A320 PRNAV, RNAV Approach

[che ci dò che ci dò!]

Dear all,
A320 Fcom - PRO part - Special OPS - RNP-1 states that, in case of FMS1/FMS2 pos diff. during PRNAV operations, the pilot has to inform ATC of the loss of RNP capability.
In case of RNAV GNSS approach, Fcom - PRO - SOP - says:

"If the FMS1/FMS2 POS DIFF message is triggered during the approach, discontinue the approach, unless radio NAVAID raw data is available and indicates correct navigation to continue the approach by using the AP/FD associated to the non-affected FMGC side."

The question is: why in a RNAV approach (which is more critical than a PRNAV STAR) I'm allowed to check if one FMGC still has a good position to continue the approach, whereas in a PRNAV STAR, Fcom says I just have to inform ATC of the loss of PRNAV capability?

Last question: why in the SOP for RNAV GNSS approaches there's no guideline in case of "Check IR 1(2)(3)/FM position" message on MCDU whereas there's a guideline for this in case of PRNAV operations?
Thanks!

[737Jock]

Because it's not an RNAV GPS approach, but an RNAV overlay of a conventional approach.

So when you get FMS1/FMS2 pos dif, and you can continue the approach on basic rawdata navigation. Then you are allowed to continue the overlay approach with the correct FMS.

In case of PRNAV STAR, you are doing a PRNAV (gps) procedure that has no navaid backup.

[FoxForce44]

Last question: why in the SOP for RNAV GNSS approaches there's no guideline in case of "Check IR 1(2)(3)/FM position" message on MCDU whereas there's a guideline for this in case of PRNAV operations?

Well, remember that the aircraft has "figures of merit" to consider the most accurate aircraft position : first, of course, GPS position received for MMR's, after comes the Radio update crossing DME DME, DME ILS....and finally, GPIRS position. So, during the PRNAV the FMS uses the most accurate (MMR), and it's the ONLY allowed source of data on the procedure (at least of all the procedures i've seen).

The use of precise signal is SO important on PRNAV procedures, that all procedures request the deactivation of the RADIO UPDATE option, to ensure that only MMR's data will feed the FMS's.

[FE Hoppy]

PRNAV - RNP 1 sids and stars - GPS/DME/IRS coasting

RNAV (GNSS) - RNP APRCH - Final Segment of approach - GPS only