Narrative:

The first officer was the flying pilot and the captain was the pilot monitoring. We were cleared for the ILS 8R into iah. I don't really remember what the ceilings were but I would guess around 1500 feet. We were IMC when the deviation took place. We were outside eelpo but approaching it when the autopilot pitched nose down around 9-10 degrees to hold the glide slope. Then the plane started pitching back up then back down again. At that point I disconnected the autopilot. The glide slope start[ed] moving all around. At that point I asked the captain if we had the right freq. Which we did and he noticed that we were below the published minimum outside of eelpo. I want to say I was about 500 or more below the minimum of 3000 feet but approaching eelpo. I made corrections by climbing and around that point; the glide slope centered back up and seemed to be working normal.we asked tower if anyone else was having problems with the glide slope. Tower said they get that sometimes when an aircraft taxis across the ILS critical area on the ground. Another aircraft behind us reported the same issue. Tower was not having aircraft hold short of the ILS critical area even though aircraft were shooting ILS's. Houston tower told us that they don't hold aircraft clear of the ILS critical area unless the ceiling are at 800 feet or below. I can't believe that they allow aircraft to pass through the ILS critical area while they assigning an ILS to all aircraft because you are starting the approach off in IMC conditions. I'm not sure why 800 feet is the magic number for using the ILS critical area when they know it cause issues on the glide slope. I really think that need[s] to be addressed with houston tower.

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Original NASA ASRS Text

Title: Air carrier flight crew encountered an unstable ILS glide slope during approach.

Narrative: The First Officer was the flying pilot and the captain was the Pilot Monitoring. We were cleared for the ILS 8R into IAH. I don't really remember what the ceilings were but I would guess around 1500 feet. We were IMC when the deviation took place. We were outside EELPO but approaching it when the autopilot pitched nose down around 9-10 degrees to hold the glide slope. Then the plane started pitching back up then back down again. At that point I disconnected the autopilot. The glide slope start[ed] moving all around. At that point I asked the captain if we had the right freq. which we did and he noticed that we were below the published minimum outside of EELPO. I want to say I was about 500 or more below the minimum of 3000 feet but approaching EELPO. I made corrections by climbing and around that point; the glide slope centered back up and seemed to be working normal.We asked tower if anyone else was having problems with the glide slope. Tower said they get that sometimes when an aircraft taxis across the ILS critical area on the ground. Another aircraft behind us reported the same issue. Tower was not having aircraft hold short of the ILS critical area even though aircraft were shooting ILS's. Houston Tower told us that they don't hold aircraft clear of the ILS critical area unless the ceiling are at 800 feet or below. I can't believe that they allow aircraft to pass through the ILS critical area while they assigning an ILS to all aircraft because you are starting the approach off in IMC conditions. I'm not sure why 800 feet is the magic number for using the ILS critical area when they know it cause issues on the glide slope. I really think that need[s] to be addressed with Houston Tower.

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.