A320 FMGS des calculation in NPA

[che ci dò che ci dò!]

I couldn't find anything in the A320 FCOM concerning how the FMGS calculates the final descent profile in a NPA.
I believe final descent is calculated starting at MDA above either the RWY threshold or the MPA, whichever comes first, and going backward at the pubblished descent angle. This is why in certain "old style IAP" with MAP after the RWY, the FMGS' calculated descent profile has to be disregarded.
Did I get it right?

[rudderrudderrat]

Hi che ci dò che ci dò!,

It's hidden in OEB 31.
" This OEB replaces the A320 OEB 189.

This OEB is issued to provide Operators with the operational recommendations to apply in cases where the flight crew performs an RNAV or a LOC or LOC Back Course (B/C) approach with the MAP located before the runway (RWY) threshold.

This is because in such cases, the FMGC does not compute the vertical flight path correctly. As a result, it may cause the aircraft, when flown in managed vertical guidance, during an RNAV approach, to fly a vertical flight path lower than the published one on the approach procedure chart.

This anomaly also applies to the vertical deviation indication symbol, VDEV.

These recommendations were originally published in Refer to FCOM Standard Operating Procedure - Non Precision Approach section. Due to the fact that more and more RNAV procedures are being published in the Instrument Approach Procedures (lAP), Airbus found it necessary to publish this OEB in order to highlight these recommendations.
Applicable to:
All A320 family aircraft fitted with the Honeywell FMS."

[che ci dò che ci dò!]

Thanks for your answer, but I think this OEB (now in the LPC browser) explains descent calculation for RNAVapproaches only. Concerning other NPAs such as VOR, NDB, etc, I’ve found the explanation here below:



“When the MAP is located after the rwy threshold, an FPA (#0°) must be coded at the MAP. For these old style NPA with the MAP after the rwy threshold, and depending on the position of the approach axis relative to the rwy, FMGC guidance may start the final approach descent slightly before the FAF. In most cases, the crossing altitude difference at the FAF is not significant but, sometimes,this difference might be higher. Therefore, as it’s not acceptable for the use of FINAL APP mode, we recommend validating the IAP either in a simulator or with the a/c in good weather conditions.”



To my understanding, this means that final descent is calculated starting at MDA above either the RWY threshold or the MAP, whichever comes first, and going backward at the published descent angle.


Is this correct or do I miss anything?

[TyroPicard]

Surely it is calculated in exactly the same way as between any two waypoints which have altitude constraints? From alt to alt..... What you insert as MDA is irrelevant to the calculation.

That is one reason why you are not allowed to modify the profile for a managed NPA...

[rudderrudderrat]

Hi che ci dò che ci dò!

I've attached the original oeb-189-1.pdf (http://www.blackholes.org.uk/PP/oeb-189-1.pdf), which explains it better.

The FMGC calculates an approach from 50 ft above the runway threshold, backwards. In some cases, it mistakenly calculated from 50ft at the MAP - which if before the runway, would put you below the published profile - hence the warning.

[Microburst2002]

Such FMGS calculated NPA vertical descent profiles are the ones termed Baro VNAV, right?

[Bula]

Just get your airline to spend the money on the FMGC 1A Mod. Problem solved :)