FLC DSNDS 400 FT BELOW ASSIGNED ALT AND TURNS OFF RTE TO AVOID C-5.

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HAVING RECEIVED A DSCNT CLRNC FROM ZBW FROM FL220 TO FL200; I RESET THE ALT REMINDER AND RECEIVED ACKNOWLEDGEMENT FROM MY COPLT. MY COPLT STARTED THE AUTOPLT ENGAGED DSCNT FROM FL220. AT THIS TIME I TOLD MY COPLT THAT I WOULD BE OFF ATC FREQ TO GET ATIS AT SWF AND ALSO TO CALL COMPANY WITH ARR MESSAGE. MY COPLT ACKNOWLEDGED. I THEN LOOKED DOWN INTO MY FLT BAG TO MY L FOR THE FREQ INFO; SHORTLY THEREAFTER; I HEARD ALT WARNING GO OFF. I LOOKED UP; IMMEDIATELY WENT BACK TO ATC FREQ AND CTLR SAID 'VERIFY YOUR ALT AT FL200.' SEEING FL196 ON MY ALTIMETER; I SAID 'GET ON YOUR ALT' AND THEN TOLD ATC THAT WE WERE 'LEVELING AT FL200.' MY COPLT TOLD ME THAT SHE HAD A TA WARNING AND THAT SHE HAD BEEN VISUALLY WATCHING A C-5A AHEAD AND TO THE R. APPARENTLY NOT COMFORTABLE WITH HIS ALT AND/OR OUR RATE OF CLOSURE; SHE HAD MADE A SLIGHT L TURN AND TOOK AN EVASIVE POSTURE IN RELATION TO THE C-5A; INCLUDING THE ALTDEV. AT THIS POINT; THE TA WAS OVER AND WE WERE WELL CLR OF THE TFC. I PERSONALLY DID NOT SEE OR HEAR THE TA. IN THAT MY HEAD WAS DOWN TO THE L AND; IN THAT THE TFC AHEAD WAS TO THE R; I COULD NOT EVALUATE MY COPLT'S EVASIVE ACTION BUT SHE SAID THAT SHE DID WHAT SHE HAD DONE EVASIVELY FOR WHAT SHE THOUGHT WAS FOR GOOD REASON. SHORTLY THEREAFTER; I TOLD THE CTLR THAT WE HAD A TFC ALERT AND HE THEN GAVE ME ZBW TELEPHONE NUMBER FOR QUALITY ASSURANCE. I WAS TOLD TO CALL DURING NORMAL BUSINESS HRS DURING THE WEEK. I COMPLIED BY SPEAKING WITH ATC REPRESENTATIVE ON OCT/XX/95. I DO BELIEVE THAT BECAUSE OF THE MASSIVE SIZE OF THE C-5A; IT WOULD APPEAR LARGER AND THUS CLOSER THAN IT REALLY WAS. SINCE VISIBILITY WAS SO GOOD AND BECAUSE MY COPLT WAS INTENTLY; VISUALLY FIXED ON THE TFC IN ORDER TO ASSURE BEING WELL-CLR; THE ALTDEV OCCURRED. WITH SAFETY IN MIND; I BELIEVE MY COPLT USED THE 'SEE AND AVOID' PRINCIPLE AND CONSIDERED THAT PARAMOUNT IN FLYING THE AIRPLANE IN ORDER TO BE SURE OF AVOIDING ANY POSSIBLE CONFLICT.