Narrative:

I was the cfii on this flight for an instrument rated commercial student of mine. What happened: glideslope came alive on ILS; and got stuck. While my student thought she was flying a perfect glideslope; we were on a collision course for the ground. Background info: this flight was the 300 NM cross country flight required for the commercial rating. We were on our way back to our home base. I am a newly minted cfii. 600 hours tt. I have 100+ hour of actual instrument time from flying a G1000 C208. While I have 40+ hours in the C210 we were flying; I've flown only one or two approaches in it. Similarly; my student had 10+ hours in the airplane; but little recent instrument time. While we both were inclined to scrub the flight and try and come home the next day; it was spring break in key west; and there was no place left to stay overnight; so we decided to fly home. Though my student lacked much current instrument experience; I was confident in my ability to shoot the ILS. Well; I'm letting my student fly the ILS. We turned inbound... Glideslope came alive and moved to 'one dot high.' student started descending; fast. Vsi and altimeter were decreasing. By the time the altimeter reached 600 ft I knew something was wrong. Luckily I had the GPS procedure loaded as well; so I could see our distance from the map was still 2+ miles. I grabbed the controls; leveled off. Less than 2 seconds later tower comes on the radio and tells us they're getting an altitude alert. I leveled off; kept flying; and sure enough the glideslope stayed pegged right where it was; indicating one dot high. I just kept my altitude (600 MSL) because I knew it was above the localizer minimums (480 MSL) and 1 mi from the missed approach point I was about to go around when my student caught the maslrs. Conclusion: I am thoroughly convinced that almost anyone in this situation could have flown themselves into the ground following a false glideslope indication. For me; typically flying ILS approaches on glass avionics; the idea of the glideslope indicator getting stuck was totally foreign to me. Thankfully I was able to recognize things were not right; but I don't think everyone might have been that lucky. If my student was alone and the ILS was unmonitored; they could have ended up in the trees.

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Original NASA ASRS Text

Title: Flying a G1000 equipped C210; an instructor and commercial student pilot flew a night IMC ILS to the their home base and descended to 600 FT on a false glideslope. ATC called a low altitude alert concurrent with the instructor identifying the condition on his backup GPS.

Narrative: I was the CFII on this flight for an instrument rated commercial student of mine. What happened: Glideslope came alive on ILS; and got STUCK. While my student thought she was flying a perfect glideslope; we were on a collision course for the ground. Background info: This flight was the 300 NM cross country flight required for the commercial rating. We were on our way back to our home base. I am a newly minted CFII. 600 hours TT. I have 100+ hour of actual instrument time from flying a G1000 C208. While I have 40+ hours in the C210 we were flying; I've flown only one or two approaches in it. Similarly; my student had 10+ hours in the airplane; but little recent instrument time. While we both were inclined to scrub the flight and try and come home the next day; it was Spring Break in Key West; and there was NO place left to stay overnight; so we decided to fly home. Though my student lacked much current instrument experience; I was confident in my ability to shoot the ILS. Well; I'm letting my student fly the ILS. We turned inbound... glideslope came alive and moved to 'one dot high.' Student started descending; fast. VSI and altimeter were decreasing. By the time the altimeter reached 600 FT I knew something was wrong. Luckily I had the GPS procedure loaded as well; so I could see our distance from the MAP was still 2+ miles. I grabbed the controls; leveled off. Less than 2 seconds later Tower comes on the radio and tells us they're getting an altitude alert. I leveled off; kept flying; and sure enough the glideslope stayed pegged right where it was; indicating one dot high. I just kept my altitude (600 MSL) because I knew it was above the LOC minimums (480 MSL) and 1 MI from the missed approach point I was about to go around when my student caught the MASLRS. Conclusion: I am thoroughly convinced that almost anyone in this situation could have flown themselves into the ground following a false glideslope indication. For me; typically flying ILS approaches on glass avionics; the idea of the glideslope indicator getting stuck was totally foreign to me. Thankfully I was able to recognize things were not right; but I don't think everyone might have been that lucky. If my student was alone and the ILS was unmonitored; they could have ended up in the trees.

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.