B727 AND A LIGHT TWIN HAVE A CONFLICT RESOLVED BY TCASII RA.

Narrative

WE WERE UNDER IFR CTL PROCEEDING DIRECT TO BWI; LEVEL AT 10000 FT MSL. BWI APCH INFORMED US OF TFC; 12 O'CLOCK; OPPOSITE DIRECTION CLBING VFR 'THROUGH' 9500 FT MSL. ALL 3 OF US UNDERSTOOD THE CTLR TO SAY CLBING 'TO' 9500 FT MSL. AT APPROX 6 MI; THE VFR TFC APPEARED ON OUR TCASII SCREEN 600 FT BELOW US AND CLBING. WHEN HE REACHED ABOUT 4 MI; 400 FT BELOW AND STILL CLBING; WE HEARD THE TCASII ANNOUNCE 'TFC; TFC.' AT LESS THAN 2 MI; WE STILL HAD NOT VISUALLY ACQUIRED THE TFC WHICH WAS NOW CO-ALT. THAT WAS WHEN THE TCASII COMMANDED; 'DSND; DSND.' I DISCONNECTED THE AUTOPLT AND PUSHED FORWARD AND THEN LOOKED UP TO SEE THE SMALL TWIN ENG PLANE DIRECTLY IN FRONT OF ME AT ABOUT 1 - 1 1/2 MI. I LOOKED DOWN AT THE TCASII AS I CONTINUED MY DSCNT AND SAW THAT THE PLANE WAS ABOUT 100 FT ABOVE ME. AS THE PLANE PASSED OVERHEAD; WE HAD ABOUT 600 FT OF VERT SEPARATION AND NO LATERAL SEPARATION. JUST THEN; THE CTLR WARNED US TO MAINTAIN 10000 FT AND THE TCASII RA COMMANDED US TO 'MONITOR VERT SPD.' OUR DSCNT BOTTOMED OUT AT APPROX 9200 FT MSL AND I CLBED IMMEDIATELY BACK TO 10000 FT.' WE ADVISED BWI APCH THAT WE WERE RESPONDING TO AN RA. UPON LNDG AT DCA; THERE WERE NO COMMENTS FROM THE 4 FLT ATTENDANTS OR 115+ PAX THAT INDICATED THEY NOTICED THE INCIDENT. THE CAPT CALLED A SUPVR AT BWI APCH WHO SAID THE PLT OF THE OTHER ACFT HAD US IN SIGHT AND WAS CLRED 'THROUGH' 9500 FT (VICE 'TO' 9500 FT). I BELIEVE THAT THE EVASIVE MANEUVER THAT I PERFORMED IN RESPONSE TO THE TCASII RA PREVENTED A NEAR MISS THAT WOULD HAVE BEEN WITHIN 150 FT OF THE OTHER ACFT. I DO NOT BELIEVE THAT THE PLT OF THE OTHER ACFT HAD US IN SIGHT; AND I DO NOT THINK THAT THE APCH CTLR SHOULD HAVE ALLOWED US TO GET THAT CLOSE ANYWAY.