ACR FLT APCHING BOS HELD HIGH AT FL270 REQUIRES RAPID DSCNT TO MAKE XING AT 11000 FT. TAKE EVASIVE ACTION TO AVOID VFR TFC AT 15500 FT.

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FLT XXXX (ZZZ-BOS) DSNDED FROM CRUISE ALT TO FL270 WITH ZNY WHILE ENRTE FROM CHAMP INTXN TO PVD VOR. AFTER HDOF TO ZBW; ACR X WAS KEPT AT FL270 UNTIL PASSING OVER AN EBOUND ACR Y FLT. PASSING THE EASTERN TIP OF LONG ISLAND; ACR X REQUESTED LOWER ALT IN ANTICIPATION OF A NORMAL CLRNC TO CROSS PVD AT 11000 FT. THE ZBW CTLR GAVE ACR X A SLIGHT L VECTOR TO AVOID TFC TO THE E; THEN CLRED US TO FL210 AND TO PROCEED DIRECT PVD. HE THEN HANDED ACR X OFF TO ANOTHER ZBW CTLR. THIS CTLR CLRED ACR X DIRECT TO INNDY INTXN (PVD 093 DEG RADIAL/15 DM) AND GAVE A CLRNC TO CROSS INNDY AT 11000 FT. THIS WAS VERY HELPFUL; AS ACR X WAS ALREADY HIGH IF IT HAD BEEN NECESSARY TO CROSS PVD AT 11000 FT. SHORTLY THEREAFTER; THE CTLR SAID SOMETHING TO THE EFFECT OF: 'BOSTON IS LNDG RWY 4R; WE NEED YOU TO CROSS PVD AT 11000 FT.' AT THIS POINT ACR X WAS ABOUT 20 (?) MI SSE OF PVD HDG NE TOWARD INNDY. AS THE FO (PF) EXTENDED THE SPD BRAKES; WENT TO IDLE PWR AND BEGAN A L TURN; THE CAPT QUERIED THE CTLR OVER THE CLRNC SINCE IT NECESSITATED A STEEP DSCNT TO PVD. THE ZBW CTLR REAFFIRMED THE CLRNC. AT THIS TIME; ACR X WAS ENTERING A L 25-30 DEG BANK FROM A HDG OF ABOUT 040 DEGS TO 340 DEGS AND DSNDING AT ABOUT 3500 FPM. AS ACR X APCHED 17000 FT IN A RAPID DSCNT; THE ZBW CTLR CALLED OUT VFR TFC AHEAD AT 15500 FT. NEAR SIMULTANEOUSLY; A TCASII EMPTY DIAMOND CONTACT FIRST APPEARED ON THE SCOPE AT 11-12 O'CLOCK POS. THIS CONTACT THEN IMMEDIATELY TURNED INTO A SOLID DIAMOND; INDICATING LESS THAN 6 MI LATERAL DISTANCE AND WITHIN 1200 FT ALT. BECAUSE OF THE L TURN TOWARD PVD; WAS BELLY UP TO THE TFC; CONVERGING (DUE TO TURN RADIUS); AND APCHING ITS ALT. ROLLING OUT TO GAIN VISUAL CONTACT WOULD HAVE INCREASED THE RATE OF LATERAL CLOSURE. THE FLT CREW THEN HAD TO MAKE A SNAP JUDGEMENT WITH ZERO TIME FOR DISCUSSION: DSND THROUGH THE ALT OF THE VFR TFC WHILE ON A CONVERGING COURSE; OR LEVELOFF ABOVE THE VFR TFC. CAPT (PNF) ELECTED THE LATTER COURSE OF ACTION; TOOK THE ACFT AND BEGAN TO AGGRESSIVELY LEVEL OFF. ACR X LEVELED AT 15800 FT (300 FT VERT SEPARATION ON TCASII); THEN CLBED ABOUT 500 FT WHILE CLRING TO THE N OF THE VFR TFC. BASED ON THE TCASII DISPLAY; IT APPEARED THAT THE MINIMUM SEPARATION WAS ABOUT 3 MI. THERE WAS NO TCASII TA OR RA THROUGHOUT THIS SEQUENCE OF EVENTS. ACR X ASKED THE ZBW CTLR WHETHER SHE WAS AWARE OF THE TFC; AND A MALE VOICE ANSWERED STATING THEY HAD ISSUED A TA. PRESUMABLY; THIS WAS THE ONE CALL ACR X RECEIVED WHILE DSNDING INTO THE VFR ACFT APPROX 2 MI ON THE NOSE. ACR X ADVISED ZBW THAT THE FLT WOULD NOT CROSS PVD AT 11000 FT AND ASKED THE CTLR TO MARK THE TIME AND SAVE THE (RADAR) TAPES. THE ZBW CTLR THEN HANDED ACR X OFF TO BOS APCH; WHICH VECTORED US FOR AN ILS RWY 4R. BASED ON OTHER TFC OBSERVED AFTER THE HDOF; IT DID NOT APPEAR TO THE FLT CREW THAT THE VECTORING OVER PVD WAS DRIVEN BY A NEED TO ATTAIN SPACING WITH OTHER ACFT INBOUND TO BOS AHEAD OF US. A NORMAL APCH AND LNDG WAS SUBSEQUENTLY ACCOMPLISHED. ACR X FLT REGULARLY RECEIVES EXCELLENT SVC FROM ZBW. THIS INCIDENT WAS AN ABERRATION; ALBEIT A POTENTIALLY DANGEROUS ONE. HOPEFULLY; IT WILL SERVE AS A LESSON LEARNED FOR THE FLT CREW AND CTLR. HOWEVER; IT IS ALSO A WARNING OF A LARGER PROB THAT MUST BE ADDRESSED BY FAA AND THE INDUSTRY. OUTDATED ATC EGS THAT CONTINUE TO ALLOW UNCTLED VFR ACFT TO FLY AT HIGH ALT (EG; ABOVE 10000 FT) IN INCREASINGLY CONGESTED AIRSPACE ARE AN INVITATION TO DISASTER.