Airbus Calculation of Final Path Profile
Hello everyone,
I have to admit that this hasn't been a big one for me as a 'classically-experienced' driver: the approaches I most usually fly are ILS or overlay VOR/NDB, where raw data are available and authoritative.
By that I mean, particularly for the overlay approaches, the not-below heights derive from primary altimeter and raw data fix. If the target profile has put you low against the raw data, well, you don't continue descent. Cold weather and nav accuracy without GPS spring to mind as possible factors.
But, as my new airline starts to do RNAV/RNP approaches, I need to revisit how the aircraft derives its target profile, to achieve the same sort of interdependent 2D/ALT crosscheck. I'm thinking of the old presentation slide where the FMGC defines its end-of-approach point as a fix and altitude and then projects a 3deg slope backward from that point in space.
What I want to know is whether the FMGC takes current primary altimeter QNH into account when calculating the fix and derived profile, or whether the FMGC looks at QNH set in the APP page. (I have my thoughts, but I'd like them confirmed...)
FCOM states that the APP page QNH is used for repressurisation profile, but seems (to my research, anyways) to be silent as to how it derives the target profile.
You can see how this becomes problematic when the GNSS approaches have no raw data fixes to trap errors in altimeter setting. (I know: they should be trapped by SOPs; but, but, but...)
This is actually the first part of a (possible) two part question. Thanks, in advance.
As an aside, it also makes a very good case for compensated altimeters in any Airbus, as of - I don't know - 30 years ago...