ACR B757 RPTR EXPERIENCED A NEAR ENCOUNTER WITH A MISSED APCH BE90 NEAR SEA. CREW REACTED TO A TCASII INCIDENT AND AFTER CLBING OBSERVED TFC PASS 300 FT BELOW THEM. SEA SUPVR ADVISED THAT THE BE90 WAS A MISSED APCH AT BFI.

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THE ACFT WAS A FULLY LOADED B757; ROUTINE FLT FROM MSP TO SEA ON APR/XX/96. SOME TOWERING CUMULONIMBUS CLOUDS NECESSITATED DEVS DURING VECTORS FOR APCH AT SEA. AFTER JAKSN INTXN; WE FLEW WESTERLY (HDG APPROX 270-275 DEGS) TO AVOID BUILD-UPS. (I WAS FLYING FO.) WE VECTORED W OF LOC COURSE; THEN INTERCEPTED GS/LOC BTWN ANVIL AND FAF AND WERE CLRED FOR RWY 16R APCH. WE HAD BOEING FIELD AND SEATAC VISUALLY DURING THE APCH. THE CAPT AND I COMMENTED ON THAT FACT; AND I NOTED THE 'USUAL' TCASII DISPLAY OF TARGETS ASSOCIATED WITH BOEING AND SEA AIRSPACE. THE CAPT NOTED THERE WAS ANOTHER B757 ON FINAL TO BOEING RWY 13R (I NEVER SAW THIS ACFT DUE TO THE FACT I WAS FLYING AND INTERCEPTING FINAL; ETC). I ESTIMATE THIS ACFT WAS AT OR NEAR THE MM TO RWY 13R AS WE INTERCEPTED FINAL. AT PARKK WE WERE LOCKED UP AND ON GS. SHORTLY AFTER PARKK I DISCONNECTED AND BEGAN MANUAL FLYING; WE WERE FULLY CONFIGURED FOR LNDG; CHKLIST COMPLETE; CLRED TO LAND. I NOTICED A TARGET ON TCASII (BLUE) TURNING FROM 9 TO 10 TO 10:30 O'CLOCK; RAPIDLY; APPROX 4 1/2 MI OFF THE L. AS IT SEEMED TO BE TURNING INTO US; I LOOKED OUT THE CAPT'S #1 WINDOW; AND OBSERVED A TWIN ENG PROP IN A STEEP (AT LEAST 30 DEGS) ANGLE OF BANK TURN; COMING TOWARD US. I CALLED OUT 'HEY WHAT'S HE DOING' AND BOTH THE CAPT AND JUMP SEATER (ANOTHER COMMUTING CAPT) PICKED HIM UP IMMEDIATELY. WE IMMEDIATELY THEN GOT THE TA FOLLOWED SHORTLY BY THE RA; CLB; INCREASE CLB! WHEN I SAW HIM; I IMMEDIATELY BEGAN LEVELING OFF (DEVIATING ABOVE GS) BECAUSE I COULD TELL HE WAS GOING TO BE CLOSE; AND I KNEW FROM MANY OTHER APCHS AT SEA; THIS WAS NOT NORMAL. SO WE GOT THE TA AND RA ABOVE GS. I TOOK EVASIVE (VERT CLB) ACTION TO AVOID HIM; AND THE CAPT NOTED HIS ANGLE OF BANK INCREASING; AS HE MUST ALSO HAVE BEEN TRYING TO 'WRAP-IT- UP.' I BELIEVE I NOTED HIM PASS BELOW US ON THE TCASII AT MINUS 300 FT. AFTER THE CLR OF CONFLICT; I REESTABLISHED ACFT ON GS AND LANDED. SUBSEQUENT PHONE CALL WITH TWR AT SEA REVEALED THEY BELIEVE WE WERE AT 1400 FT AND THE BEECH 1900 WAS AT 800 FT ON THE RADAR TRACKS. MY OPINION IS WE WERE CLOSER. THE SEATAC TWR SUPVR; MR X; SAID THE BEECH 1900 WAVED OFF OF BOEING DUE TO LOSS OF SEPARATION WITH PREVIOUS LNDG TFC AT BOEING (POSSIBLY THE B757 WE NOTED AS WE INTERCEPTED RWY 16R LOC/GS). THE RADAR TAPES NEED TO BE ANALYZED; AND I AM FORMALLY REQUESTING YOU DO THAT; PLEASE. PLEASE SEND ME A COPY AND TELL ME JUST WHAT THE VERT SEPARATION WAS BTWN OUR ACFT. (MR X PROMISED US A COPY OF THE RADIO XMISSIONS BUT LATER RESCINDED THAT OFFER). I WANT TO KNOW WHAT HAPPENED. THANKFULLY WE WERE VMC AND SAW HIM EARLY; UNDER IMC (VINTAGE SEATTLE) CONDITIONS; THIS EVENT WOULD HAVE BEEN TRAGIC. I THINK; AFTER LOOKING AT THE MISSED APCH FOR BOEING RWY 13R THAT EITHER THE BEECH PLT EXECUTED AN INCORRECT MISSED APCH PROC; OR HE WAS INSTRUCTED BY BOEING TWR TO EXECUTE A PROC THAT PUT HIM IN CONFLICT WITH US. PLEASE DO YOUR BEST TO INVESTIGATE THIS MATTER. I PERSONALLY DO NOT BELIEVE THAT WE EVER WERE GOING TO COLLIDE; HOWEVER; FOR MY OWN SATISFACTION; I'D LIKE TO KNOW IT WON'T REOCCUR.