ACR MLG ALTDEV EXCURSION FROM CLRNC ALT IN RESPONSE TO TCASII RA.

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2 TCASII ALERTS (RA CLB; AND RA MONITOR DSCNT) ON APCH TO SEA. WE DID INITIALLY START A CLB OFF THE GS; BUT I DECIDED TO DISREGARD THE WARNING DUE TO BOEING FIELD TFC PATTERN. I THOUGHT THAT WAS THE CAUSE OF THE ALERT. WE LEVELED FOR A FEW MOMENTS AND THEN STARTED A DSCNT AGAIN. THE SECOND RA WENT OFF; AND I TURNED TCASII OFF. PRIOR TO THE 2 RA ALERTS; WE HAD A TA BUT THAT WAS BEHIND US. MY INITIAL BELIEF THAT THE ALERTS WERE CAUSED BY BOEING FIELD TFC; I FEEL WAS INCORRECT. ABOUT THE TIME WE PASSED PARKK; A COMMUTER LTT PASSED US UP. HE WAS VISUAL FOR 16L. HE WAS THE CAUSE OF THE ALERTS; BUT HIS CLRNC WAS TO MAINTAIN VISUAL WITH US; AND WE WERE NOT TOLD OF HIS POS; OR OF HIM PASSING US. I THINK HIS SPD WAS A GOOD 25-30 KTS FASTER THAN US. WE WERE TOLD NOT TO EXCEED 190 KTS (TO PARKK) IF APCH CTL HAD ADVISED US OF LTT TFC; WE COULD HAVE REASONED THE CAUSE OF THE RA ALERTS. TCASII IS A NEW SYS; AND SOME BUGS NEED TO BE WORKED OUT. I FEEL THAT TURNING OFF TCASII; AS I DID; IS DEFEATING THE SYS; AND REMOVING A SAFETY FACTOR; HOWEVER; IN THE APCH MODE; DOING A GAR FOR EVERY RA ALERT IS NOT THE ANSWER EITHER. WITH TIME AND EXPERIENCE. WE CAN LEARN TO USE THE TCASII TO ITS GREATEST ADVANTAGE. IN OUR PARTICULAR SITUATION THOUGH; BETTER COM WOULD HAVE (PROBABLY) SOLVED OUR CONFUSION.