Narrative:

Being vectored from south of the airport--being vectored through the final approach course (for traffic sequencing) and thus vectored to intercept the runway 10 ILS course on a heading of 120 degrees to maintain 1500' until intercepting the localizer. As our aircraft reached the heading and altitude (about simultaneously) an small transport aircraft passed immediately beneath our aircraft at our (crew) estimate of 100'. Heading of the small transport aircraft was estimated to be 90 degrees. We queried the radar controller as to whether (he) had any radar information on the second aircraft, to which he replied he did not and that his radar had dropped our own aircraft's transponder. We didn't propose a near miss on the radio--tried to telephone while on the ground, but unable to get the ATC facility on the telephone. Callback conversation with reporter revealed the following: reporter states there is an airport about 5 mi west which is used by many private and corp aircraft. The approach to second airport conflicts with sju approachs. Usually traffic is called and easily avoided. Reporter feels it was convenient to drop the 2 targets from the radar at that time. When he was finally able to make contact with approach, 4 or 5 days later, a supervisor indicated he had reviewed the tapes and the targets had indeed dropped. Said no report would be filed as there was no information to substantiate. Incident happened so fast, no time for any crew action to occur.

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Original NASA ASRS Text

Title: NMAC WITH SMT, NO TRAFFIC CALL.

Narrative: BEING VECTORED FROM S OF THE ARPT--BEING VECTORED THROUGH THE FINAL APCH COURSE (FOR TFC SEQUENCING) AND THUS VECTORED TO INTERCEPT THE RWY 10 ILS COURSE ON A HDG OF 120 DEGS TO MAINTAIN 1500' UNTIL INTERCEPTING THE LOC. AS OUR ACFT REACHED THE HDG AND ALT (ABOUT SIMULTANEOUSLY) AN SMT ACFT PASSED IMMEDIATELY BENEATH OUR ACFT AT OUR (CREW) ESTIMATE OF 100'. HDG OF THE SMT ACFT WAS ESTIMATED TO BE 90 DEGS. WE QUERIED THE RADAR CTLR AS TO WHETHER (HE) HAD ANY RADAR INFO ON THE SECOND ACFT, TO WHICH HE REPLIED HE DID NOT AND THAT HIS RADAR HAD DROPPED OUR OWN ACFT'S XPONDER. WE DIDN'T PROPOSE A NEAR MISS ON THE RADIO--TRIED TO TELEPHONE WHILE ON THE GND, BUT UNABLE TO GET THE ATC FAC ON THE TELEPHONE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR STATES THERE IS AN ARPT ABOUT 5 MI W WHICH IS USED BY MANY PVT AND CORP ACFT. THE APCH TO SECOND ARPT CONFLICTS WITH SJU APCHS. USUALLY TFC IS CALLED AND EASILY AVOIDED. RPTR FEELS IT WAS CONVENIENT TO DROP THE 2 TARGETS FROM THE RADAR AT THAT TIME. WHEN HE WAS FINALLY ABLE TO MAKE CONTACT WITH APCH, 4 OR 5 DAYS LATER, A SUPVR INDICATED HE HAD REVIEWED THE TAPES AND THE TARGETS HAD INDEED DROPPED. SAID NO RPT WOULD BE FILED AS THERE WAS NO INFO TO SUBSTANTIATE. INCIDENT HAPPENED SO FAST, NO TIME FOR ANY CREW ACTION TO OCCUR.

Data retrieved from NASA's ASRS site as of August 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.