AN ACR CAPT RPTS A NMAC DURING CLB FROM PHL. TCASII ALERTED THE CREW BEFORE ATC ISSUED TFC. ACFT CAME WITHIN 50-100 FT VERTLY AND ZERO HORIZLY.

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WHILE ON A ROUTINE FLT FROM PHL TO MHT I ENCOUNTERED A 'NEAR HIT' WITH A KING AIR; ACFT Y. OUR ACFT (ACR Z) WAS ON AN EASTERLY HDG (090 DEGS) ESTABLISHED ON THE COYLE 274 DEG RADIAL TRACKING TO THE E. I; THE CAPT; WAS DOING THE COMS WORK; WHILE THE FO WAS FLYING THE ACFT WITH THE AUTOPLT ENGAGED. WE WERE LEVEL AT 5000 FT MSL. ACR Z WAS GIVEN A CLB CLRNC FROM 5000 FT MSL TO 10000 FT MSL. AT THAT TIME; A DISPLAY OF A 'YELLOW' WARNING ON OUR RADAR TCASII WAS SHOWN INDICATING A PERPETRATOR IN THE AREA. I QUESTIONED THE ATC AND WAS ISSUED A TA 'TFC 10 O'CLOCK; 3 MI.' WE WERE ALSO GIVEN A R TURN TO 130 DEGS. MY MIND CANNOT RECALL IF IT WAS WITH THE INITIAL CALL OR A SUBSEQUENT ISSUANCE IN REGARD TO THIS TURN. AT ANY RATE; MANY EVENTS WERE ABOUT TO TAKE PLACE IN THE NEXT 7-10 SECONDS. THE FO WAS ROTATING THE ACFT NOSE UP AND STARTING A R TURN. THE TCASII WARNING HAD CHANGED FROM YELLOW TO RED. OUR ALT WAS ESTIMATED AT 5050 FT MSL WITH TCASII RED INTRUDER PLUS 200 FT. TCASII WAS ADVISING 'DSND' VOCALLY; RESOLVE AT 2000 FT RED DOTS ON VSI. AT THAT TIME BEING UNABLE TO LOCATE ACFT VISUALLY I MOVED FORWARD IN SEAT. THE INTRUDER HAD BEEN VISUALLY BLOCKED BY THE L SIDE FORWARD WINDSCREEN POST. FO HAD BEEN RESPONDING TO ALL TCASII ADVISORY IMMEDIATELY. THE MASS; INERTIA; AND VELOCITY OF OUR ACFT WERE BEING TRANSFERRED TO A DSND MODE FROM ASCENSION MODE. I; CAPT TOOK PHYSICAL CTL OF ACFT BECAUSE I HAD MADE VISUAL CONTACT WITH THE CONFLICT TFC WHICH EMERGED FROM THE FORWARD WINDSCREEN POST. TCASII CLRNC WAS AT 'ZERO FT' AND A SINGULAR 'RED' DIAMOND ON THE RADAR SCREEN AT THAT POINT. IT WAS MY INTENTION TO EVADE THE INTRUDER BY CORRECTING BEHIND AND BELOW HIM. NO OTHER ESCAPE WAS PLAUSIBLE. I ROLLED THE ACFT TO THE L AND CONTINUED NOSE DOWN EFFORT ALREADY ESTABLISHED BY FO. I BELIEVE WE WERE AT THE TRANSITIONAL ARC IN FLT AT THIS TIME. THE NEXT MOMENT WAS MOST TERRIFYING TO ME; AS I SAW THE 'KING AIR' BELLY IN THE FORWARD WINDOW ALONG WITH HEARING THE 'DOPPLER EFFECT' CREATED FROM THE 'KING AIR' ENGS. AT THAT POINT I FELT HE WOULD SURELY HIT OUR R WING WHICH WAS UP FOR THE L DSNDING TURN. IT IS MY BELIEF I USED 30 DEG L BANK IN THE EVASIVE MANEUVER WITH 10-15 DEGS NOSE DOWN. AT 4900 FT MSL I ROLLED LEVEL. FROM THE TIME I SAW THE 'KING AIR' UNTIL WE HAD MISSED IT WAS ESTIMATED 3-4 SECONDS. THIS EVENT LEFT ME NOT ONLY UNNERVED BUT PRACTICALLY SPEECHLESS. WHEN I WAS ABLE TO SPEAK I ASKED THE CTLR IF HE REALIZED WE JUST ABOUT HIT A KING AIR. WE THEN CLBED TO 10000 FT MSL AND CONTINUED OUR FLT TO MHT. UPON REACHING MHT I CALLED ATC AND TALKED ABOUT THE INCIDENT. I ALSO NOTIFIED THE CHIEF PLT'S OFFICE AS WELL AS DISPATCH. UPON RETURNING TO PHL; ATC TWR WISHED TO SPEAK TO ME AND HE WAS APPRISED OF THE SIT.