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What happens with an ILS (or LPV) glideslope below the DH?

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Needles crossed? With my calibrator’s hat on, I can say with absolute confidence that the GP info you are seeing is junk after about 175ft. During calibration we hand-fly to 50ft and after 200ft, even on a CAT III, we stop cross-checking against the ILS GP information presented on the HSI.

PS. Are you always looking to keep the needles crossed after DA? What about offset localisers such as Lydd? Do you follow the lateral guidance which is happily diverging from the actual centreline?

Last Edited by Dave_Phillips at 26 Jan 23:25
Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

Dave_Phillips wrote:

Needles crossed? With my calibrator’s hat on, I can say with absolute confidence that the GP info you are seeing is junk after about 175ft. During calibration we hand-fly to 50ft and after 200ft, even on a CAT III, we stop cross-checking against the ILS GP information presented on the HSI.

When I originally did my IR, one of the exercises by my enlightened instructor was to land completely blind. He said I might need it one day, if everywhere was fogged out. Both localiser and GS worked to and along the runway.

If I have (as I am sure you have had) IR students who get twitchy at 300’, I teach them to fly to 100’ on the ILS, so they can fly to 200’ with confidence (with all the caveats of course.)

I have also, in IMC, allowed the autopilot to fly LPV to 100’, out of academic interest.

I have never seen a GS, whether ILS (Cat I, II or III) or LPV to be junk after 200’. I would love you to direct me to one in Southern England I could go and try. I have certainly monitored GS vs PAPI to below 100’ at Southend, Biggin, Cranfield, Cambridge, Gatwick, Heathrow, Southampton, Bournemouth, East Midlands, Birmingham, Coventry and Stansted, and they have been bang on all the way down.

I’d actually be very interested to know the theoretical reason why the GS should be junk below 200’ (especially even on Cat II). It is short range line of sight, I can think of no mechanism for junking it, even if you wanted to, short of having separate aerials among the approach lights deliberately to interfere. Is that what you are suggesting. And how is the LPV GS interfered with?

PS. Are you always looking to keep the needles crossed after DA? What about offset localisers such as Lydd? Do you follow the lateral guidance which is happily diverging from the actual centreline?

The DA at Lydd is, as you know, very high (which is why it is of very limited training value). You have to jink right 10° at DA. It is barely more than a cloudbreak. But, by the same token, the Cat A & B RVR is 1500m (ie VMC) and the Cat C vis is 2100m, so the question of an approach to minima at Lydd is a very different one to the 200’, 550m situation we have been discussing.

EGKB Biggin Hill

Timothy, I can’t be bothered. My professional role is calibrating ILS. I’m also an IRE (of which you are not). Feel free to teach habits to chaps without the associated technical background knowledge. Feel free to wonder why you may get a jittery needle, GP flag at about 150ft or an unexpected fly-up (sometimes down) at 130ft. Ask yourself why a CAT III autopilot is programmed to switch-out the GP at 170ft and then maintains a rate of descent observed at that point (averaged over the last 8 seconds) for the second phase of the auto land (choosing to completely ignore your assumed perfect GP signal) before the RADALT controlled segment cuts in for the third phase at 50ft. If you want to go really technical, look at how the GP signs is bounced off the ground immediately in front of the antenna before shooting into space on an approximate 3 deg slop. Ask yourself how the 90 & 150hz signals interact with each other immediately after the ‘bounce’ and why some airports require additional GP calibrations when the ground is waterlogged. Have a look a minimum required DDMs and RF levels (the things that make the needles move on your HSI). etc etc etc.

Timothy wrote:

I would love you to direct me to one in Southern England I could go and try. I have certainly monitored GS vs PAPI to below 100’ at Southend, Biggin, Cranfield, Cambridge, Gatwick, Heathrow, Southampton, Bournemouth, East Midlands, Birmingham, Coventry and Stansted, and they have been bang on all the way down.

Well, all the airports you have listed there have appeared in may logbook as calibration flights over the past 18 month. Who do the airport believe – the professional calibration company or Timothy N?

Unfortunately I’ve allowed myself to get dragged down a Timothy rabbit hole. I’m sorry.

Last Edited by Dave_Phillips at 27 Jan 11:28
Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

[quote]
Ask yourself why a CAT III autopilot is programmed to switch-out the GP at 170ft and then maintains a rate of descent observed at that point (averaged over the last 8 seconds) for the second phase of the auto land (choosing to completely ignore your assumed perfect GP signal) before the RADALT controlled segment cuts in for the third phase at 50ft.[/quote]

That was also my understanding, which I mentioned here on another thread. I then started to look for a reference for this A/P behaviour. Despite trawling the Internet for an extensive time, I couldn’t find any. So I would be very interested if you had a source that explains this. Thank you.

Apart from that, I would also like to understand the technical reason why a GS becomes unusable below 200 ft. So if you had anything other than “professional experience” to quote, that would also be of great interest.

Last Edited by Rwy20 at 27 Jan 12:11

Dave_Phillips, I am now even more eagerly awaiting to read on calibration, if you have the time (or even can) for it one of these days!

Also curious to know, would be following an LPV in the conditions Timothy talks to safer than following an ILS? (Academically speaking, I have no intent to go below minima). And would there be other issues leaving the auto pilot on below minima when following an LPV? When looking at the picture what_next is showing, I think I might just go-around, but if had to land, I think one might have the temptation to leave the AP on a bit longer (if safe) until the visual picture is a bit better.

Dave_Phillips wrote:

Unfortunately I’ve allowed myself to get dragged down a Timothy rabbit hole. I’m sorry.

It was worth it (to me) because this was very interesting. Thanks for sharing.

I know a guy who claimed that he was doing 0/0 landings (ILS) with an STEC55X. I just looked at him in disbelief and wondered how he managed not to get himself killed, and never sought to discuss further

Last Edited by at 27 Jan 12:55
LFPT, LFPN

Noe wrote:

Also curious to know, would be following an LPV in the conditions Timothy talks to safer than following an ILS? (Academically speaking, I have no intent to go below minima). And would there be other issues leaving the auto pilot on below minima when following an LPV? When looking at the picture what_next is showing, I think I might just go-around, but if had to land, I think one might have the temptation to leave the AP on a bit longer (if safe) until the visual picture is a bit better.

From a technical perspective LPV is different in that it is a line in space and not subject to propogation issues as Dave is describing. Autopilots normally have a min altitude which is often effectively the minima of the approach or 1000ft AGL. It is hard to say that the autopilot stops following the FD or HSI indications below that altitude (it will follow them right to the ground) but I think it is more about if you get a sudden fly up or down indication, the autopilot will just do it. These systems (cat I) are certified assuming you hand fly from the DA/MDA.

EGTK Oxford

JasonC wrote:

It is hard to say that the autopilot stops following the FD or HSI indications below that altitude (it will follow them right to the ground) but I think it is more about if you get a sudden fly up or down indication, the autopilot will just do it.

Or if there is an autopilot failure causing an incorrect roll or pitch change you should have time to regain control.

ESKC (Uppsala/Sundbro), Sweden

Yes, being so close to the ground and having the autopilot possibly going “crazy” is a bit scary. As we both seem to think, the probability of a bad indication on the LPV is low (and I imagine possibly next to nil). Given that, would it be unwise to leave the AP on “a bit longer” until better reference aquired (in case visibility really bad), and if so, what would be the potential things killing you? Very hard for me to think what I would actually do once in that situation (thinking from my desk, it would be disconnect AP at DA if I decide to land), but I can see how it’s possible that flying AP on on the LPV for a bit after that wouldn’t be a stupid thing to do.

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