Airbus Back Up Navigation

[Islander63]

I'm in doubt if in the event of a double FMGS failure could we continue flying RNAV5 airspace using Back Up navigation supported by GPS. Has anybody any idea about?
Accordingly with the procedures in the Special Operations of the FCOM we need at least one FMGC to enter in RNAV5 airspace but it doesn't say if you can remain inside if such a failure happens once you have already enter. Thank you.

[mothergoose1]

inevent of double fmgs fail,u gotta lift that guarded nav switch.tune the rmp and have good situational awareness.u cant be in that airspace no more.also u are unable rvsm as no autopilot aswell buddy.

[Islander63]

inevent of double fmgs fail,u gotta lift that guarded nav switch.tune the rmp and have good situational awareness.u cant be in that airspace no more.also u are unable rvsm as no autopilot aswell buddy.

Thank you mothergoose. I know you have your RMP available for radio tuning, but you have also the back up navigation with the remaining waypoints of your flight plan and with GPS available your accuracy would be probably High. My question is more related to the "legality" rather than "what to do". As you commented regarding the RVSM no doubts, no autopilots no RVSM, cristal clear; but in the RNAV I didn't find it.

[FlightDetent]

Not so crystal.:E I do not think mothergoose1 understands fully the technology behind MCDU BACK-UP NAV.
In BACK–UP NAV, AP can be engaged in selected modes if the FG part is available.So, AP yes, RNAV yes-some. The whole feature as far as I understand it was developed to increase redundancy for ocean crossings, keeping the RNP capability as a sole objective.

Specifically about RNP5, I do not know. RVSM no problem.

[mothergoose1]

Well i guess not all the a320s have mcdu back up navigation feature.The newer a320 ones have them.So i dont know about the autopilot and legality with that.

[mothergoose1]

Il get back to you.its a very practical question.If youre getting a gps primary with acc high.it wont be an issue

[Islander63]

Not so crystal.:E I do not think mothergoose1 understands fully the technology behind MCDU BACK-UP NAV.
So, AP yes, RNAV yes-some. The whole feature as far as I understand it was developed to increase redundancy for ocean crossings, keeping the RNP capability as a sole objective.

Specifically about RNP5, I do not know. RVSM no problem.

Yes, you are right. Could happen that the AP is available because the FG part of the FMGC is working. In that case no problem with RVSM.

[Islander63]

Il get back to you.its a very practical question.If youre getting a gps primary with acc high.it wont be an issue

I think I'm wrong here. In Back-up navigation there is not information if the accuracy is high or low. To have this value you need the EPU, estimated position uncertainly, normally displayed in the right bottom corner of the PROG page. But in the Back Up Navigation PROG page we don't have it, so no possible to compare with the Requested Navigation Performance (RNP).

We have the accuracy of the GPS in terms of Figure of Merit, nothing else. I guess is not enough for RNAV5 navigation.

Thank you anyway.