Airbus DME's

[Check Airman]

Hi,

I was looking at an A330 FCOM in the RADIO NAV section, and I saw something interesting. To paraphrase, it said that the aircraft had 2 DME's, and the frequency is paired with the VOR / ILS. It went on to say that up to 5 ground stations can be tuned by the FMC.

My question is if there are only 2 DME's installed, how can the FMC tune 5 ground stations?

I remember reading somewhere that the 744 also has 5 DME inputs available, so maybe it's not just an Airbus thing.

[Field In Sight]

1 at a time ( or more correctly 2 ). I don't know how quickly though.

[grebllaw123d]

Check Airman,
Tuning up to 5 DMEs is not an Airbus or a Boeing feature, but a general FMC feature.
Read chapter 15.2.1.2 in the following link and you will get info about this feature:
http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_15.pdf

[Check Airman]

Thanks for that link. That document seems to be an excellent reference. I'll have to see if I can find the entire thing.

A standard DME can only accept one tuning command at a time, an agility-capable DME can accept two tuning commands at a time, and a scanning DME can accept up to five tuning commands at a time.

If I'm interpreting this properly, the scanning DME fitted to modern planes are simply capable of interrogating multiple ground stations at once, hence the pilot can select whatever DME in the cockpit, and FMS position updating is unaffected?

[grebllaw123d]

To the best of my knowledge the answer to your question is yes.

[Check Airman]

thanks for the info

[Microburst2002]

I think they call it multiplexing. The interrogator can search, lock and track several transponders simultaneously. This radar pulses are microseconds long, only...

A single DME interrogator can track 2 DMEs for FMGC position (one for onside FMGC and one for offside one) plus a different one for display, plus one coupldmto the ILS and you still have one left.

VOR can't multiplex, so if FMGC is calculating position with VOR DME IRS, you must select it for display in RADNAV or see TUNE ZZZ in the scratch pad.