Narrative:

Mdt X landing gso from the northeast descending from washington center south boston sector. I owned 10000 to 6000' on the south high scope. I issued a descent clearance to 6000'. I initially assigned a heading of 230 degrees for a vector to the traffic pattern. Shortly after a VFR 1200 code appeared less than 1 mi from mdt X. His mode indicated 9100', while mdt X altitude indicated 9200' descending. I issued the TCA as 12 O'clock less than 1 mi indicated 9100'. Mdt X didn't acknowledge. Mdt X next transmission was we had a close one with an small transport Y. I told mdt X that I called the traffic as soon as I saw it appear. He rogered that and continued inbound. Before the mdt X was switched to the lower sector he asked what altitude the small transport Y indicated and I said 9100'.

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Original NASA ASRS Text

Title: MDT DESCENDING FOR APCH HAD NMAC WITH UNKNOWN VFR SMT, ATC CALLED TRAFFIC AS SOON AS IT WAS OBSERVED.

Narrative: MDT X LNDG GSO FROM THE NE DSNDING FROM WASHINGTON CENTER S BOSTON SECTOR. I OWNED 10000 TO 6000' ON THE S HIGH SCOPE. I ISSUED A DSNT CLRNC TO 6000'. I INITIALLY ASSIGNED A HDG OF 230 DEGS FOR A VECTOR TO THE TFC PATTERN. SHORTLY AFTER A VFR 1200 CODE APPEARED LESS THAN 1 MI FROM MDT X. HIS MODE INDICATED 9100', WHILE MDT X ALT INDICATED 9200' DSNDING. I ISSUED THE TCA AS 12 O'CLOCK LESS THAN 1 MI INDICATED 9100'. MDT X DIDN'T ACKNOWLEDGE. MDT X NEXT XMISSION WAS WE HAD A CLOSE ONE WITH AN SMT Y. I TOLD MDT X THAT I CALLED THE TFC AS SOON AS I SAW IT APPEAR. HE ROGERED THAT AND CONTINUED INBND. BEFORE THE MDT X WAS SWITCHED TO THE LOWER SECTOR HE ASKED WHAT ALT THE SMT Y INDICATED AND I SAID 9100'.

Data retrieved from NASA's ASRS site as of July 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.