Flight Crew Air Transport Pilot (ATP)
Situational Awareness
ATC Issue All Types
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification
Procedure
On approach we were level at 6;000 FT cleared for the ILS 16L. With the LOC captured the airplane also gave us GS* [glideslope acquire]. As the PF was setting Go Around Altitude; we both noticed the Glide Slope jump 1 dot above. The plane began to add power to climb to the Glide Slope. The PF selected a vertical descent of 300 to stop the Climb. The plane's peak altitude was 6;260 FT. We rearmed the glide path and proceeded on the approach. Initially we attributed this to a distance of 28 miles. Proceeding further down glide path we noticed descent rates of 200 to 1;400 FPM. We asked the Controller about any spurious glide slope reports for 16L. He initially responded that there were no reports of spurious glide path. The Controller came back a minute or so later to respond that Tower was crossing airplanes across the ILS critical area from 16L to 16C on the C taxiway. Makes sense. Suggest Tower write a NOTAM when they are doing simultaneous approaches with a scattered layer obscuring the airport advising us of a spurious glide path.
Although we had the instantaneous 'Dog looking at the TV stare.' Call outs were made by me the PM and action was taken by the PF the Captain. We were paying total attention to the situation and monitoring the approach on the final phase like we were supposed to do. Suggest Tower write a NOTAM when they are doing simultaneous approaches with a scattered layer obscuring the airport advising us of a spurious glide path.
A320 flight crew experiences an unstable glideslope during approach to Runway 16L at SEA. It is ultimately determined that the Tower is crossing aircraft to depart Runway 16C which is causing the instability.