Narrative:

While on approach to pdx while cleared for the visual to runway 28L; approach control notified us of traffic ahead at an unknown altitude; a primary target only on approachs radar. We were in a gradual descent approximately 12-15 mi southeast of the field on a 015 degree heading. I was looking out the left side window while the first officer was looking out the right. At about 4500 ft MSL the first officer jerked the yoke. When I looked over at him; he said we almost hit an aircraft and he instinctively tried to maneuver away. I asked him what it was and how far. He indicated that it was very close; about 50 ft. I told ATC that we almost hit another aircraft. They indicated that they had no altitude on it; primary radar return only. We received no further information than that from ATC. We continued the visual approach into pdx and landed uneventfully. The first officer indicated that the aircraft appeared to be a high wing red and white cessna heading in the same direction. All parties were following established rules and procedures. Maybe approach could have and maybe we should have asked for vectors away from their target. Being a primary target only; that according to ATC sometime shows and sometimes doesn't; it is hard to know exactly where VFR traffic is and what it is doing; especially when the aircraft is not squawking. Further discussion on the ground with ATC indicated that this airspace is very busy and that an upgrade to a class B would be appropriate; something they seemed to be requesting for a long time. Also; discussion with our ATC representative confirmed the frequency of these types of events. This is apparently a high risk airspace and increased vigilance is required. Notification to all company crews through a rbf and notes on the 10-7 page should be accomplished if these are indeed regular occurrences in this airspace. Supplemental information from acn 802609: while on approach into portland; the approach controller called traffic north of our position with altitude unknown. We were searching visually for the target and there was nothing showing on TCAS. About 15 mi south of runway 28L descending through approximately 4500 ft; the aircraft comes into view slightly above and to the right. I estimate the other aircraft to have passed about 50 ft above and 100 ft to the right. The event happened so quickly that I was only able to turn slightly left and the autoplt stayed engaged. The aircraft was a single engine high wing with fixed gear and traveling in the same direction. The only threat is that an aircraft can fly that close to a busy airport with no transponder or radio communication. The information we received from pdx approach was that this is a high threat area and we would all benefit if the airspace was class B instead of class C.

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Original NASA ASRS Text

Title: A B737 EXPERIENCED A NMAC WITH A SMALL AIRCRAFT ON APPROACH TO PDX.

Narrative: WHILE ON APCH TO PDX WHILE CLRED FOR THE VISUAL TO RWY 28L; APCH CTL NOTIFIED US OF TFC AHEAD AT AN UNKNOWN ALT; A PRIMARY TARGET ONLY ON APCHS RADAR. WE WERE IN A GRADUAL DSCNT APPROX 12-15 MI SE OF THE FIELD ON A 015 DEG HDG. I WAS LOOKING OUT THE L SIDE WINDOW WHILE THE FO WAS LOOKING OUT THE R. AT ABOUT 4500 FT MSL THE FO JERKED THE YOKE. WHEN I LOOKED OVER AT HIM; HE SAID WE ALMOST HIT AN ACFT AND HE INSTINCTIVELY TRIED TO MANEUVER AWAY. I ASKED HIM WHAT IT WAS AND HOW FAR. HE INDICATED THAT IT WAS VERY CLOSE; ABOUT 50 FT. I TOLD ATC THAT WE ALMOST HIT ANOTHER ACFT. THEY INDICATED THAT THEY HAD NO ALT ON IT; PRIMARY RADAR RETURN ONLY. WE RECEIVED NO FURTHER INFO THAN THAT FROM ATC. WE CONTINUED THE VISUAL APCH INTO PDX AND LANDED UNEVENTFULLY. THE FO INDICATED THAT THE ACFT APPEARED TO BE A HIGH WING RED AND WHITE CESSNA HDG IN THE SAME DIRECTION. ALL PARTIES WERE FOLLOWING ESTABLISHED RULES AND PROCS. MAYBE APCH COULD HAVE AND MAYBE WE SHOULD HAVE ASKED FOR VECTORS AWAY FROM THEIR TARGET. BEING A PRIMARY TARGET ONLY; THAT ACCORDING TO ATC SOMETIME SHOWS AND SOMETIMES DOESN'T; IT IS HARD TO KNOW EXACTLY WHERE VFR TFC IS AND WHAT IT IS DOING; ESPECIALLY WHEN THE ACFT IS NOT SQUAWKING. FURTHER DISCUSSION ON THE GND WITH ATC INDICATED THAT THIS AIRSPACE IS VERY BUSY AND THAT AN UPGRADE TO A CLASS B WOULD BE APPROPRIATE; SOMETHING THEY SEEMED TO BE REQUESTING FOR A LONG TIME. ALSO; DISCUSSION WITH OUR ATC REPRESENTATIVE CONFIRMED THE FREQ OF THESE TYPES OF EVENTS. THIS IS APPARENTLY A HIGH RISK AIRSPACE AND INCREASED VIGILANCE IS REQUIRED. NOTIFICATION TO ALL COMPANY CREWS THROUGH A RBF AND NOTES ON THE 10-7 PAGE SHOULD BE ACCOMPLISHED IF THESE ARE INDEED REGULAR OCCURRENCES IN THIS AIRSPACE. SUPPLEMENTAL INFO FROM ACN 802609: WHILE ON APCH INTO PORTLAND; THE APCH CTLR CALLED TFC N OF OUR POS WITH ALT UNKNOWN. WE WERE SEARCHING VISUALLY FOR THE TARGET AND THERE WAS NOTHING SHOWING ON TCAS. ABOUT 15 MI S OF RWY 28L DSNDING THROUGH APPROX 4500 FT; THE ACFT COMES INTO VIEW SLIGHTLY ABOVE AND TO THE R. I ESTIMATE THE OTHER ACFT TO HAVE PASSED ABOUT 50 FT ABOVE AND 100 FT TO THE R. THE EVENT HAPPENED SO QUICKLY THAT I WAS ONLY ABLE TO TURN SLIGHTLY L AND THE AUTOPLT STAYED ENGAGED. THE ACFT WAS A SINGLE ENG HIGH WING WITH FIXED GEAR AND TRAVELING IN THE SAME DIRECTION. THE ONLY THREAT IS THAT AN ACFT CAN FLY THAT CLOSE TO A BUSY ARPT WITH NO XPONDER OR RADIO COM. THE INFO WE RECEIVED FROM PDX APCH WAS THAT THIS IS A HIGH THREAT AREA AND WE WOULD ALL BENEFIT IF THE AIRSPACE WAS CLASS B INSTEAD OF CLASS C.

Data retrieved from NASA's ASRS site as of May 2009 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.