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TOPIC: ILS Config

ILS Config 21 Jun 2018 21:08 #1

  • jpfive
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On some photos i found this type of ILS in the 60s.this was placed abreast the runway in a form of a semi circle. I dont know if this was part of the localiser or not. I am trying to find more information about this type of ILS and if there was gludepath or marker beacons with this. Here is a photo of the system installed at raf abingdon.

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ILS Config 22 Jun 2018 10:09 #2

  • ChrisTheAncient
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Yes... That is an ILS Localiser building and aerial array.

The two 'little aerials' are monitors which would drive an ILS indicator in the GRSF area which also had indicator lights showing the serviceability (or otherwise) of all the transmitters.

The ILS glidepath aerial would be at the other end of the runway (close to the touchdown point... obviously) and looks rather like a mast with with two tea trays mounted on it! It also had a small building with it for the transmitter. There was also a monitor aerial to drive the ILS indicator in the GRSF area.

The usual 'marker' beacons used in an RAF configuration were the 'middle marker' circa 1NM from the touchdown and an 'outer maker' circa 6-7NM from touchdown (middle marker - usually 'decision height' and outer marker usually 'start descent height'.
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Last Edit: by ChrisTheAncient.

ILS Config 22 Jun 2018 10:48 #3

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Thanks for the information. However i would like to ask you why the localiser was not situated at the end of the runway but in the middle and on the side of the runway. Does the pilot had to disengage the autopilot and correct the heading? Can it handle autoland?

Thanks appreciated

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ILS Config 22 Jun 2018 13:26 #4

  • TerryClark
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jpfive wrote: Thanks for the information. However i would like to ask you why the localiser was not situated at the end of the runway but in the middle and on the side of the runway. Does the pilot had to disengage the autopilot and correct the heading? Can it handle autoland?

Thanks appreciated

This old type semi circular localiser was common on RAF airfields but not civil ones; it was to try to ensure the beam was as narrow as possible however since the mid '60s, this has fallen out of favour as a narrow beam can also be produced by making the aerial array wider.
Having the localiser aerial at one side of the runway rather than on the extended centreline of the runway could be for various reasons such as the end of the runway being too close to the airfield boundary, the presence of an arrestor barrier which, when raised, would have 'distorted' the integrity of the beam, or having a road crossing just off the end of the runway where large vehicles might also distort the beam.
There is a penalty for what are called 'offset localisers' in that the decision height is often (but not always) 50ft higher than with an in-line localiser; I think the criteria is if it is more than 3 deg offset ie if the localiser final approach track is more than 3 deg different from the runway QDM.
The use of 75 mhz 'fan' markers has largely died out nowadays, they have been replaced by a DME situated midway between the landing thresholds and set to read zero at either threshold and normally where there is an offset loc, instructions on the approach plate issued to pilots generally point this out and instruct the pilot not to use an autocoupled approach but to fly manually.
A good example of offset ILS is at Sumburgh on runway 09/27 where the runway ends close to the sea at both ends; runway 27 loc is 2.6 deg offset and runway 09 is 2.2 deg offset.
Another is RAF Odiham runway 27 where the loc is 3 deg offset due to the presence of the airfield boundary and a public road just off the end of the runway.
An exception is Cranfield runway 22 where the loc is on the far side of a public road from the airfield itself!!
As for the glidepath; there are now usually 3 'tea trays' on the mast rather than 2, the top one being configured to bounce the radiated signal off the ground thus interfering with the signal formed by the lower two and ensuring a much more accurate beam; there is also a requirement for the ground in front of the GP aerial to be absolutely flat for (I think) at least 400m in front of the aerial, this being called the 'beam forming area'.
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Last Edit: by TerryClark.

ILS Config 22 Jun 2018 15:22 #5

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Some good stuff from Terry there!

And... a simple reason for having the localiser at the side of the runway. It would usually make it within easy reach (i.e. a short path) of the taxiway without the need to 'invade' the bits where aircraft might want to be during take-off or landing.

And, if they put it on the centreline, it would probably be in the undershoot/overshoot area and create an obstruction just where they don't want one! On airfields where I had been that had that sort of ILS, inevitably the undershoot/overshoot area at the end of the runway was right up to the boundary fence so that they could make the runway as loooooooong as possible!
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ILS Config 22 Jun 2018 15:36 #6

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Great thanks for the info both of you. So were was the localiser pointing. I mean how far from runway threshold before turning to final runway heading or the intersection of the localiser path and runway track.

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ILS Config 22 Jun 2018 15:57 #7

  • TerryClark
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jpfive wrote: Great thanks for the info both of you. So were was the localiser pointing. I mean how far from runway threshold before turning to final runway heading or the intersection of the localiser path and runway track.

There will usually be a note on the approach plate advising at what range the Loc beam intercepts the runway centreline; this is normally at or just before decision height.
Thinking back to when I was planning the systems at Farnborough, I think the RAF apply a 50ft 'penalty' on any approach using an offset localiser (but I could be wrong) so that instead of a decision height of 200ft for an in-line localiser or PAR, the minimum DH with an offset localiser is 250ft.
I should perhaps point out that the 'norm' for a localiser is for it to be positioned on the centreline; offset localisers are only installed nowadays when it is not possible to install an in-line one.
As a localiser is classed as an aid to air navigation, it is disregarded when it comes to taking obstructions into account, as is the glidepath aerial which is usually about 30 ft high not far from the edge of the runway.

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Last Edit: by TerryClark.

ILS Config 22 Jun 2018 16:26 #8

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Thanks very much for this information. Do you know a book or something similar to icao annex 14 about designing aerodromes in the 50s and 60s?

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ILS Config 22 Jun 2018 17:08 #9

  • TerryClark
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Lydd ILS Runway 21 Note 1
'Localiser offset 5 deg and intercepts runway centrline 900m before thr Rwy 21'
A railway line runs a short distance from the end of the runway.

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Last Edit: by TerryClark.

ILS Config 22 Jun 2018 17:13 #10

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jpfive wrote: Thanks very much for this information. Do you know a book or something similar to icao annex 14 about designing aerodromes in the 50s and 60s?

Sorry I don't have access to reference material like that.
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