CLOSE PROX ACR-MLG GA-SMA AT THE EDGE OF DCA TCA.

Narrative

WHILE ON AN ATC VECTOR TRANSITIONING FROM ARMEL VOR TO THE 10 DME FIX FOR THE RIVER APCH AT DCA; WASH APCH CALLED VFR TFC AT 2 O'CLOCK AND 5 MI; ALT UNKNOWN (THIS WAS THE ONLY ADVISORY WE RECEIVED). THE F/O AND I LOOKED INTENSIVELY SINCE VIS WAS GOOD AND WE BOTH THOUGHT WE SHOULD BE ABLE TO FIND HIM. THE F/O LOOKED DOWN TO CHANGE NAV FREQS AND I REACHED UP TO CHK THAT THE LNDG LIGHTS WERE ALL ON (THEY WERE). I NOTICED MOVEMENT IN THE WINDSHIELD AND AN SMA (WHITE WITH A RED STRIPE) FILLED IT. THE ACFT WAS SO CLOSE AND AT OUR ALT (5500') THAT I DIDN'T BANK THINKING HE WOULD HIT OUR WINGTIP. I PUSHED SLIGHTLY FORWARD ON THE WHEEL NOT WANTING TO RAISE MY RUDDER INTO HIM WITH ABRUPT MOVEMENT. I WOULD ESTIMATE VERT SEP AT NO MORE THAN 100' AND HE PASSED RIGHT OVER US. WE CAME SO CLOSE I ACTUALLY HEARD HIS ENG AS WE PASSED. WE ADVISED ATC OF A NEAR MISS AND CONTINUED A NORMAL APCH AND LNDG AT DCA. I DON'T THINK THE PLT EVER SAW US! I TURNED TO GIVE HIS POS TO ATC AND HE WAS STILL ON COURSE AND ALT. I BELIEVE HE WAS SIGHTSEEING JUST OUTSIDE OF THE TCA AND ALL EYES WERE ON DOWNTOWN WASHINGTON. THIS IS A PERFECT CASE FOR MANDATORY MODE C REQUIREMENTS IN SMA. HAD ATC RPTED HIS ALT TO ME THIS NEAR MISS WOULD NOT HAVE HAPPENED. SUPPLEMENTAL INFO FROM ACN 83316: AT THE TIME WE WERE IN A GRADUAL DSCNT FOR OUR APCH INTO DCA. NO SUBSTANTIAL EVASIVE ACTION WAS TAKEN; OTHER THAN A SLIGHT LOWERING OF THE NOSE; AS THE CAPT FELT THAT TO BANK THE ACFT OR MAKE TOO RADICAL A NOSE DOWN PITCH MIGHT INCREASE THE CHANCE OF A COLLISION DUE TO THE PROX OF THE OTHER ACFT.