AN AIRCARRIER PILOT REPORTS HIS AIRCRAFT PITCHING AND ROLLING IN RESPONSE TO GLIDESLOPE AND LOCALIZER FLUCTUATIONS AT 2000 FT WHILE FLYING A COUPLED ILS TO SEA RUNWAY 16L.

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WE CAPTURED THE ILS RWY 16L AND NEAR THE OUTER MARKER THE AIRPLANE BEGAN TO PITCH AND ROLL WHILE ON THE AUTOPILOT. NOW WE ARE A DOT LOW SO I DISENGAGED THE AUTOPILOT AND STABILIZED THE APCH. A WEEK EARLIER FLT TO SEA SAME RWY AUTOPILOT ON AND CAPTURED SAME EVENT. THIS IS DUE TO CROSSING TRAFFIC ON RWY 16L FOR DEP ON RWY 16C.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT THIS WAS HIS THIRD ILS RUNWAY 16L APPROACH IN WHICH HE HAS ENCOUNTERED INTERFERENCE FROM AIRCRAFT TAXIING IN THE PROTECTED AREA. THE REASON IS IF THE CEILING IS 800 FT OR ABOVE AND VISIBILITY AT 2 NM OR MORE CONTROLLERS WILL ALLOW GROUND TRAFFIC IN THE PROTECTED AREA. THE REPORTER KNOWS OF MANY OTHER PILOTS WHO HAVE ENCOUNTERED THE SAME INTERFERENCE. WHILE DEPARTING AFTER THIS EVENT; THE REPORTER HEARD ANOTHER PILOT TALKING WITH ATC ABOUT INTERFERENCE THAT CREW HAD EXPERIENCED INBOUND AND ATC HELD THAT THEY WERE WITHIN THEIR RIGHTS FOR TRAFFIC FLOW TO ALLOW AIRCRAFT IN THE PROTECTED AREA IF THE WEATHER CRITERIA WERE MET. IT IS THE REPORTER'S BELIEF THAT FOR SOME REASON INTERFERENCE TO THE SEA RUNWAY 16L ILS HAS A MORE PRONOUNCED EFFECT THAN INTERFERENCE AT SOME OTHER AIRPORTS. THE EVENT REPORTED HERE WAS THE MOST PRONOUNCED HE HAS SEEN WITH THE ALTITUDE LOSS AND ROLL MOTION.