Glideslope ends about 200' below the runway

I suspect you have a problem with one of the 90/150hz filters on the UHF element of your navbox.

Quite unlikely, as the ground check worked ok.

Hmmm, the ground test equipment simulates a relatively weak signal, normally about 3dbm above the threshold where your avionics would present a flag. I’m wondering whether a stronger signal is causing the problem.

Last time I was at the said company (~2 years ago) they only had a very old piece of kit which from memory looked like this

and didn’t work properly. You need somebody with a calibrated IFR4000 or similar.

I wondered if perhaps the GS at the airport in question is set up to guide you to the start of the runway (where the PAPIs will of course indicate all-reds) but I doubt that is allowed. But maybe it is?

Another thing is that PAPIs are not supposed to be used below a certain height.

Certainly a very interesting problem!

An ILS signal should be harmonised with the PAPIs at system minima such that the nominal touchdown point is somewhere close to being abeam the ILS G/P & PAPIs. Below system (procedure) minima on a Cat I there will be a slight discrepancy which for the average GA aircraft pilot would notice a slight below-the-GP indication on the PAPIs (i.e. something akin to two and a half reds) due to MEHT differences.

As it happens, I personally flew ILS & PAPI calibration flights at Oxford (21 Jan 15) and Gloucestershire (18 Feb 15) and both systems passed.

Is the indicator a King KI 525 ?

Some of these units had a capacitor fitted that breaks down and makes the glide slope indication voltage sensitive. There is a mod to stop this problem.

Don’t know but you’re only one aircraft which has noticed an issue in hundreds, if not thousands, of aircraft. I’m due to calibrate Manchester on Thursday night and, if tasked, could easily take a quick look at Gloucestershire and/or Oxford on the way up North.

My recommendation would be to file an MOR (Mandatory Occurrence Report) which will force the system to check the integrity of ILS signals. At the same time, I would still recommend that you get your own equipment thoroughly tested. False/wrong ILS indications are not the sort of things which should only be discussed in internet forum.

The IFR-4000 can do different signal levels, to do a full avionics test you will test both with low level signal and a very strong signal.

The older equipment Peter showed is quite ok as well (as long as it functional within specs) the IFR-4000 has the large advantage that it can be used for bench and ramp testing, and speeds up the testing, as you can program test. This also insures you always do the same test, where with an older test their are quite some settings to be changed at each step, while the IFR-4000 is only using the next button for the next test item.

This also insures you always do the same test

This certainly is the best you can do, but it’s still not 100% repeatable, as the tests are usually performed over the air, not plugged in.

So you still have the influence of antenna positions, aircraft positions, people and machinery close by, etc.

And most importantly, most people seem to do these checks with the engine off, which means they won’t catch prop modulation issues (and apparently many don’t even understand this issue).