ILS DME location

[cockpitvisit]

Hello,

is it true that in some parts of the world, DME shows the distance to the touchdown point, and not the distance to the localizer at the far end of the runway?

How is this achieved - do they co-locate DME with the glidepath transmitter instead of the localizer, or do they somehow manipulate the DME transmitter to show a shorter distance?

How do you (as a pilot) find out whether a specific ILS DME shows the distance to the localizer, or to the touchdown point?

Thanks :)

[FlyingStone]

It's actually the other way around.

Most ILS/DME installations around the world have DME collocated with the glideslope antenna, which means DME reads 0 at touchdown point.

Sometimes (namely in the UK) DME reads 0 at landing threshold. If that's the case, a note on the approach chart will advise the pilot that's the case.

[Dan Dare]

I'm no expert, but I think if you look at the aerodrome charts you will still see a DME transmitter somewhere near the mid-point of the runway. That would be quite clever so you only need (the costs and complexity of) one transmitter aerial to do both directions. The clever bit is merely tweaking appropriate DME timing so that it shows zero at e.g. the threshold.

[ShyTorque]

Read the plate.

[eglnyt]

Within the ICAO specification for ground DME equipment is a fixed delay from Interrogation to Reply. To allow for any local differences in cable length etc it has to be possible to adjust the equipment to ensure that delay is within prescribed tolerances. If you want to show zero at a point other than the aerial location it is a relatively easy matter to use that adjustment to offset the point at which zero is displayed in the aircraft provided you know what the difference is. That is just application of physics and a flight check to confirm. If you can locate the DME near enough halfway between the two thresholds you can use the same equipment and setting for both and all you need to arrange is for the ident to change appropriately. That is what is generally done in the UK. If you have a reasonably long runway and the DME isn't halfway along you may need two different delay settings. Although many years ago Gatwick did have a DME with the ability to select different preset delays depending upon the runway in use that was, as far as I'm aware, unique in the UK and when it was replaced they moved the DME to the middle as the replacement equipment didn't have that facility. Nowadays these things are set in software and programmed with a laptop so it would be much easier if you wanted to do it.
In the UK the DME position is usually shown on the Aerodrome Chart along with its frequency and ident. I don't think the charts in the AIP say where zero is but the DME text entry in the AD.19 for the Aerodrome will.

[cockpitvisit]

Thanks for the infos! :)

[Fly3]

Beware ILS/DME in the USA. At some airports such as LAX the DME reads zero at the upwind end of the runway. Check the charts carefully.

[Capt Fathom]

Not always. It would depend on which runway you are using!

[mustafagander]

Capt Fathom,

That's the point, read the charts. Many USA airports ILS DME reads zero at the upwind end. Very useful!
In other parts of the world they read zero at the G/S intercept point.

[Capt Fathom]

Yep, That point has been made numerous times already!