HS125 FLC CLOSED MORE RAPIDLY THAN EXPECTED ON AN MD88 TO WHICH APCH CTLR INSTRUCTED THEM TO MAINTAIN VISUAL SEPARATION FOR A VISUAL APCH TO PARALLEL RWYS RESULTING IN THE MD88 TURNING AND CLBING IN RESPONSE TO A TCASII RA.

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ON AN IFR FLT PLAN TO PORTLAND; OR; OUT OF APPROX 5000 FT MSL; WE CALLED THE MILL AND ARPT IN SIGHT. AT THIS TIME; WE WERE GIVEN A VECTOR OF 120 DEGS AND INSTRUCTIONS TO LOOK FOR AN MD88 APCHING TROUTDALE FROM THE S FOR THE MILL VISUAL TO RWY 28L AT PORTLAND. WE CALLED THE ACFT IN SIGHT AND WERE CLRED TO FOLLOW HIM ON THE MILL VISUAL TO RWY 28R. AT THIS TIME; WE WERE AT LEAST 5 MI FROM THE MD88. HE BEGAN HIS TURN TOWARD THE MILL AND WE STARTED OUR TURN TO FOLLOW HIM. SINCE HE WAS HDG N; HE BEGAN HIS TURN TO THE W. WE WERE NOW IN A TURN TO THE S; HEADING FOR THE MILL. AT THIS TIME; I BELIEVE HE BEGAN TO SLOW TO HIS APCH SPD; BUT SINCE HIS DIRECTION OF FLT WAS 90 DEGS TO OURS; WE FAILED TO NOTICE HIS SLOWING. WE WERE PASSING THROUGH 220 KTS WHEN THE CAPT CALLED FOR FLAPS 15 DEGS AND GEAR DOWN. AT THIS SAME TIME; THE MD88 CALLED HE HAD AN RA AND WAS COMPLYING WITH IT AND TURNED TO THE S AND BEGAN A CLB. APCH ADVISED THAT WE WERE THE RA AND WE WERE BEHIND HIM. THEY THEN CALLED US TO VERIFY WE WERE FOLLOWING HIM AND THAT WE HAD HIM IN SIGHT. WE ADVISED THEM WE DID HAVE HIM IN SIGHT AND WERE FOLLOWING HIM. APCH THEN ASKED HIM IF HE COULD CONTINUE FOR THE ARPT FROM HIS POS AND HE REPLIED HE COULD. APCH NEVER DID SAY HOW CLOSE WE WERE; BUT IT LOOKED TO ME AND THE CAPT THAT WE WERE WITHIN 3 MI. APCH CALLED THE CAPT LATER AND HE SAID THEY HAD LISTENED TO THE TAPES AND HE SAID NOTHING ABOUT HOW CLOSE WE WERE TO THE MD88. WHAT MAY HAVE CAUSED THE RA IS THE INITIAL CLOSURE RATE AS THE MD88 SLOWED AND THE MOMENT BEFORE WE SELECTED FLAPS AND GEAR DOWN; WHICH IMMEDIATELY INCREASED OUR SLOWING. IT WAS ONLY A SECOND OR 2 BEFORE THE MD88 WAS ABLE TO ACCEPT THE CLRNC AGAIN. IT LOOKED THAT HE MAY HAVE ONLY CLBED A FEW HUNDRED FT AND SLIGHTLY PULLED AWAY ON THE SPACING. WE BOTH LANDED WITH NO FURTHER PROBS. SUPPLEMENTAL INFO FROM ACN 558848: SEVERAL FACTORS: 1) THE SLOW THAN ANTICIPATED MD88; MOST PROBABLY A NORMAL APCH SPD FOR THEM; BUT NOT KNOWING WHAT THAT IS WAS DIFFICULT TO JUDGE. 2) NO SPD CLUE FROM APCH; IE; 'YOU HAVE 50 KTS OVERTAKE ON HIM;' OR SOMETHING TO THAT EFFECT. 3) ATC CLRING BOTH ACFT OVER THE SAME FIX FOR THE APCH TO DIFFERENT RWYS. 4) MY SPD BEING 'NORMAL' FOR THAT PHASE OF THE APCH; I SHOULD HAVE SLOWED SOONER TO MAINTAIN THE IN-TRAIL SPACING. HAD THE MD88 NOT INITIATED THE RA MANEUVER; WE WOULD NOT HAVE CLOSED ON EACH OTHER BECAUSE OF MY TURN AND SLOWING TREND. THE MD88 OBVIOUSLY HAD TO RESPOND TO THE RA; HOWEVER; WE WERE NOT CLOSING ANY LONGER AT THAT POINT.