A320 DME/DME not authorized
 

Question:


How would you know if on the A320, your FMGC is receiving DME/DME updating? Clearly, since this is not authorized on some RNAV (GNSS) approaches, there must be a way for the crew to identify DME/DME updating?


thanks !

To use an old term, "you have the cart before the horse".

What it is saying is that it it is not approved for aircraft without GPS sensor update of the FMGC.

DME/DME/IRU is the default update for aircraft without GPS (GNSS). There was originally an approval in certain countries that allowed DME/DME/IRU RNAV approaches as long as an EPE/ANP of better than 0.3 was available.

Actually, there are approaches where it says on the chart "DME/DME, LOC and VOR/DME updating not authorized". Therefore you have to disable those update methods before starting the approach.

Most those are RNP AR approaches and are part of the training for the AR crews to turn the DME updating off in some aircraft.

Check if you have "GPS Primary" indicated in the PROG page.
If you don't, you can not fly the approach.
And if you get the message "GPS Primary lost" on both FMSs during the approach, you need to go around, unless you are visual with the runway.
If you get the "GPS Primary lost" message on one FMS, use the other AP/FMS.

There's no need to disable anything in Airbus. (Unless your company SOP is different.)
As long as the GPS is the primary navigation source, it doesn't matter if you still receive DME updates. Normally, you will always receive DME signals as backup, but the GPS signal will be considered more accurate.
The DME/DME NOT AUTHORIZED, only means that it can not be the primary navigation method and that you need a GPS.

And as far as I know, there are not that many RNAV (DME/DME) approaches anyway. Most of them require GPS.

You can also check the Position Monitor page to see what the current update method is. (e.g. GPIRS/DME/DME for GPS Primary, or IRS/DME/DME without GPS Primary.)