Guernsey : ILS frequencies...

[DamienInFrance]

Hello,

I flew to Guernsey this week and I was quite surprised by the ILS. I'm just a PPL, the weather was great, ATC was very helpful ... so the ILS was useless ... but I'm just looking for answers to several questions that enoys me since then :ugh: .

1) first I was surprised to read the same ILS frequency (108.1) on both side of the runway (R09 & R27) - I would tell that there is some HF compatibility issues with such a configuration - is this common in other airports ?

2) I am not used to the ILS instruments but since I had one onboard this time (on the HSI) I displayed the ILS frequency (108.1) and set the course pointer to the R09 QFU (I remember reading that man should always set the course pointer to the published front course of the ILS) ... the CDI deflected to the wrong direction - I must admit that I did not spend much time trying to understand what was wrong (may be the HSI was not properly configured ... I may be forget to switch NAV1/GPS to NAV1 :ooh: ) ?

Anyway, coming back to the fact that R09 and R27 ILS have the same frequency, I would like to make I understand how it works. To the best of my knowledge I would say that :
=> R09 ILS has a Front Beam and no Back Beam - Front Course 091 (R09 QFU) - Glide Slope & DME available
=> R27 ILS has a Front Beam and no Back Beam - Front Course 271 (R27 QFU) - Glide Slope & DME available
=> BTW, isn't the DME supposed to display 0 on the ILS threshold (but how since both DME use Ch 18X) :confused:

Did I miss something ?

The ILS configurations I saw so far were a lot simpler (just 1 ILS fully available available on one side of the runway and sometimes a back beam LLZ available on the other side but without GS).

Thanks in advance for your answers.

Damien.

[chevvron]

Panic ye not; ILS should only transmit in one direction ie from one localiser at a time, so if you're on the 'wrong' end, you will get an incorrect CDI deflection. DME is normally positioned midway between thresholds and the time base of the reply is adjusted to make your instruments think you've reached the DME when you're actually at the ILS touchdown point.

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[DamienInFrance]

Panic ye not; ILS should only transmit in one direction ie from one localiser at a time, so if you're on the 'wrong' end, you will get an incorrect CDI deflection.

Ok but how man could see the front course (or the 'right' end) of the LLZ. When I read the AIP (EGJB AD 2.19), I see :
=> LLZ 09 ILS CAT I (Ident I-UY) 108.1
=> LLZ 27 ILS CAT I (Ident I-GH) 108.1

That's why my first guess was that 2 LLZ were in use !?

[gcolyer]

Last time I was in Guernsey all ILS approaches were on 27 and GA traffic was going out on 09.

So it could be that the ILS for the day was infact 27 even though you were told to land on 09. That would explain the delfection.

[Chilli Monster]

Ok but how man could see the front course (or the 'right' end) of the LLZ. When I read the AIP (EGJB AD 2.19), I see :
=> LLZ 09 ILS CAT I (Ident I-UY) 108.1
=> LLZ 27 ILS CAT I (Ident I-GH) 108.1

That's why my first guess was that 2 LLZ were in use !?

1) Only one ILS can radiate at a time

2) Because this is the case it makes sense to use the same frequency in what is a congested spectrum

3) ILS will be selected by a switch in the tower - either 09 OR 27. The different idents are so that you, as the pilot, can cross check the ident you have is for the runway you're using.

Simple

[Jinkster]

Always a good idea to ident it - ATC might tell you to use ILS 09 when ident is actually showing 27!

When I did my IR the sim instructor deliberaltely did that at East Mids - runway 09 and 27.....

:ok:

[mark147]

I'd always wondered the same thing. It's obvious now you mention it that only one would be radiating at any one time.

Do they change the DME ident at the same time as the ILS ident, even though nothing else needs to change with the DME? Or can the DME somehow present a different ident in different directions (seems unlikely).

Mark

[HR200]

At my home airport, EGNM, the ILS freq. for both Runway 14/32 is the same freq at 110.9MHz, but only one is in use at any one time.

[chevvron]

LLZ and DME ident should both change when the other runway is selected.