LESS THAN STANDARD SEPARATION BETWEEN 2 ACR MAKING PARALLEL APCHS. OPERATIONAL ERROR.

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THIS INCIDENT OCCURRED WHILE BEING RADAR VECTORED FOR A LNDG IN VISUAL FLT CONDITIONS. THE LAST ACKNOWLEDGED CLRNC/XMISSION FROM DETROIT APCH CTL TO MY FLT BEFORE THE INCIDENT WAS TO TURN RIGHT TO 180 DEGS; DSND TO 4000' AND MAINTAIN A SPD OF 190 KTS. I ACKNOWLEDGED THIS CLRNC AND THE F/O WHO WAS FLYING COMPLIED. BOTH THE F/O AND MYSELF OBSERVED THIS HDG WAS TAKING US THROUGH THE LOC COURSE THE PREVIOUS CTLR HAD TOLD US TO EXPECT (21R). FREQ CONGESTION WAS SUCH THAT I WAS UNABLE TO QUERY THE CTLR ABOUT THIS IMPENDING SITUATION. THE CTLR THEN CALLED US AND SAID WE APPEARED TO BE LINING UP FOR RWY 21L; AND TO TURN TO A HDG OF 240 DEGS. THE F/O STARTED THE TURN; BUT BEFORE I COULD ACKNOWLEDGE THE XMISSION; ANOTHER CTLR CAME ON THE FREQ ABRUPTLY AND TOLD US TO TURN TO A HDG OF 250 DEGS AND DSND TO 3000' IMMEDIATELY. THE F/O CONTINUED HIS TURN AND BEGAN A DSCNT TO COMPLY WITH THE CLRNC. I THEN COMMENTED TO THE CTLR THAT WE HAD NOT BEEN CLRED FOR AN APCH. AS WE WERE IN OUR TURN I NOTICED ACR Y AT OUR 10 O'CLOCK POS; ABOUT 1 MI AWAY IN A LEFT BANKED TURN TO A S WESTERLY HDG; AND HE APPEARED TO BE IN A CLB. I THEN COMMENTED TO THE CTLR THAT I HAD ACR Y IN SIGHT AND THE ARPT IN SIGHT BUT WE STILL HAD NOT BEEN CLRED FOR THE VISUAL APCH TO RWY 21R. AT THIS TIME; THE CTLR CLRED US FOR THE APCH TO RWY 21R AND ACR Y RESUMED HIS APCH TO RWY 12L. CONTRIBUTING FACTORS: LARGE VOL OF TFC IN APCH AREA; RAPID NONCEASING RADIO XMISSIONS; MAJOR AIRLINE PUSH IN PROGRESS; AND CTLR TRNING IN PROGRESS. I FULLY UNDERSTAND THE NEED FOR CTLR TRNING UNDER ACTUAL CONDITIONS; HOWEVER SUPVRS MUST NOT LET ACTUAL CONDITIONS DETERIORATE TO THIS POINT IN ORDER TO MAKE A LESSON PLAN POINT. IT WAS UNNERVING ENOUGH TO THE FLT CREWS INVOLVED; NOT TO MENTION THE PAX WHO OBVIOUSLY SAW WHAT HAD HAPPENED. SUPPLEMENTAL INFO FROM ACN 83059: AFTER FLYING APPROX 1/2 MI E OF THE RWY 21R EXTENDED CENTERLINE AND 8 ILS DME; THE CTLR DIRECTED US TO TURN TO A 240 DEG HDG AND SAID WE WERE LINED UP WITH RWY 21L; AND TO CHK OUR ILS FREQ AS IF HE THOUGHT WE HAD BEEN TOLD TO INTERCEPT THE LOC TO RWY 21R.