOST IMMEDIATELY AFTER THE INITIAL AURAL WARNING; TCASII REVERSED ITSELF; DEMANDING 'DSND; DSND NOW!' I SHARPLY DOVE OUR ACFT 200-300 FT LOWER; AND DURING THIS DIVE; TCASII ANNOUNCED 'CLR OF CONFLICT.' I GOT A GLIMPSE OF THE TURBOPROP; THEN AT MY 8:30 O'CLOCK POS SLIGHTLY ABOVE US AT THIS POINT. WHEW! I S-L-O-W-L-Y ALIGNED WITH RWY XYR BY TURNING SLIGHTLY L. WE DID NOT VIOLATE CLASS B AIRSPACE DURING OUR SLIGHT DIVE. WE DID NOT PUSH OVER ANY MORE THAN 1/2 G OR SO. NO MENTION OF THE RA WAS GIVEN TO ATC. WE LANDED UNEVENTFULLY. ONLY AFTER 2 NIGHTS OF REVIEWING THE SIT (IN MY MIND) DO I BELIEVE IT MIGHT QUALITY AS AN NMAC; AND AM SUBMITTING A DUPLICATE RPT VIA NASA CLAIMING SAME. PROBS: ATC LEFT US HANGING. POOR INFO XFER. WE HAD NO INFO AS TO 'WHERE' THE TFC WAS (SIZE; TYPE; CLOCK POS; ALT). WE WERE RELYING ON HIM TO AVOID US; AND DID NOT SEE HIM UNTIL ALMOST TOO LATE. HAD THIS SCENARIO BEEN PLAYED OUT IN THE AFTERNOON HAZE INTO THE SETTING SUN; ION IMC; OR HAD I NOT SEEN THIS ACFT; AND ALLOWED OUR ACFT TO INTERCEPT THE EXTENDED CTRLINE OUR SEPARATION WOULD HAVE BEEN LESS OR NONE AT ALL; RESULTING IN A MIDAIR COLLISION. SUSPECT: INTRUDER ACFT; ALTHOUGH ON A FINAL APCH; WAS NOT EXACTLY ALIGNED WITH THE EXTENDED CTRLINE FOR RWY YY-L; ALTHOUGH ADMITTEDLY THE RWYS ARE EXTREMELY CLOSELY SPACED.