Narrative:

During ILS to runway 16L sea. Aircraft was cleared to descend to 4000 ft intercept the localizer and cleared the ILS runway 16L. Autopilot was engaged. Aircraft intercepted the localizer and glideslope simultaneously at approximately 4000 ft MSL. Aircraft immediately pitched nose down perhaps 4 degrees below the horizon and accelerated; then pitched nose high perhaps 5 degrees above the horizon; then stabilized out on the normal glideslope. At first I thought we captured the glideslope a little high and the airplane did a aggressive maneuver to track then overshoot; later I realized that seattle tower had taxied a large aircraft across our landing runway and that had disrupted the glideslope beam. Just outside the outer marker (dglas) it happened again; same aircraft erratic pitching motion as before. This time I identified it as an aircraft crossing the runway and disconnected the autopilot to correct the deviation. The ILS was flown to normal minimums without further incident and a safe landing was made. As previously reported on other flights; a large aircraft taxiing across the approach end of runway 16L at sea will disrupt the glideslope signal on that ILS. Suggestions: while conducting approaches to sea runways 16L & 16R and departures on runway 16C; do not use the ILS runway 16L glideslope until all large aircraft have crossed. ATC can warn aircraft on approach of the crossing aircraft and clear approaching aircraft down to 1900 ft MSL to intercept the glideslope at that altitude. The approaching aircraft need to have the glideslope protected while within 3 miles of the dglas intersection (or glideslope intercept) and while on the glideslope. Another method that could work is use the RNAV (GPS) runway 16L rather than the ILS. At the very least the chart (ILS runway 16L sea)should outline this problem and the ATIS should also warn aircraft of this problem.

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Original NASA ASRS Text

Title: B737 First Officer experiences pitch oscillations while attempting to fly the ILS to Runway 16L at SEA with autopilot engaged. Large aircraft crossing Runway 16L was identified as possible cause as second pitch excursion began approaching DGLAS and autopilot was disengaged.

Narrative: During ILS to Runway 16L SEA. Aircraft was cleared to descend to 4000 FT intercept the localizer and cleared the ILS Runway 16L. Autopilot was engaged. Aircraft intercepted the localizer and glideslope simultaneously at approximately 4000 FT MSL. Aircraft immediately pitched nose down perhaps 4 degrees below the horizon and accelerated; then pitched nose high perhaps 5 degrees above the horizon; then stabilized out on the normal glideslope. At first I thought we captured the glideslope a little high and the airplane did a aggressive maneuver to track then overshoot; later I realized that Seattle Tower had taxied a large aircraft across our landing runway and that had disrupted the glideslope beam. Just outside the outer marker (DGLAS) it happened again; same aircraft erratic pitching motion as before. This time I identified it as an aircraft crossing the runway and disconnected the autopilot to correct the deviation. The ILS was flown to normal minimums without further incident and a safe landing was made. As previously reported on other flights; a large aircraft taxiing across the approach end of Runway 16L at SEA will disrupt the glideslope signal on that ILS. Suggestions: While conducting approaches to SEA Runways 16L & 16R and departures on Runway 16C; do not use the ILS Runway 16L glideslope until all large aircraft have crossed. ATC can warn aircraft on approach of the crossing aircraft and clear approaching aircraft down to 1900 FT MSL to intercept the glideslope at that altitude. The approaching aircraft need to have the glideslope protected while within 3 miles of the DGLAS intersection (or glideslope intercept) and while on the glideslope. Another method that could work is use the RNAV (GPS) Runway 16L rather than the ILS. At the very least the chart (ILS Runway 16L SEA)should outline this problem and the ATIS should also warn aircraft of this problem.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.