My first and only time in working with .air files was for Eaglesoft's Citation X version 2.
I had a heck of a time reworking it's performance data to get the aircraft to match performance data tables. The hardest part is that when I input the weight and balance data in the .cfg file... the aircraft wouldn't sit correctly on the runway. However, we were able to adjust the contact points and get the aircraft to react correctly based on weight and balance.
It provided for a rather realistic behavior in the end. :)