BE02 CREW INITIATES EVASIVE ACTION AFTER RECEIVING TCASI WARNING ON DEP FROM PWM.

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WE WERE ON AN IFR FLT PLAN FROM AUGUSTA; ME; TO HYANNIS; MA. WE OBTAINED IFR CLRNC FROM PORTLAND APCH CTL; INITIAL ALT 3000 FT. ON TAXI OUT TO RWY 35 AT AUGUSTA; SEVERAL BROADCASTS WERE MADE 'IN THE BLIND' ON CTAF. RPTED WX AT THE AUGUSTA ARPT WAS AS FOLLOWS: WIND 020 DEGS AT 13 KTS; 2000 FT SCATTERED; 2800 FT BROKEN WITH A LOWER CEILING N OF THE ARPT; ESTIMATED BY PLTS AT 1000 FT OVCST. IFR RELEASE WAS OBTAINED FROM PORTLAND APCH. AS WE BEGAN TO TAXI ONTO RWY 35; ANOTHER XMISSION WAS MADE ON CTAF DECLARING OUR INTENTIONS FOR DEP. ANOTHER 2 XMISSIONS WERE MADE ON CTAF AS ACFT ALIGNED WITH RWY AND DURING TKOF ROLL. THE CAPT WAS THE PF. ON CLBOUT FROM TKOF; THE ACFT ENTERED INST CONDITIONS AT 900 FT AGL. PASSING THROUGH 1200 FT AGL (ACFT PITCH ATTITUDE 11-12 DEGS) THE TCASI BECAME ACTIVE IN THE TA MODE; 'TFC; TFC.' (MANUFACTURER STATES 'IN ARPT AREAS CRITERIA FOR A TA 15- 20 SECONDS; LESS THAN OR EQUAL TO .2 MI; +/-600 FT FROM ACFT.) THE TCASI SHOWED ALT SEPARATION 0 FT -- HORIZ LESS THAN OR EQUAL TO 1/4 MI. THE PF INITIATED AN AGGRESSIVE ESCAPE MANEUVER PITCHING THE ACFT TO 25 DEGS; VERT SPD INDICATOR +4000 FPM; THE TA KEPT GOING FOR AT LEAST 20-30 SECONDS -- SOLID. AT THE TIME WE WERE IN SOLID IFR CONDITIONS; THERE IS NO POSSIBLE WAY FOR AN ACFT TO MAINTAIN VFR CONDITIONS. CHKING ON WITH PORTLAND APCH; THE CTLR SAID NOTHING ABOUT ANY TFC. WHEN ASKED BY THE CAPT; THE CTLR RELUCTANTLY SAID 'THERE WAS A VFR TARGET OFF OF THE DEP END; HE'S NOT TALKING TO ANYONE.' THE TFC WAS NEVER CALLED TO US WHEN WE OBTAINED IFR RELEASE; NOR WAS THE CONFLICTING 'VFR' ACFT ON CTAF. THE CTLR DID NOT HAVE A TAIL NUMBER. WE ALMOST HAD A CATASTROPHIC MIDAIR COLLISION BECAUSE OF AN ACFT OPERATING VFR IN IFR CONDITIONS. TCASI PROBABLY PREVENTED A TOTAL LOSS OF ACFT AND CREW.