Narrative:

I was the pilot flying. Captain was the pilot monitoring. Cleared ILS approach to runway 16L at sea. Weather was changing rapidly as rain showers moved through the area. Altitude was between 4;000 and 5;000 MSL between karfo and kenmo. During this phase of the approach weather was solid IMC with light rain. Winds aloft were generally off the nose between 45 and 50 KTS. Autopilots a and B were engaged along with autothrottles. Localizer and GS were captured. Command speed was 150 KTS (assigned). Configuration was gear down with 15 degrees of flaps. Approach was stable prior to the event. Aircraft suddenly pitched up towards 20 degrees nose high. Power came in appropriately as speed started to slow and was near max continuous. Flight director commands were appropriate to maintain the GS and localizer courses. After the pitch up; the aircraft pitched down toward 10 degrees nose low to maintain the GS. Power reduced to idle to maintain commanded speed. Another up/down oscillation occurred (staying right on GS and localizer) and then autopilot was disconnected and the approach flown manually to landing. Remainder of approach and landing was uneventful. Throughout the event the autopilot attempted to remain on the indicated gp and course and was successful in doing so. However; the pitch and power adjustments required were excessive and uncomfortable. It was like being on a roller coaster. My first thought was windshear. However; no windshear was reported on the ATIS or by ATC. There were no on board windshear alerts. The wind did not suddenly shift speed or direction. The only other theory I have is that the ILS GS signal caused the significant vertical path deviations. After the event; the captain queried ATC if the ILS critical area was protected. The response was yes. ATC said that perhaps the preceding aircraft caused the anomaly. (We were 5 miles behind the preceding jet). He asked if there were any reports of problems with the GS and the answer was no. We reported a suspected problem with the GS and an air carrier jet (behind us I believe) said he was also noticing a problem. After roll out; tower asked for a PIREP on turbulence on final. I told them the final was smooth but the GS inside of 15 NM was unstable and oscillating and I would not recommend an ap coupled approach. The next day when we departed sea; we saw the following NOTAM: sea 12/202 sea runway 16L ILS gp possible fluctuations flight attendant when weather is 800/2 or better/prior to dglas when weather is less than 800/2 this NOTAM may have been there on the day of the flight but I didn't remember seeing it. (I could not decode the 'flight attendant' in the above NOTAM and I found the entire NOTAM a bit confusing). Apparently sea is aware of a problem with the glidepath. What I don't know is if they are aware of how drastic the fluctuations are. The fluctuations were so severe; that ap coupled approaches should not be made until the problem is fixed. In fact; no ILS approaches should be made in IMC conditions. In my 35 years of flying; I have never experienced a glidepath oscillation of this magnitude. The fact that we were in solid IMC made it even more disconcerting. Sea ATC should immediately suspend ILS approaches to runway 16L in sea until a thorough inspection of the ILS can be accomplished. I suggest a new flight inspection of the approach by the FAA.

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Original NASA ASRS Text

Title: B737 First Officer experienced violent pitch oscillations during a coupled ILS approach to Runway 16L at SEA. After two cycles the autopilot was disconnected and the approach flown manually to a normal landing.

Narrative: I was the pilot flying. Captain was the pilot monitoring. Cleared ILS approach to Runway 16L at SEA. Weather was changing rapidly as rain showers moved through the area. Altitude was between 4;000 and 5;000 MSL between KARFO and KENMO. During this phase of the approach weather was solid IMC with light rain. Winds aloft were generally off the nose between 45 and 50 KTS. Autopilots A and B were engaged along with autothrottles. LOC and GS were captured. Command speed was 150 KTS (assigned). Configuration was gear down with 15 degrees of flaps. Approach was stable prior to the event. Aircraft suddenly pitched up towards 20 degrees nose high. Power came in appropriately as speed started to slow and was near max continuous. Flight director commands were appropriate to maintain the GS and LOC courses. After the pitch up; the aircraft pitched down toward 10 degrees nose low to maintain the GS. Power reduced to idle to maintain commanded speed. Another up/down oscillation occurred (staying right on GS and LOC) and then autopilot was disconnected and the approach flown manually to landing. Remainder of approach and landing was uneventful. Throughout the event the Autopilot attempted to remain on the indicated GP and course and was successful in doing so. However; the pitch and power adjustments required were excessive and uncomfortable. It was like being on a roller coaster. My first thought was windshear. However; no windshear was reported on the ATIS or by ATC. There were no on board windshear alerts. The wind did not suddenly shift speed or direction. The only other theory I have is that the ILS GS signal caused the significant vertical path deviations. After the event; the Captain queried ATC if the ILS critical area was protected. The response was yes. ATC said that perhaps the preceding aircraft caused the anomaly. (We were 5 miles behind the preceding jet). He asked if there were any reports of problems with the GS and the answer was no. We reported a suspected problem with the GS and an air carrier jet (behind us I believe) said he was also noticing a problem. After roll out; tower asked for a PIREP on turbulence on final. I told them the final was smooth but the GS inside of 15 NM was unstable and oscillating and I would not recommend an AP coupled approach. The next day when we departed SEA; we saw the following NOTAM: SEA 12/202 SEA RWY 16L ILS GP POSSIBLE FLUCTUATIONS FA WHEN WEATHER IS 800/2 OR BETTER/PRIOR TO DGLAS WHEN WEATHER IS LESS THAN 800/2 This NOTAM may have been there on the day of the flight but I didn't remember seeing it. (I could not decode the 'FA' in the above NOTAM and I found the entire NOTAM a bit confusing). Apparently SEA is aware of a problem with the glidepath. What I don't know is if they are aware of how drastic the fluctuations are. The fluctuations were so severe; that AP coupled approaches should not be made until the problem is fixed. In fact; no ILS approaches should be made in IMC conditions. In my 35 years of flying; I have never experienced a glidepath oscillation of this magnitude. The fact that we were in solid IMC made it even more disconcerting. SEA ATC should immediately suspend ILS approaches to Runway 16L in SEA until a thorough inspection of the ILS can be accomplished. I suggest a new flight inspection of the approach by the FAA.

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.