A B737-700 first officer believes that a radio tower south of the ILS 28L at pdx may be interfering ...

Attributes
ACN	1274346
Time
Date	201506
Local Time Of Day	0601-1200
Place
Locale Reference	PDX.Airport
State Reference	OR
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Route In Use	Visual Approach
Flight Plan	IFR
Component
Aircraft Component	ILS/VOR
Person 1
Function	First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 219
Events
Anomaly	Aircraft Equipment Problem Less Severe

Narrative:

The past four times I have flown an approach to runway 28L at pdx; the aircraft with autopilot engaged; on glideslope; in approach mode with glideslope captured; at 3.5 DME at approximately 1000 feet AGL; the aircraft pitches nose down with a corresponding GS showing 1/4 dot high. The aircraft recovers; then continues normally on the approach.the first time it was day IMC several months ago. The last three times; it was day VMC on a visual approach. At first I thought it was just a 'glitch.' now I think there may be an anomaly on the approach. There is an antenna on a ridge at the 10 o'clock position; from the aircraft perspective; that is nearly level to the aircraft on the rising ridgeline south of the airport. Perhaps it interferes with the ILS signal?

Original NASA ASRS Text

Title: A B737-700 First Officer believes that a radio tower south of the ILS 28L at PDX may be interfering with the glideslope signal causing a bump at about 1000 feet.

Narrative: The past four times I have flown an approach to Runway 28L at PDX; the aircraft with autopilot engaged; on glideslope; in approach mode with glideslope captured; at 3.5 DME at approximately 1000 feet AGL; the aircraft pitches nose down with a corresponding GS showing 1/4 dot high. The aircraft recovers; then continues normally on the approach.The first time it was day IMC several months ago. The last three times; it was day VMC on a visual approach. At first I thought it was just a 'glitch.' Now I think there may be an anomaly on the approach. There is an antenna on a ridge at the 10 o'clock position; from the aircraft perspective; that is nearly level to the aircraft on the rising ridgeline south of the airport. Perhaps it interferes with the ILS signal?

Data retrieved from NASA's ASRS site 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.

A B737-700 first officer believes that a radio tower south of the ILS 28L at pdx may be interfering with the glideslope signal causing a bump at about 1000 feet.