To put it bluntly, I find the vertical deviation calculated by the FMGC useless for the later part of your approach.
If it is your companies policy or preferred technique to extend the runway centerline once on vectors, the vertical deviation again is useless since it calculates a profile from your current position directly towards the CF without taking in to account the realistic amount of track miles, which you as a pilot can guesstimate with the help of TCAS, or even better, ATC might just tell you. Often your TCAS gives you a good clue as to how many people are ahead of you on finals, and if there is nobody ahead of you most controllers will give you at least a 7 or 8 mile final, if not 10.
Also, most people activate the approach once they start using selected speed, if ATC wants, say 220KTS. With the approach activated the vertical deviation is again useless since it works out a descent at green dot speed, if you're still doing 220 or 240, or whatever, then in open descent you're going down a little bit quicker then at green dot.
The FMGC also works out your descent profile with only half the headwind you're experiencing and double the tailwind, it also assumes the engine anti-ice is switched on and assumes a thrust value for that, which equally is more often then not the case.
The Airbus profiles are definately safe and conservative, but I find it a total waste and poor airmanship to fly a descent with thrust on, which is what you'll often get when you go down at, or a mile or two, before the descent arrow, hence again I find most people will fly past the descent arrow to force the autothrust to idle. Next time you fly go 10 / 15 miles past the arrow and pull open descent, you'll be on profile again around 20000ft, which proves my point.
What I find works best is track miles to run x 3 + 10% (1NM = 330ft on a 3 degree slope) + airfield elevation to get a mental model of a 3 degree slope, then you add whatever you feel comfortable with for your deceleration etc, I would say 5-10 miles depending on how heavy you are and which Airbus you're flying.
This has served me well on the 319/320/330/340 and 380.
Last edited by PGA; 23rd April 2013 at 15:20.