Narrative:

Air carrier X climbing wbound out of 12000 ft for 17000 ft approximately 15 mi southwest of dulles international airport, reported an aircraft pass under him approximately 300 ft. I observed no traffic. 10 seconds later I observed a 'primary target only' sbound. I advised air carrier X that I was tracking a target 5 mi south of his position, with no transponder or altitude squawk, and not in contact with this aircraft. He gave me a full description of the aircraft in question and I passed on the information appropriately.

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Original NASA ASRS Text

Title: ACR X HAD NMAC WITH NON XPONDER ACFT WITHIN 30 MI VEIL OF PRIMARY TCA ARPT. SEE AND AVOID CONCEPT.

Narrative: ACR X CLBING WBOUND OUT OF 12000 FT FOR 17000 FT APPROX 15 MI SW OF DULLES INTL ARPT, RPTED AN ACFT PASS UNDER HIM APPROX 300 FT. I OBSERVED NO TFC. 10 SECONDS LATER I OBSERVED A 'PRIMARY TARGET ONLY' SBOUND. I ADVISED ACR X THAT I WAS TRACKING A TARGET 5 MI S OF HIS POS, WITH NO XPONDER OR ALT SQUAWK, AND NOT IN CONTACT WITH THIS ACFT. HE GAVE ME A FULL DESCRIPTION OF THE ACFT IN QUESTION AND I PASSED ON THE INFO APPROPRIATELY.

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.