THIS ACR B727 FLC HAD A CLOSE CONFLICT WITH AN AA5 PLT WHO DSNDED WITHOUT A CLRNC IN CLASS B AIRSPACE IN AN ATTEMPT TO REMAIN IN VISUAL CONDITIONS. THE B727 FLC CREDITS THEIR TCASII EQUIP WITH PREVENTING A MIDAIR COLLISION.

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ON JUN/XA/97 WE DEPARTED LGA ON AN ACR FLT. WE WERE GIVEN THE MASPETH CLB OFF OF RWY 13. DEP CTL GAVE US A CLB TO 6000 FT. SHORTLY THEREAFTER WE WERE GIVEN A R TURN TO 360 DEGS. OUT OF 4500-5000 FT (APPROX) THE CTLR CALLED VFR TFC FOR US AT OUR 2 O'CLOCK AT 6500 FT. AS WE WERE APCHING CLOUDS THE CHANCES OF OUR SPOTTING THE TFC WERE SLIM. WHEN WE ACKNOWLEDGED THE TFC CALL; WE INFORMED THE CTLR THAT WE WERE ABOUT TO ENTER THE CLOUDS. AT ABOUT THIS TIME; WE RECEIVED A TCASII RA TO MONITOR VERT SPD. I FOLLOWED THE TCASII RA WHILE THE FO AND FE ATTEMPTED TO ACQUIRE THE TFC VISUALLY. WE OBTAINED VISUAL WHEN THE VFR ACFT POPPED OUT OF THE CLOUDS AT ABOUT 1-2 O'CLOCK POS; AND ABOUT 1 MI HORIZ. THE VFR ACFT DSNDED AS IT PASSED CLOSE BEHIND US (APPROX 1/2 MI) WITHOUT CLRNC IN ORDER 'TO MAINTAIN VFR' AS THE PLT RPTED. THE CTLR THEN ADMONISHED THE PLT NOT TO DSND WITHOUT CLRNC IN CLASS B AIRSPACE. THE CTLR STATED TO THE VFR PLT; 'THAT COULD HAVE BEEN REALLY BAD.' WE ARE CONVINCED THAT WITHOUT TCASII WE WOULD HAVE HAD A MIDAIR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR IS AN ACR CAPT OF A B727-200. HE SAID THAT AFTER THE FLT HE CALLED N90 AND TALKED TO A SUPVR WHO ALLEGEDLY TOLD HIM THAT THE AA5 PLT WAS LOST ON BOTH XPONDER AND SKIN PAINT SHORTLY AFTER THE INCIDENT ONLY TO REAPPEAR NEAR EWR AND AGAIN WAS LOST WHEN QUESTIONED ABOUT HIS UNAUTH RTE OF FLT AND CHANGE OF ALT. THE SUPVR INDICATED THAT THE FAA WAS INTERESTED IN THAT PLT'S ACTIVITIES. THE CAPT WAS IMPRESSED WITH THE TCASII'S REACTION AS THE SIT DEVELOPED IN THAT IT INITIALLY ADVISED THAT THE FLC LEVELOFF WHEN THE AA5 WAS DSNDING AND THEN ADVISED A CLB WHEN THE AA5 CONTINUED TO DSND TOWARD THE B727'S ALT. HE WAS APPRECIATIVE THAT TCASII WORKED AS DESIGNED.