Re possible LLZ (localiser) interference from preceding a/c, eginyt is right. If you are on the GS, you should always have line of sight on the LLZ antenna, unless either another a/c is departing, or you are at a platform height on long final.
HEATHROW DIRECTOR,
Agreed for the UK.
bubbers44 and eginyt,
Think you are both partly right on the DME location, because bubbers seems to be thinking American and eginyt British.
Can't quote the relevant books, and I'm not currently flying, but in the 1980s, most large US airfields seemed to have the DME TRx antennae at the LLZ antenna. No bias is applied to the readout, so the reading at the touchdown point had to be quoted on the chart (say, 2.4).
In the UK, the DME is generally somewhere at the side of the Rwy, but not normally abeam the touchdown point. A bias is then built into the reading transmitted to the a/c, so that a/c on the LLZ will receive a distance to touchdown. If there is an ILS/DME approach in both directions on the same Rwy, it would make sense for the DME to be placed equidistant from both thresholds, but that used not to be the case at Gatwick, where the bias had to be changed when they switched from 26L to 08R. I think that may have changed now.
The bias sysyem can create confusion when an a/c is using the DME readings for another purpose, such as a SID. And the DME reads negative when you are between the threshold and the ground antenna.
I notice at least one airfield in the US with multiple runways, LAX, has decided to reduce the number of DME installations. They seem still to have the DME with the LLZ, but only at one end of each Rwy. So, to give a very rough example (in the absence of a chart), if you are touching down in one direction the DME reading might be 2.4, reducing. Landing the other way, it might be 0.5, increasing. (The latter case means that the DME would read zero 0.5 nm before touchdown.)
Confused? Just do what it says on the chart!
Last edited by Jetdriver; 9th September 2013 at 20:32.