Air carrier landing bwi experienced glideslope oscillations while on the ILS runway 10 approach. The...

Narrative:

This is the third write up for this particular issue. I have seen this issue almost consistently over the last nine years. The glide slope for ILS runway 10 at bwi oscillates up and down. It is very noticeable very far out (more than 20 miles). As you get closer the total deviation is smaller (approximately one 'dot' amplitude cycle every one second at around 12 miles). However; even lined up with the localizer; the deviations are still apparent and the flight director's command nose up and down movements in sync with the glide path. Had we had the autopilot on; it would have moved the nose up and down and probably would have kicked the autopilot off. It's as if there is a harmonic effect from interference; or an installation problem; or maybe something emitting near the airport that affects the glide slope. My big concern is that this approach is the only CAT ii/III approach at bwi; and is frankly unusable as a CAT I ILS. I certainly don't trust it. Bwi used runway 33L ILS approaches the vast majority of the time.

Original NASA ASRS Text

Title: Air Carrier landing BWI experienced glideslope oscillations while on the ILS Runway 10 approach. The reporter noted this anomaly had been experienced on a number of other occasions.

Narrative: This is the third write up for this particular issue. I have seen this issue almost consistently over the last nine years. The glide slope for ILS Runway 10 at BWI oscillates up and down. It is very noticeable very far out (more than 20 miles). As you get closer the total deviation is smaller (approximately one 'dot' amplitude cycle every one second at around 12 miles). However; even lined up with the localizer; the deviations are still apparent and the flight director's command nose up and down movements in sync with the glide path. Had we had the autopilot on; it would have moved the nose up and down and probably would have kicked the autopilot off. It's as if there is a harmonic effect from interference; or an installation problem; or maybe something emitting near the airport that affects the glide slope. My big concern is that this approach is the only CAT II/III approach at BWI; and is frankly unusable as a CAT I ILS. I certainly don't trust it. BWI used Runway 33L ILS approaches the vast majority of the time.

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	1092253
Time
Date	201306
Local Time Of Day	1201-1800
Place
Locale Reference	BWI.Airport
State Reference	MD
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Route In Use	Visual Approach
Flight Plan	IFR
Person 1
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Flight Engineer Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 220 Flight Crew Total 14000 Flight Crew Type 11000
Events
Anomaly	No Specific Anomaly Occurred All Types

Air carrier landing bwi experienced glideslope oscillations while on the ILS runway 10 approach. The reporter noted this anomaly had been experienced on a number of other occasions.