ACR X HAD NMAC WITH SIGHTSEEING HELI WHILE ON APCH. VISUAL SEPARATION IN USE. SEE AND AVOID CONCEPT.

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I WAS WORKING THE LCL CTL (TWR) POS DURING THE TIME OF THE INCIDENT. TFC WAS VERY HVY AS IT WAS FOR MOST OF THE DAY. DEP DELAYS WERE IN EXCESS OF 30 MINS. ACR X WAS ABOUT 3 MI FROM THE THRESHOLD OF RWY 13 WHEN I CLRED HIM TO LAND. AT THAT MOMENT THERE WERE NO TFC TARGETS ON MY D-BRIGHT RADAR DISPLAY; AND I WAS NOT TOLD OF ANY TFC THAT MAY BE A FACTOR. AT ABOUT 2 MI FROM THE RWY X ASKED ME IF I HAD A HELI OR ANY TFC FOR HIM. LOOKING UP AT THE RADAR AGAIN I DID NOT SEE ANY TARGETS OTHER THAN X. I THEN TURNED THE CAB COORDINATOR (A SUPVR; WORKING CBA N AND S; THE ASSOCIATED HDOF AS WELL AS THE CAB COORDINATOR POS) AND ASKED HIM IF HE HAD ANYTHING OUT THERE NEAR MY FINAL? I WAS TOLD 'YEAH; A (HELI) Y; HE HAS YOU IN SIGHT.' I LOOKED BACK AT THE BRITE AND STILL SAW NO TFC TARGETS. I TOLD X OF TFC AND ADVISED HIM THAT THE HELI HAD HIM IN SIGHT. THEN ADVISED ME THAT THIS HELI CAME VERY CLOSE; AND INDICATED THAT HE SHOULD NOT HAVE BEEN THERE. AFTER X LANDED AND EXITED THE RWY HE ASKED FOR THE TWR PHONE NUMBER AS HE WAS SWITCHED TO GND CTL. X TELEPHONED THE TWR SHORTLY AFTER LNDG SAYING THAT HE PASSED OVER THE HELI BY 200 FT AND THAT HE WOULD BE FILING A NMAC RPT. SUBSEQUENT ARR ACR ASKED IF HE HAD TFC ALSO. I ASKED IF THERE WAS ANYTHING ELSE OUT THERE AND WAS TOLD NO. RADAR NOW DISPLAYED PREVIOUS HELI Y AT 800 FT; 1 1/2 MI N OF FINAL APCH COURSE AND NO FACTOR. I TOLD SECOND ACR THAT HE HAD NO TFC BELOW HIM. AMONG THE CONTRIBUTING FACTORS ARE: THE POOR QUALITY OF THE LGA'S RADAR PRESENTATION. TOO OFTEN ACFT TARGETS DO NOT ACQUIRE. TOO OFTEN RADAR TARGETS 'DROP OFF' AND ONLY APPEAR INTERMITTENTLY. TOO MANY GHOST TARGETS APPEAR AND TRAVEL ACROSS THE AIRSPACE. MANY PORTIONS OF LGA'S CLASS B AIRSPACE DO NOT DISPLAY ACFT AT LOW ALTS. THESE PROBS HAVE BEEN WITH US FOR A WHILE; AND HAVE BEEN RPTED TO THE FAA. SEVERAL UNSATISFACTORY CONDITION RPTS (UCR'S) HAVE BEEN FILED ON THIS PROB; THE MOST RECENT WAS IN JUN 1995. THE STAFFING SHORTAGES AT LGA ROUTINELY REQUIRE THE COMBINING OF SEVERAL POS. FLC IN A TIME MANNER. THE OVERUSE OF VISUAL SEPARATION; WHEN WX CONDITIONS AND OTHER FACTORS WOULD INDICATE THAT MAINTAINING VISUAL CONTACT IS NOT PRACTICAL. Y; A SIGHTSEEING HELI WAS ON A RTE THAT CIRCLES MANHATTAN; THE SPUYTEN DUYVIL RTE. PART OF THIS RTE IS TO TRAVEL NBOUND ON THE E AND HARLEM RIVERS AT 500 FT; AN AREA WHERE THE ILS APCH TO RWY 13 GS DSNDS ARRS TO BTWN 700 AND 800 FT.