Narrative:

While flying an ILS approach to runway 6; and while inside the final approach fix; our glideslope CDI began showing a series of significant deviations despite that the airplane was maintaining a consistent descent rate. The deviations are consistent with interference or a glideslope antenna issue at the airport. Beginning at approximately 2;600 feet; our glideslope course deviation indicator (CDI) showed a 1/2 scale downward deflection seemingly out of nowhere; followed by a 1/2 scale upward deflection. The pilot flying (PF) had configured the airplane to fly the approach by autopilot; and it had been doing a good job tracking the vertical path until that point. At approximately 2;000 MSL; our glideslope CDI showed an almost full deflection downward. The PF disconnected the autopilot; and then we immediately saw the airport and continued our approach in visual conditions. Had we not seen the airport; we would have gone around because the glideslope seemed incapable of properly guiding us to the decision altitude; and the deviation was exceeding our limitations for a stable approach in IMC. The sudden nature of the glideslope swings made us both think that it was not an issue with our airplane's equipment. In talking to other pilots at our company; this seems to be a known issue at this airport; but it also seems as if nobody has bothered to bring it up in a safety report or inquiry with the control tower. The PF and I notified the control tower of possible glideslope problems; and we also spoke with an airport ops tech once parked at the terminal about the issue. A contributing factor may have been low level wind shear; however; we had been dealing with this all day at other airports and not had it impact our autopilot's ability to track the glideslope on an ILS approach.

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Original NASA ASRS Text

Title: Air carrier First Officer reported the aircraft experienced glideslope signal interference during an instrument approach.

Narrative: While flying an ILS approach to Runway 6; and while inside the final approach fix; our glideslope CDI began showing a series of significant deviations despite that the airplane was maintaining a consistent descent rate. The deviations are consistent with interference or a glideslope antenna issue at the airport. Beginning at approximately 2;600 feet; our glideslope course deviation indicator (CDI) showed a 1/2 scale downward deflection seemingly out of nowhere; followed by a 1/2 scale upward deflection. The Pilot Flying (PF) had configured the airplane to fly the approach by autopilot; and it had been doing a good job tracking the vertical path until that point. At approximately 2;000 MSL; our glideslope CDI showed an almost full deflection downward. The PF disconnected the autopilot; and then we immediately saw the airport and continued our approach in visual conditions. Had we not seen the airport; we would have gone around because the glideslope seemed incapable of properly guiding us to the decision altitude; and the deviation was exceeding our limitations for a stable approach in IMC. The sudden nature of the glideslope swings made us both think that it was not an issue with our airplane's equipment. In talking to other pilots at our company; this seems to be a known issue at this airport; but it also seems as if nobody has bothered to bring it up in a safety report or inquiry with the control tower. The PF and I notified the control tower of possible glideslope problems; and we also spoke with an airport ops tech once parked at the terminal about the issue. A contributing factor may have been low level wind shear; however; we had been dealing with this all day at other airports and not had it impact our autopilot's ability to track the glideslope on an ILS approach.

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.