MLG OBSERVED AN SMT CROSS HIS FLT PATH IN CLOSE PROX. NO ADVISORY FROM ARTCC CTLR AND NO TIME FOR EVASIVE ACTION. NMAC REPORTED BY MLG.

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OUR FLT WAS LEVEL AT 17000' AS ASSIGNED; NAVIGATING DIRECT CMK VOR AS CLRED. NO TFC HAD BEEN ISSUED. I WAS FLYING THE ACFT FROM THE LEFT SEAT. THE F/O WAS READING BACK A CHANGE OF FREQ TO THE NEXT CENTER SECTOR. I LOOKED UP FROM A SCAN OF THE ENG GAUGES (CENTER PANEL) AND OBSERVED; THROUGH THE CENTER WINDSCREEN SECTION; AN UNIDENTIFIED ACFT PASSED ACROSS OUR FRONT LEFT TO RIGHT. I CALLED THE TFC AND THE F/O OBSERVED IT AS THE TARGET OPENED VERY RAPIDLY TO OUR RIGHT. WE DID NOT MAKE THE ASSIGNED FREQ SWITCH; INSTEAD I ASKED OUR SECTOR CTL (133.1) IF ANY TFC WAS OBSERVED OUR VICINITY. THE INITIAL RESPONSE WAS NEGATIVE TFC. I THEN DECLARED THE NEAR MISS. BOS CENTER THEN CAME BACK SAYING A 1200 CODE HAD JUST THEN APPEARED ON THE RADAR; AND TO STAY WITH THEM ON THE PRESENT FREQ WHILE THEY TOOK DOWN DETAILS. OUR VIEW OF THE OTHER ACFT WAS EXTREMELY BRIEF; MAKING IDENT DIFFICULT; SINCE IT WENT ALMOST IMMEDIATELY TO A TAIL ON ASPECT AND FALLING RAPIDLY BEHIND OUR RIGHT SIDE. OUR IMPRESSION WAS OF A MEDIUM SIZED; LIGHT COLORED LOW WING TWIN PROP. WE TOOK NO EVASIVE ACTION; THERE WAS NO TIME TO DO SO. MY ESTIMATE OF THE TARGET TRACK; 90 DEGS LEFT TO RIGHT TO OUR 140 DEG MAXIMUM ALLOWABLE COURSE; WHICH WOULD PUT IT ON A S WESTERLY HDG. MY MAIN CONCERN IS THE FAILURE OF ATC RADAR TO ADEQUATELY WARN US OF A POSSIBLE CONFLICT. I BELIEVE IT REASONABLE TO ASSUME THE ACFT HAD BEEN SHOWING 1200 ON THE TRANSPONDER PRIOR TO PASSING OUR POS. A POSSIBLE FACTOR: THE ACFT WE WERE FLYING HAD BEEN RELEASED IN ACCORDANCE TO APPROVED MEL; WITH AN INOP AUTOPLT PITCH AXIS. THIS NECESSITATED EXTRA HEAD IN THE COCKPIT TIME TO MANUALLY MAINTAIN ALT (ESPECIALLY WITH THE SWITCH PROGRAM WAITING TO VIOLATE US!). WHETHER OR NOT I WOULD HAVE SEEN THE TFC EARLIER W/O THIS SITUATION IS; OF COURSE; UNKNOWN. BUT; GIVEN THE TFC ENVIRONMENT WE HAVE TODAY; PERHAPS THIS TYPE OF MEL IS TOO LIBERAL.