ACR MAKING PRACTICE CAT IIIA APCH HAS TCASII RA TO DSND. TERMINATED DSCNT AT 1500 FT.

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WX CONDITIONS WERE VFR AND VISUAL APCHS TO BOTH RWYS WERE IN PROGRESS TO RWYS 16L/R. THE PRACTICE CAT IIIA AUTOLAND APCH WAS REQUESTED BY THE CREW TO SATISFY COMPANY OPS POLICY REQUIREMENTS AND FOR CREW PROFICIENCY. APCHING FOURT INTXN ON THE OLYMPIA 1 ARR; SEATTLE APCH CTL REQUESTED WE KEEP OUR SPD UP TO OVERTAKE SLOWER AND LOWER TFC ALSO LNDG AT SEA-TAC. PASSING FOURT INTXN WE REQUESTED A PRACTICE CAT IIIA AUTOLAND APCH AT SEA. APCH CTL VECTORED OUR ACFT FOR THE ILS APCH 13 DME N OF THE ARPT; CLRED US FOR THE APCH; AND INSTRUCTED US TO CONTACT SEA TWR. ESTABLISHED ON THE ILS APCH OVER ANVIL (ISZI 11 DME); WE CONTACTED SEA TWR AND WAS CLRED TO LAND. APCHING DONDO OM; 4.3 DME FROM OUR LNDG RWY; WE RECEIVED A TA; FOLLOWED IMMEDIATELY BY AN RA TO DSND 1500-2000 FPM. IT IS IMPORTANT TO NOTE THAT THE RA WAS RECEIVED APPROX 2500 FT ASL AND GND LEVEL; BUT ONLY APPROX 2000 FT ABOVE OUR LNDG RWY AT SEA. WE IMMEDIATELY COMPLIED WITH THE RA; WHILE TRYING TO GAIN VISUAL CONTACT WITH THE INTRUDER. OUR TCASII DISPLAY WAS SO CLUTTERED WITH TARGETS IN THE TFC PATTERN AT BOEING/KING COUNTY ARPT THAT WE WERE UNABLE TO DETERMINE THE LOCATION OF THE INTRUDER ACFT. WHILE DSNDING THROUGH 1500 FT MSL THE DECISION WAS MADE TO TERMINATE COMPLIANCE WITH TCASII INSTRUCTIONS AT 1000 FT. APCHING 1000 FT AND WITH THE TCASII STILL GIVING INSTRUCTIONS TO DSND 1500-2000 FPM; THE TCASII MODE SELECTOR WAS POSITIONED TO XPONDER ON (TA AND RA MODE DESELECTED). UPON LEVELING AT 1000 FT APPEARING AT ABOUT OUR 9:30 O'CLOCK POS WAS AN ACFT. THE SAME ACFT WHICH WE HAD PASSED OVER FOURT INTXN HAD CROSSED OUR FLT PATH FROM R TO L FROM ABOVE AND BEHIND; AND AT A MUCH HIGHER RATE OF DSCNT AND FORWARD SPD. IT WAS THIS ACFT; ON A VISUAL APCH TO RWY 16L AT SEA THAT HAD GIVEN US OUR RA. IT IS MY ASSUMPTION THAT THE INTRUDER ACFT'S CREW HAD GAINED; AND WAS MAINTAINING VISUAL CONTACT WITH OUR ACFT WHILE POSITIONING THEMSELVES FOR A LNDG ON THE PARALLEL RWY. HOWEVER; BECAUSE WE WERE NOT AWARE OF THE EXPECTED FLT PATH OF THE INTRUDER ACFT WE WERE BOUND TO COMPLY WITH THE INSTRUCTIONS OF THE TCASII. IT WAS A VERY STRESSFUL SIT TO BE GIVEN INSTRUCTION TO DSND AT 1500 FPM TO AVOID A TARGET WE COULD NOT SEE; WHILE APCHING AN ALT THAT IS ONLY 1000 FT AGL AT THE ACFT'S PRESENT FLT POS; AND ONLY 500 ABOVE THE ELEVATION OF THE DEST ARPT. HAD WE BEEN ADVISED OF THE PROJECTED FLT PATH OF THE INTRUDER ACFT WE COULD HAVE DESELECTED THE RA MODE OF TCASII AND AVOIDED A STRESSFUL EXPERIENCE FOR THE PAX; WHO WERE VERY ALARMED BY THE CONSTANT VOICE OF THE TCASII TELLING US TO 'DSND; DSND; DSND;' FOR THE FOLKS IN THE TWR AT BOEING FIELD WHO MUST HAVE FOUND IT QUITE INTERESTING TO WATCH THIS AIRLINER DIVING TOWARD ITS TFC PATTERN; AND ESPECIALLY FOR THE CREW; WHO WAS STRESSED TO THE MAX WHILE COMPLYING WITH AN RA THAT WOULD NOT HAVE OCCURRED HAD MORE EFFECTIVE COMS OCCURRED. THIS WAS INDEED A SIT WHEREIN THE LEADING ACFT SHOULD HAVE BEEN INFORMED OF THE INTENTIONS OF THE FOLLOWING ACFT.