OJT INSTRUCTOR INTERVENED WHEN TRAINEE CTLR ACTIONS WERE BELIEVED INSUFFICIENT WHEN DISCONTINUING VERT SEPARATION BTWN A B737 AND A XING DA20. THE B737 AND DA20 INDIVIDUALLY REACTED TO TCASII RA'S. INSTRUCTOR ASSERTS TCASII RA'S HINDERED CTLR ACTIONS; MAKING SIT CLOSER THAN IT SHOULD HAVE BEEN.

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ON FRI; NOV/XX/97 AT XX10; A SIT DEVELOPED AND A LOSS OF STANDARD SEPARATION (5 MI HORIZ; 2000 FT VERT ABOVE FL290) OCCURRED BTWN ACR X AND CPR Y ABOUT 20 MI NW OF THE MIP VOR. I AM A RADAR INSTRUCTOR AND AT THE TIME OF OCCURRENCE I WAS GIVING TRAINING TO A DEVELOPMENTAL. CPR Y WAS SWBOUND ON J49 ABOUT 30 MI NE OF PSB VOR AT FL310. ACR X WAS 20 MI W OF THE MIP VOR FLYING THE MIP 1 ARR INTO LGA AT FL330. THE DEVELOPMENTAL ISSUED ACR X A L TURN TO 360 DEGS; A SPD OF 250 KTS; AND A DSCNT CLRNC TO FL220. ACR X READ THIS CLRNC BACK CORRECTLY. AS THE INSTRUCTOR; I INITIATED AN EXPEDITE DSCNT CLRNC THROUGH FL290 TO ACR X WHEN BOTH ACFT WERE APPROX 8-9 MI APART CONVERGING. ACR X ACKNOWLEDGED THIS CLRNC. AT THE TIME THE EXPEDITE CLRNC WAS ISSUED; ACR X DATA BLOCK SHOWED AN ALT MODE C OF FL312. (REMEMBER; THE INFO DISPLAYED IS 6- 12 SECONDS OLD; ALSO ACR X DSCNT RATE PRIOR TO THE EXPEDITE CLRNC WAS APPROX 1500 FPM.) IF I HAD DONE NOTHING AND BOTH CPR Y AND ACR X HAD NOT REACTED TO THEIR INDIVIDUAL RESOLUTIONS; THEY WOULD HAVE CROSSED WITH AT LEAST 1000-1500 FT SEPARATION. KNOWING OF TCASII SENSITIVITY AND PROX OF BOTH ACFT; I INITIATED A TA TO CPR Y FIRST; THEN A TA TO ACR X. ACR X ACKNOWLEDGED WITH REACTING TO A TCASII RESOLUTION AND WAS CLBING. CPR Y RESPONDED ALSO BY REACTING TO AN RA. I FEEL ACR X WAS ALREADY REACTING TO TCASII PRIOR TO EXPEDITE CLRNC THROUGH FL290 BUT STILL ACKNOWLEDGED THE EXPEDITE CLRNC. I ALSO FEEL THAT EVEN THOUGH STANDARD SEPARATION MAY HAVE BEEN STRAINED; TCASII HINDERED THIS SIT AND MADE IT A LOT CLOSER THAN IT SHOULD HAVE. IT IS MY UNDERSTANDING THAT ACR X FILED A NEAR MISS RPT.