I'm really curious about the calculation of the possible climb gradient on departure. This question came up the other day when we departed from Chicago o'Hare Airport. In the departure plate it says the o'hare departure needs you to be in 5NM at 3000ft and in 8NM at 5000ft. How can you calculate if you are able to do it if you fly an e.g. B 777 or 787 etc. I check the AFM but no answer ...
Since I fly 767/757 we never worry about this stuff because we know they have A321s doing the same thing. Sorry couldn't resist :-)
SIMPLIFIED:
Let's just say you are climbing out at 150 knots, that's 2.5 miles per min, you will reach 5NM in 2 minutes so you need 1500 FPM.
If I recall in the Airbus, if you put the constraints in the FMS, and you cannot make that constraint, the Airbus will call you a retard, OK kidding again, but it will turn amber or red, a circle around that fix.