MLG HAS NMAC WITH ANOTHER ACFT; AN SMT OR LARGER; OUTSIDE THE BOS TCA.

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FLT FROM PORTLAND; ME; WAS APPROX 20 MI OUT OF BOSTON; DSNDING THROUGH ABOUT 6400' AT 250 KTS ASSIGNED TO INTERCEPT THE ILS RWY 27 LOC WHEN I SPOTTED AN ACFT AT ABOUT THE 10:30 POS ON A COLLISION COURSE. THE ACFT HAD AN SMA TYPE OF PROFILE; BUT LOOKED BIGGER AND I JUDGED IT TO BE AT THE SAME ALT WE WERE AND AT MOST 1-2 MI AWAY. THERE WAS NO TIME TO TURN. I CLOSED THE THROTTLES IMMEDIATELY AND PUT THE PLANE INTO A DIVE AS RAPIDLY AS POSSIBLE. IT APPEARED THAT THE OTHER ACFT STARTED TO CLB AND ITS RIGHT WING WENT UP. ITS DIRECTION OF FLT WAS TOWARD THE NNE; WHILE WE WERE COMING FROM THE E AND EITHER ON THE RWY 27 LOC OR ABOUT TO INTERCEPT IT. THE WHOLE INCIDENT HAPPENED SO FAST (ABOUT 5-15 SECS) THAT THE F/O DIDN'T EVEN SEE THE OTHER ACFT UNTIL IT WAS PASSING DIRECTLY OVERHEAD. HE ESTIMATED ITS CLRNC TO BE 100-200'. ABOUT 20 SECS LATER; I REGAINED ENOUGH COMPOSURE TO ASK APCH CTL WHY THEY HAD NOT CALLED OUT THAT TFC. APCH CTL REPLIED THAT THE OTHER ACFT WAS 900' ABOVE US AND THAT SHE HADN'T SEEN IT. 'IT'S OUTSIDE THE TCA;' SHE ADDED. HER TONE OF VOICE CLEARLY IMPLIED; 'BIG DEAL! WHAT'S YOUR PROB?' OF COURSE BY THEN THE OTHER ACFT WOULD BE HIGHER THAN US; HE WAS OBVIOUSLY STILL CLBING AT MAX RATE AND WE WERE DSNDING AS SHARPLY AS WE COULD W/O TURNING THE AIRPLANE UPSIDE DOWN. IT IS DIFFICULT TO SAY WHY I HAD NOT SPOTTED THE OTHER ACFT SOONER. IT ALL HAPPENED WITH SUCH STARTLING SPD THAT I CANNOT EVEN SAY WHETHER IT FIRST APPEARED IN THE FRONT OR THE SIDE WINDOW. IT MIGHT HAVE BEEN BEHIND THE METAL FRAME SEPARATING THE 2. WE CONTINUED IN TO LAND AT BOS W/O FURTHER INCIDENT. THERE WAS NO INJURY TO PAX OR CREW; BUT THE F/A'S ALL STATED THAT THEY NOTICED THE SHARP PITCHOVER AND THAT IF THEY HADN'T BEEN STRAPPED IN THEIR SEATS; THEY WOULD HAVE COME OFF THE FLOOR. I AM SURE THEY WOULD HAVE; BUT A MIDAIR COLLISION WAS AVOIDED. THE CTLR IN THIS INSTANCE WAS BUSY AND AS SHE POINTED OUT; WE WERE OUTSIDE THE TCA. IT IS COMFORTING TO KNOW THAT SHE WAS IN NO WAY RESPONSIBLE FOR POINTING OUT VFR TFC. AS A P.S.; I WOULD LIKE TO NOTE THAT WHILE CLBING OR DSNDING; A PLT'S JUDGEMENT OF VERT SEP IS AS LEAST IMPAIRED AND MAY BE COMPLETELY ERRONEOUS. IT WOULD NOT BE ACCURATE UNTIL YOU ACTUALLY HIT. WITH THE HORIZON DISPLACED ON THE WINDSHIELD DUE TO THE DSCNT AND SOMEWHAT OBSCURED DUE TO A SLIGHT HAZE; ANY ESTIMATE OF OUR INITIAL VERT SEP COULD HAVE TAKEN A FEW SECS; AND AT THAT CLOSURE RATE; YOU DO NOT HAVE SEVERAL SECS TO INITIATE ACTION. I RECALL AN ACCIDENT SOME 30 YRS AGO WHERE AN ACFT DEPARTING NY SBND AT NIGHT BECAME INVERTED IN ITS ATTEMPT TO MISS THE APCHING LNDG LIGHTS OF ANOTHER PLANE THAT WAS ACTUALLY 1000 OR MORE FT VERT SEPARATED FROM HIM. THE ACFT CRASHED IN THE OCEAN JUST S OF NYC WITH THE LOSS OF ALL SOULS.