Same ILS frequency for Parallel runways.
 

Came across an airport with parallel runways and the ILS frequency is the same for both different ident.

Now I assume this works ok in practice but both runways cant be in use with the ILS at the same time.:ooh:

So assuming seperate ground equipment for each runway and a big switch in the twr to choose 25 R or 25 L etc.

Would appreciate more advice on how this works in practice ?

Parallel runways. One for departures, one for arrivals. ILS active on only the arrivals runway?

Even when the ILS serves opposite ends of the same runway there normally has to be an interlock that not only ensures only one transmitts at a time but if I remember correctly there is a specified delay between one being shut off and the other starting to transmitt.

Technically it would be possible to do the same with parallel runways however, one would have to ask why bother?

Such an arrangement removes the posibility of offering the "departure runway" to an arrival on an opportunity basic.

Such an arrangement also gives a single point of failure that has the potential to efffectively make both runways unavailable at the same moment i.e. runway A ILS fails and due to a switching failure runway B not working.

I don't think that ILS frequencies are at such a premium.

The only saving I could think of is in respect of the associated DME whereby a suitable located single DME could indicated zero at all four thresholds.

Overall while physically possible (with the appropriate protections), it does not sound like a good idea to me.

There are few airports in the US that have one ILS for two runways, you come down the ILS to a certain DME and Alt and turn towards the non ils run way. I could never find out exactly why , there are a lot of theories .

Cost of the ground equipment or physical constraints around the runway which prevent installation or mean that it can't meet the specs for an instrument strip??

Maybe just not worth it on the CBA.