Essentially yes I think your summary is correct. Re it "thinking" it is higher that it is - let me explain my logic and terminology.
Assume:
- Sea level airport
- METAR calling -20°C at the runway
- That's ISA-35
- Uniform atmosphere, and approach overwater.
- NPA FAF@ 2000' at 6.3nm from threshold (3°slope).
- Minima 250 (ignoring PEC) 0.8nm from threshold (I know there's another 50' over the threshold but it doesn't really matter here)
If you did
no temp correction, the aircraft would fly at 2000 on the FCU, but be actually around 1720' on the RADALT. (The 1720 is calculated by 2000-(4*35*2)). If you set -3° on the FPA, the aircraft "thinks" it's at 2000' (its getting baro info, not RADALT or geometric info) and would fly a profile to the threshold quite successfully (assuming it didn't hit anything on the way
). It, however, has not flown a geometric 3 degree slope, rather it has flown a slope of 1720' over 6.3*6072' which is about 2.57° geometric.
If you
temp correct:
- For the FAF: to have the RADALT at 2000' you'd need to set 2000+(4*35*2)=2280' on the FCU. Call this FAFcorr. The cold weather correction is 280' at the FAF
- For the minima: to have the RADALT at 250' you'd need to be at 250+(4*35*.25) = 285. Call this MINcorr The cold weather correction at the minima is 35' (the correction is less the closer to the ground).
When corrected, the aircraft "thinks" it's flying from 2280' (but is actually only 2000') over 6.3nm. It needs to tell itself that it is flying 2280' over 6.3*6072 which is 3.41° (but it is actually geometrically flying 3°). I would set FPA-3.4° on the FCU - and have the PM calling out high/low from the chart at each nm to the threshold. In practice you should use the delta alt / distance between the FAFcorr and the MINcorr, but for simplicity you get the drift if I've explained it well enough!
That said, I think the figures in the table above are slightly conservative. I don't know why (nor why they are not in the A320 FCOM) The manuals seem to be updated in late 2017 for the A32F and interestingly not in the Main FCOM/FCTM changes documents from that period that I can find.