B757 pilot reports recurring GS oscillations for ILS runway 10 at bwi.

Attributes
ACN	757365
Time
Date	200710
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : bwi.tower
State Reference	MD
Altitude	agl bound lower : 0 agl bound upper : 300
Aircraft 1
Controlling Facilities	tower : bwi.tower
Operator	common carrier : air carrier
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 10
Flight Phase	descent : approach
Route In Use	approach : instrument precision
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 220 flight time total : 10000 flight time type : 8000
ASRS Report	757365
Events
Anomaly	other anomaly other
Independent Detector	other flight crewa
Resolutory Action	none taken : unable
Supplementary
Problem Areas	Navigational Facility
Primary Problem	Navigational Facility

Narrative:

The glideslope seemed to oscillate up and down a bit between 450 ft or so and close to touchdown; only about 1/4 or so; but definitely seemed to have interference. I only write this up because I have consistently seen higher frequency oscillations of the glideslope on runway 10 ILS; around 200-100 feet; regardless of weather; usually in the evening. I've been flying offirst officern from bwi for 3 and a half years. Runway 10 often displays glidescope oscillations; and tonight I noticed runway 33L also moving around; albeit at a higher altitude and a lower frequency cycle. Is there any chance all the radiation from a commercial facility is interfering with the signals? I think runway 10 is a CAT ii/III; and I would hate to shoot one with those deviations.callback conversation with reporter revealed the following information: reporter stated that the GS oscillations for the ILS runway 33L were not as pronounced and frequent as those encountered on approach to runway 10. He has consistently experienced GS oscillations affecting raw data displays for ILS runway 10. The GS seems to oscillate at the rate of 2 per second with a 1/2 DOT deviation. The flight director and autopilot are unaffected by these oscillations. The oscillations were encountered on numerous B757's flown into bwi by the reporter and occur below 300 ft AGL.

Original NASA ASRS Text

Title: B757 PLT REPORTS RECURRING GS OSCILLATIONS FOR ILS RWY 10 AT BWI.

Narrative: THE GLIDESLOPE SEEMED TO OSCILLATE UP AND DOWN A BIT BETWEEN 450 FT OR SO AND CLOSE TO TOUCHDOWN; ONLY ABOUT 1/4 OR SO; BUT DEFINITELY SEEMED TO HAVE INTERFERENCE. I ONLY WRITE THIS UP BECAUSE I HAVE CONSISTENTLY SEEN HIGHER FREQUENCY OSCILLATIONS OF THE GLIDESLOPE ON RWY 10 ILS; AROUND 200-100 FEET; REGARDLESS OF WEATHER; USUALLY IN THE EVENING. I'VE BEEN FLYING OFF/ON FROM BWI FOR 3 AND A HALF YEARS. RWY 10 OFTEN DISPLAYS GLIDESCOPE OSCILLATIONS; AND TONIGHT I NOTICED RWY 33L ALSO MOVING AROUND; ALBEIT AT A HIGHER ALTITUDE AND A LOWER FREQUENCY CYCLE. IS THERE ANY CHANCE ALL THE RADIATION FROM A COMMERCIAL FACILITY IS INTERFERING WITH THE SIGNALS? I THINK RWY 10 IS A CAT II/III; AND I WOULD HATE TO SHOOT ONE WITH THOSE DEVIATIONS.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THAT THE GS OSCILLATIONS FOR THE ILS RWY 33L WERE NOT AS PRONOUNCED AND FREQUENT AS THOSE ENCOUNTERED ON APCH TO RWY 10. HE HAS CONSISTENTLY EXPERIENCED GS OSCILLATIONS AFFECTING RAW DATA DISPLAYS FOR ILS RWY 10. THE GS SEEMS TO OSCILLATE AT THE RATE OF 2 PER SECOND WITH A 1/2 DOT DEVIATION. THE FLT DIRECTOR AND AUTOPILOT ARE UNAFFECTED BY THESE OSCILLATIONS. THE OSCILLATIONS WERE ENCOUNTERED ON NUMEROUS B757'S FLOWN INTO BWI BY THE REPORTER AND OCCUR BELOW 300 FT AGL.

Data retrieved from NASA's ASRS site as of January 2009 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.