Glide slope-off

[CyclicRick]

Question for the clever among you.
Why oh why at the end of an ILS approach does the glide slope fall away downhill about three quarters to half a mile from the threshold?
Is it to do with the curvature of the earth or radio wave bending/reflection etc?
Answers on a bog roll to....

Rick

[photex]

Gravity?

:hmm:

[Phrogman]

You can find the big answer in the US Airman Information manual (AIM) in section 1-1-9 where it describes the system. The threshold crossing height is established based on several factors including the largest aircraft category that normally uses the runway. There are other factors, but best to just read the big picture. Hope that helps.

[Floppy Link]

Rick

...it's 'cos you're not scanning fast enough!

Russell


p.s. come home soon, am getting nackered covering yer shifts!

[eurocopter beans]

excellent question. have'nt a clue. I think its the 'autorotate now' command thats inbuilt to ILS approach for heli pilots, just my opinion, im not saying its definitely right

[HELOFAN]

If I understand you correctly.....

It would be .....

Because the glide slope is not emitted from the same place the Localizer is transmitted from.

The glide slope is mounted from 750 to 1250 foot down the runway and about 250 - 650ft offset from the runway center line.

Also runways are built up from surrounding ground so that water runs off & so forth.

AS you pass it it shows it falling away to the side...where really you are just passing it.

Looks like a square building that is painted usually red and white checkered.

Unless I miss your point all together.


HF

[Heli-phile]

No idea, but one quiet day I did a practice ILS in a AS355 but when down to about 50' instead of hover taxying off to my hangar I actually hovered around the threshold and tried to see if I could actually find the 'sweet spot' on the runway where the ILS was aiming me.
There was no such point, it seemed to be around the 50 ' above the runway approx , after that it was actually impossible to react to the glideslope it was just hyper sensitve.

No doubt all the interfence lobes all bunch up that close to the sweetspot!!
come on techies, whats the real reason??:}

[Elan Head]

Do you mean the common tendency to lose the glideslope near the end of (but still on) an approach? (In which case the answer is: you're not scanning fast enough!)

Obviously the glideslope gets more sensitive (lateral deviation corresponding to angular deviation gets smaller) the closer you get to the antenna. I think most people see it drop out from under them on approach because the tendency is to fly the approach slightly high (the idea being that the more clearance you have from the ground, the safer you are).

Of course if you are above the non-precision MDA for any segment of an approach, obstacle clearance is assured, so there's nothing wrong with flying slightly below the glideslope until the very end of the approach, when your natural impulse to pull aft cyclic close to the runway tends to even things out.

That's the 'pilot error' interpretation. I'm always happy to blame something else, though, if there's a technical explanation for it... :)

[John Eacott]

Why oh why at the end of an ILS approach does the glide slope fall away downhill about three quarters to half a mile from the threshold?

Where does it do that? I've never come across such a phenomena (although Aberdeen used to have a nasty "hole" at the end of the ILS, IIRC).

Try a fully coupled approach (SPZ7000 or similar) and you should find the RoD remains fairly constant all the way to a 60ft height hold, commensurate with IAS, of course. I suspect Elan Head has it right: past DH, look up and continue visual and you will often gently climb above the indicated GS once all the visual cues take you further down the runway than the ILS aiming point :)

[keithl]

Elan Head, John Eacott and Helofan (line 6) combine to give the right answer. The GS doesn't "drop away", but it may appear to.

[212man]

If Glide Slopes "dropped away" how do people think Fixed Wing Aircraft carry out autolands, or even Cat III a approaches? Regarding where the transmitters are located, they are abeam the touchdown markings.

[Floppy Link]

Rick,

also as you get closer to the ground the friction effects on the wind can give you a reducing headwind component, most noticeable at night when the mixing layer is drastically reduced or nonexistant. On the 757 you had to be ready to reduce thrust slightly passing about 200ft or the glideslope would give the impression of "dropping away" as you went high. Comparing the reported surface wind with the wind you're experiencing on the way down the slope gives you the first clue, if there's a huge difference.

Having said that, it was about 200-250ft, so you'd only get it in the very final stages of the ILS.

Dunno who told me and if it's accurate or not but when I did my initial ages ago they said that the half scale deflection box you need to be in when at DA/DH is only about 10ft deep and 15ft wide.

Rapid scan, tiny corrections and wiggle your toes (helps you relax a bit)
:cool:

[CyclicRick]

I take your points but I'm sure it's got to be something to do with the ILS parabolic antennae curving the signal or the threshold drop off.
Scan is perfect and toes are really relaxed underneath the yaw pedals.

Rick.

Russel: Might take a while mate, keep up the good work :ok:

[212man]

Hey, I think there's an echo in here...

If Glide Slopes "dropped away" how do people think Fixed Wing Aircraft carry out autolands, or even Cat III a approaches? Regarding where the transmitters are located, they are abeam the touchdown markings.

[IHL]

I've never noticed it drop away.

I do have a personal tendency that I have to guard against; when I become
visual I have a tendency to dip below the glide path. If you stay on it you will touch down at about the 1000 ft marker.

[Tail-take-off]

Don't know about glideslopes dropping away but at about 50' a fixedwing aircraft enters "flare mode" & ignores the glideslope. From here on in it is all about Radaly closure rate. In fact on the A320 if the glideslope fails below 100' it will just carry on & land anyway. All you will know about it is that there is no glideslope deviation indication & the glideslope deviation scale flashes.

[[email protected]]

Rick - either because your ILS hasn't been properly calibrated or you are too high:)
It could have something to do with the RDH (reference datum height) published on the approach plates.

[Letsby Avenue]

Glad to see someone else notice this... My TRE tells me that I am 'peeking' (as if I would:)) and suffering from ground rush whereupon I sub-consciously raise the lever.

What are you doing nowadays Rick?

[Floppy Link]

...his IR :E


question Rick - is it in the sim or the aircraft?

[CyclicRick]

Don't worry, it was only a spoof to see what happened :. It's due to decreasing wind as you decend.
Got some good anwers though :p