2022-05 · NASA ASRS report 1900171
Air carrier flight crew reported an altitude deviation while on final approach after another aircraft; on the ground near the runway; was causing interference with the glide slope signal for the runway.
We were originally cleared for the ILS XYC approach. After we intercepted the ILS we spotted the field we were cleared for the visual approach with a crossing restriction. I had thought he referenced a fix we were past so everything seemed fine with where we were at. It wasn't until ATC called and told us to cross what I had thought was a different fix at 4000. We were already at 3500. He then told us to maintain present altitude until ZZZZZ. At that time we were on the glide slope. We were a little confused about what had just happened. After crossing ZZZZZ we attempted to intercept the glide slope again. At that time we didn't need to be to aggressive to do it so it didn't really concern us. It wasn't until we got a little further that we realized that we were quite low while on the glide slope. At this time Tower cleared us to land and informed us they had a low altitude alert which we responded that we were aware of and were able to correct fairly easily. We reported the glide slope error to the tower. At the same time another aircraft reported the same. Tower replied that they think it might have been due to an aircraft on the ground. Aircraft in the path of the glide slope affected the glide slope accuracy. Was not apparent until short final. No alert from Approach or Tower regarding the possible affect of this aircraft.Personally I will be more vigilant when flying a visual approach while using the ILS for guidance.
Cleared for the ILS XYC approach and shortly after intercept we had the field insight; and were cleared visual. We were given a crossing restriction for a fix that we believed we crossed at 4000 only to realize we were coming up on it at 3500. ATC then told us to maintain present altitude until ZZZZZ. It intercepted again and we thought we had corrected but we saw that we had become lower than we should on the glide slope. At this point we disconnected the automation and corrected back to where we should be. ATC cleared us to land and we told them we were correcting our altitude. Once on the ground we reported the glide slope inaccuracy and heard reports of the same from aircraft behind us.There was an aircraft on the ground believed to be the cause of the failure that we recognized on final. [I would suggest to] maintain better vigilance of the aircraft automation which the aircraft is following when on a visual approach.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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