A320 flight crew experiences an unstable glideslope during approach to runway 16L at sea. It is ulti...

Narrative:

On approach we were level at 6;000 ft cleared for the ILS 16L. With the localizer 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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	1112499
Time
Date	201308
Local Time Of Day	0601-1200
Place
Locale Reference	SEA.Airport
State Reference	WA
Environment
Flight Conditions	Marginal
Aircraft 1
Make Model Name	A320
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Not Flying First Officer
Events
Anomaly	ATC Issue All Types

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.