WHILE TURNING FINAL INTO PHX ON A VISUAL APCH TO RWY 8; AN MD80 CREW CONFLICTED WITH VFR TFC OPERATING APPROX 400 FT BELOW CLASS B AIRSPACE.

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APCHING PHX FROM THE N ON A L BASE FOR RWY 8 WE WERE CLRED FOR A VISUAL APCH. WHILE TURNING FINAL; 6 OR 7 MI OUT; TCASII ISSUED A TA AND WE SAW A CONVERGING ACR JET APCHING FROM THE S ON A VISUAL APCH TO RWY 7R. BECAUSE WE WERE TURNING; IT WAS INITIALLY UNCLR IF THE TA AND THE ACR JET WERE; IN FACT; THE SAME ACFT. AS WE WERE BEING HANDED OFF TO TWR; THE TCASII WENT TO RA AND FOLLOWING THE TCASII COMMANDS; WE ARRESTED OUR DSCNT. IT NOW BECAME CLR THAT THE ACR JET WAS NOT THE THREAT AND THAT A SECOND; UNSEEN ACFT HAD PASSED 400 FT DIRECTLY BELOW OURS. NO TA'S HAD BEEN ISSUED BY ATC FOR EITHER ACFT. BECAUSE WE WERE ABOVE THE ILS GS WHEN THE INTRUDING ACFT FLEW UNDER US; IT WOULD SEEM THAT A VFR ACFT HAD FLOWN JUST BELOW OR THROUGH THE GLIDE PATH ABOUT 5 OR 6 MI ON FINAL DURING PARALLEL RWY OPS WITH NO ADVISORIES; NO ALARMS AND NO WARNINGS OTHER THAN TCASII. ALTHOUGH THE ACR JET WAS NOT A THREAT; ITS UNANNOUNCED PRESENCE WAS A DISTR AND WASTED VALUABLE TIME WHILE ANALYZING CRITICAL SIT. LATER; AFTER SPEAKING WITH TWR SUPVR I WAS ABLE TO GAIN A BETTER UNDERSTANDING OF ATC OPS AT PHX. SINCE RWY 8 AND RWY 7R EXCEED THE 4300 FT MINIMUM CTRLINE SPACING AND SINCE THE ATIS ADVISES WHENEVER PARALLEL RWY OPS ARE IN EFFECT; THE CTLRS ARE NOT REQUIRED TO ISSUE TA'S; EVEN WITH ACFT CONVERGING AT THE SAME ALT. OF COURSE; OUR ACFT WAS CONVERGING WITH AN ACFT AT OUR ALT AS WELL AS A SECOND ACFT BELOW US AS WE WERE DSNDING. WOULDN'T BEST PRACTICE DICTATE THAT WE BE INFORMED OF THIS CRITICAL INFO? IF CTLRS ONLY NEED TO GIVE TA'S WHEN DUTIES PERMIT; I CAN ONLY WONDER WHAT OTHER DUTIES HAD A HIGHER PRIORITY THAT MORNING. ISN'T TFC SEPARATION THE ESSENCE OF ATC'S MISSION? ANYONE READING THIS RPT WILL RECOGNIZE THAT THE DOWNWARD VIEW FROM AN MD80 IS LIMITED; ESPECIALLY AT 0 DEG FLAP. ALSO; PHX; (QUITE APART FROM THE SUNRISE/SUNSET VISIBILITY AND SMOGGY HAZE PROBS) HAS A VIRTUAL MOSAIC OF VISUAL GND CLUTTER MAKING SMALL ACFT EXCEEDINGLY HARD TO PICK OUT EVEN WITH AN ADVISORY. AT 6-7 MI ON FINAL APCH; NORMAL COCKPIT WORKLOAD IS HIGH; MAINTAINING GLIDE PATH; TIMELY CONFIGURING; COMPLETING THE LNDG CHKLIST; CHANGING RADIO FREQS TO TWR; CHKING IN WITH TWR; ACKNOWLEDGING LNDG CLRNC; NOTING WIND INFO AND ADHERING TO REQUIRED ALT AND PERFORMANCE CALLOUTS MAKES THIS PHASE OF FLT CRITICAL. IT ALSO REQUIRES CONSIDERABLE HEAD DOWN; HEAD INSIDE ACTIVITY FOR BOTH PLTS. THUS; TO REVERT TO 'WE GAVE YOU 4300 FT; NOW YOU SEE AND AVOID.' WITH AN INOP TCASII WE WOULD LIKELY NEVER HAVE BEEN AWARE OF THE VFR ACFT AND HAVING NO REASON TO ARREST OUR DSCNT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED HE WAS APPROX 15 MI NW OF THE ARPT DSNDING THROUGH APPROX 4000 FT WHEN CLRED FOR A VISUAL APCH TO RWY 8. HE SAID A TCASII TA WAS RECEIVED AT 3500 FT AS THEY TURNED ON FINAL. AT THE SAME TIME THEY SAW AN ACR INBOUND FROM THE S SLIGHTLY AHEAD TURNING ONTO FINAL FOR RWY 7R AND BELIEVED AT FIRST IT CAUSED THE TCASII TA. APCHING 3000 FT; 6-7 MI ON FINAL; THE RPTR SAID A TCASII RA WAS RECEIVED INDICATING TFC WAS 400 FT BELOW THEIR ACFT.