Narrative:

On ILS 28 at 5;000 ft outside wavie on localizer; awaiting intercept of glide scope with weather at ord of approximately 1;500 ft overcast and 2.5 miles in snow. With the autopilot engaged and approach mode selected suddenly saw the glide slope go from full up to full down and stay there. Autopilot immediately tried to engage glide slope and abruptly pitched over with a firm bump and 100 ft loss of altitude before I could disengage autopilot and return at 5;000 ft. While leveling I saw the glide slope indicator go back to the correct indication of full up. Since the flight director and HUD were completely confused at this point; we cycled the flight directors off and back on and rearmed the approach mode; albeit without an autopilot. About this time we switched to tower and heard another flight asked if he wanted to continue the approach. He replied that he was visual and would continue but that he had experienced a full scale deflection of the glide slope on short final. At this point we knew that it was not our flight alone but some sort of interference with the glide slope signal. We reported our glide slope disturbance to the tower and were told that all indications were normal from their monitors. We landed without further incident; but called the tower after landing for an explanation. I was told that about that time a large cargo jet came from the south cargo ramp up for takeoff on runway 28. They had him hold short for departure on the south side of runway 28 this is right about where the glide slope antenna was located. Tower indicated that they did not need to keep the ILS critical area clear because the weather was greater than that required; but that they could see that this was having an impact on aircraft on approach and might not be a good practice. While I don't now the solution to this problem; perhaps just information to aircraft on the approach that the ILS area is not clear would have been helpful. This would have allowed us to either hand fly the approach; or use an alternate method (LNAV/VNAV) until the approach area was again clear; which would have eliminated the abrupt autopilot pitch over due to interference with a clear signal.

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Original NASA ASRS Text

Title: A large cargo aircraft was allowed to taxi in the vicinity of the ORD Runway 28 ILS glide slope antenna. This caused interference for an IMC aircraft on approach resulting in a pitch down and 100 FT altitude loss before the crew could recover.

Narrative: On ILS 28 at 5;000 FT outside WAVIE on localizer; awaiting intercept of glide scope with weather at ORD of approximately 1;500 FT overcast and 2.5 miles in snow. With the autopilot engaged and approach mode selected suddenly saw the glide slope go from full up to full down and stay there. Autopilot immediately tried to engage glide slope and abruptly pitched over with a firm bump and 100 FT loss of altitude before I could disengage autopilot and return at 5;000 FT. While leveling I saw the glide slope indicator go back to the correct indication of full up. Since the Flight Director and HUD were completely confused at this point; we cycled the flight directors off and back on and rearmed the approach mode; albeit without an autopilot. About this time we switched to Tower and heard another flight asked if he wanted to continue the approach. He replied that he was visual and would continue but that he had experienced a full scale deflection of the glide slope on short final. At this point we knew that it was not our flight alone but some sort of interference with the glide slope signal. We reported our glide slope disturbance to the Tower and were told that all indications were normal from their monitors. We landed without further incident; but called the Tower after landing for an explanation. I was told that about that time a large cargo jet came from the south cargo ramp up for takeoff on Runway 28. They had him hold short for departure on the south side of Runway 28 This is right about where the glide slope antenna was located. Tower indicated that they did not need to keep the ILS critical area clear because the weather was greater than that required; but that they could see that this was having an impact on aircraft on approach and might not be a good practice. While I don't now the solution to this problem; perhaps just information to aircraft on the approach that the ILS area is not clear would have been helpful. This would have allowed us to either hand fly the approach; or use an alternate method (LNAV/VNAV) until the approach area was again clear; which would have eliminated the abrupt autopilot pitch over due to interference with a clear signal.

Data retrieved from NASA's ASRS site as of April 2012 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.