Narrative:

My aircraft was being vectored for sequencing into the downwind leg for runway 20 at hvn. Ny approach control cleared my aircraft to maintain 4000 ft on a heading of 360 degree(?) for sequencing behind an air carrier medium large transport which was west of the airport (at 3000 ft). Both crew looking for visual sighting to the medium large transport which should have been in our 9-11 O'clock position. I made a visual scan to my right and was surprised to see an small aircraft at my 2 O'clock position approximately 1/4 mi away on an apparent collision course. His course was approximately due west. The small aircraft was exactly at my altitude of 4000 ft. I called the traffic to my copilot and said something to the effect that the small aircraft could be a problem. After approximately 2 seconds of observation of the small aircraft I determined that there was no relative movement of the small aircraft in our windscreen and that our 2 aircraft were on a collision course. I told my copilot to tell approach control we were taking evasive action, and I initiated a rapid descent to pass below the small aircraft. Apparently the transmission to approach control was blocked momentarily by other radio traffic. During the next break in chatter, the controller called us and told us to maintain 4000 ft, possibly she had seen our target deviate from our assigned altitude. We then informed her that we had taken evasive action to avoid a midair collision. She informed us that she had not observed any other traffic and commented that the unknown aircraft was probably not transponder equipped. Our aircraft went momentarily to 3750 ft MSL for less than 10 seconds while the small aircraft passed overhead and slightly aft. We then climbed back to our assigned altitude. There was no conflict with the medium large transport.

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Original NASA ASRS Text

Title: CORP ACFT HAS NMAC WITH SMA WHILE ON SEQUENCE TO APCH. NO TA GIVEN OF SMA.

Narrative: MY ACFT WAS BEING VECTORED FOR SEQUENCING INTO THE DOWNWIND LEG FOR RWY 20 AT HVN. NY APCH CTL CLRED MY ACFT TO MAINTAIN 4000 FT ON A HDG OF 360 DEG(?) FOR SEQUENCING BEHIND AN ACR MLG WHICH WAS W OF THE ARPT (AT 3000 FT). BOTH CREW LOOKING FOR VISUAL SIGHTING TO THE MLG WHICH SHOULD HAVE BEEN IN OUR 9-11 O'CLOCK POS. I MADE A VISUAL SCAN TO MY R AND WAS SURPRISED TO SEE AN SMA AT MY 2 O'CLOCK POS APPROX 1/4 MI AWAY ON AN APPARENT COLLISION COURSE. HIS COURSE WAS APPROX DUE W. THE SMA WAS EXACTLY AT MY ALT OF 4000 FT. I CALLED THE TFC TO MY COPLT AND SAID SOMETHING TO THE EFFECT THAT THE SMA COULD BE A PROBLEM. AFTER APPROX 2 SECONDS OF OBSERVATION OF THE SMA I DETERMINED THAT THERE WAS NO RELATIVE MOVEMENT OF THE SMA IN OUR WINDSCREEN AND THAT OUR 2 ACFT WERE ON A COLLISION COURSE. I TOLD MY COPLT TO TELL APCH CTL WE WERE TAKING EVASIVE ACTION, AND I INITIATED A RAPID DSCNT TO PASS BELOW THE SMA. APPARENTLY THE XMISSION TO APCH CTL WAS BLOCKED MOMENTARILY BY OTHER RADIO TFC. DURING THE NEXT BREAK IN CHATTER, THE CTLR CALLED US AND TOLD US TO MAINTAIN 4000 FT, POSSIBLY SHE HAD SEEN OUR TARGET DEVIATE FROM OUR ASSIGNED ALT. WE THEN INFORMED HER THAT WE HAD TAKEN EVASIVE ACTION TO AVOID A MIDAIR COLLISION. SHE INFORMED US THAT SHE HAD NOT OBSERVED ANY OTHER TFC AND COMMENTED THAT THE UNKNOWN ACFT WAS PROBABLY NOT TRANSPONDER EQUIPPED. OUR ACFT WENT MOMENTARILY TO 3750 FT MSL FOR LESS THAN 10 SECONDS WHILE THE SMA PASSED OVERHEAD AND SLIGHTLY AFT. WE THEN CLBED BACK TO OUR ASSIGNED ALT. THERE WAS NO CONFLICT WITH THE MLG.

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.