FAA and JAA -400s have different FMC C of G default values, JAA is 8.5%, FAA is 20%, So FAA -400s can show a higher performance limited Max CRZ ALT.
We occasionally update the FMC using the current C of G to show a new max alt closer to the FAA types, usually 1000' or less improvement. We could I suppose also just put in the FAA default value.
Only used when you're limited by the FMC Max CRZ ALT and you want a level that is for example only a couple of hundred feet above it, as opposed to say being sent down several thousand feet by ATC.
The times I've used the procedure can be counted on one hand.
Calculated by using the MACZFW and a fuel load correction.
Edit : Correct FAA value added.
Last edited by SMOC; 23rd Apr 2011 at 05:45.