LTT GIVEN TRAFFIC ADVISORY ON UNK-SMA; TRAFFIC SIGHTED AND EVASIVE ACTION TAKEN; NMAC REPORT RILED WITH APCH CTL.

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NEAR MISS WAS ENCOUNTERED 29 MI S OF THE ALLENTOWN VOR (117.5) ON THE 147 DEG R INBND AT AN ALT OF 4000'. WE JUST HAD LEVELED OFF AND HAD FINISHED OUR CRUISE CHKS; WHEN PHL DEP CTL; ON FREQ 123.8; NOTIFIED US ABOUT TFC IN OUR 10 O'CLOCK POS WITH NO ALT READ OUT. I WAS FLYING THE AIRPLANE AND AT THE MOMENT I LOOKED FOR THE TFC AND SAW A WHITE LIGHT SINGLE ENG AIRPLANE (PROBABLY AN SMA Y) WITHIN 1/2 MI AT THE SAME ALT ON A COLLISION COURSE. INSTANTLY I PULLED UP AND THE LIGHT AIRPLANE WENT DIRECTLY UNDER US AT A HDG OF 060 DEGS W/O CHANGING COURSE OR ALT. WE NOTIFIED THE RADAR CTLR ABOUT THE NEAR MISS AND TOLD HIM THAT WE WERE GOING TO FILE A RPT. WE CHKED WITH OUR F/A TO SEE IF SHE WAS ALRIGHT AND SHE RPTED TO US THAT SHE WAS STRAPPED IN AND THAT THE ABRUPT MANEUVER FELT LIKE TURB IN THE CABIN AND THAT OUR PAX DIDN'T SEEM TO BE CONCERNED. I DECIDED FOR THAT REASON NOT TO INFORM THE PAX ABOUT THIS NEAR MISS FOR SOME LOGICAL AND PSYCHOLOGICAL REASONS. IN ALLENTOWN WE CONTACTED THE PHL RADAR ROOM AND RPTED THE NEAR MISS TO THE RADAR FLOOR SUPVR; WHO WAS ALREADY NOTIFIED BY THE CTLR WHO WAS WORKING OUR FLT. HE TOOK ALL THE INFO AND ADVISED US THAT THEY ALREADY TRIED TO TRACK DOWN THE RESPECTIVE AIRPLANE IN COOPERATION WITH NY RADAR. SOLUTION: EQUIP ALL AIRPLANES WITH TRANSPONDER WITH MODE C READOUTS AND MAKE IT MANDATORY TO HAVE THEM TURNED ON AT ALL TIMES. ESTABLISH ARWY ROUTES FOR IFR TFC ONLY (SEE EUROPEAN AIRSPACE STRUCTURE).