WHILE BEING VECTORED FOR A VISUAL APCH; MLG HAS NMAC WITH SMA IN THE ARSA.

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WHILE HDG S ON THE ASSIGNED VECTOR; THE PORT COLUMBUS ARPT WAS SIGHTED BY BOTH CREW MEMBERS AT APPROX THE 10 O'CLOCK POS AND APPROX 10 MI AWAY. AT THIS TIME THE ACFT WAS FLYING AN ASSIGNED HDG OF 160-180 DEGS WHILE DSNDING TO AN ASSIGNED ALT OF 4000' MSL. APCH CTL ISSUED A TFC ADVISORY TO THE FLT OF; TO THE BEST OF MY RECOLLECTION; 'TFC 11 O'CLOCK; 3 MI; ALT UNKNOWN.' BOTH PLTS SCANNED FOR THE RPTED TFC W/O SIGHTING IT. THE LACK OF VISUAL CONTACT WITH THE RPTED TFC WAS RELAYED TO THE APCH CTL. SHORTLY THEREAFTER; THE APCH CTLR POINTED OUT THE LOCATION OF THE PORT COLUMBUS AS 10 O'CLOCK AND REPEATED THE TFC ADVISORY AS 'TFC 12 O'CLOCK LESS THAN A MILE.' BOTH PLTS AGAIN SCANNED FOR THE TFC W/O SUCCESS. THE F/O RPTED THE ARPT IN SIGHT AND NO CONTACT WITH THE RPTED TFC. A VISUAL APCH CLRNC TO RWY 10R WAS ISSUED BY APCH CTL. FLT THEN BEGAN A SHALLOW; LEFT HAND DSNDING TURN IN ORDER TO INTERCEPT THE RWY 10R FINAL APCH COURSE FOR LNDG. SHORTLY AFTER THE ACFT BEGAN ITS DSNDING LEFT HAND TURN; THE F/O SPOTTED THE ACFT. AS THE F/O EXCLAIMED THE CLOSE PROX OF THE LIGHT ACFT; CAPT INITIATED A LEFT CLBING TURN FROM APPROX 3750-3900' MSL. THE ACFT IN QUESTION WAS IDENTED BY THE F/O AS BEING AN SMA. DUE TO THE FACT THAT HE WAS FLYING THE ACFT; FOCUSING HIS ATTN TO THE LEFT OF ACFT TOWARD THE RWY; AND THE ACFT WAS IN A SHALLOW DSNDING LEFT TURN; CAPT DID NOT SEE THE SMA AT ANY TIME. ACCORDING TO F/O; WHEN HE FIRST SIGHTED THE ACFT; IT WAS SLIGHTLY FORWARD AND BELOW THE NOSE. THE SMA WAS TRAVELING IN A S WESTERLY DIRECTION. IT WAS NOT ABLE TO DETERMINE WHETHER THE SMA WAS LEVEL; CLBING; OR DSNDING DUE TO THE SHORT AMOUNT OF TIME INVOLVED WITH THIS INCIDENT. THE SMA PASSED UNDER THE RIGHT WIND OF FLT AT A DISTANCE ESTIMATED TO BE LESS THAN 300'. TWR SUPVR RELAYED SOME IMPORTANT INFO DURING HIS TELEPHONE CONVERSATION WITH CAPT: AT THE POS OF THE NEAR COLLISION; THE SMA IN QUESTION WAS WITHIN THE COLUMBUS ARSA. THE SMA WAS NOT IN RADIO CONTACT WITH ANYONE AND IN CLEAR VIOLATION OF THE RULES CONCERNING FLT WITHIN THE ARSA. THE SMA IN QUESTION HAD A TRANSPONDER AND WAS SQUAWKING CODE 1200. NO ALT RPTING INFO WAS AVAILABLE FROM THE SMA.