I don't see any practical use for it while you're in the air...its more of a planning tool. I guess you could use it to double check dispatch to make sure that they gave you a legal T/O alternate...other than that I can't come up with anytime's you'd use it...It calculates it based on yoru groundspeed...but since it's doing so in still air it just takes your indicated airspeed, and corrects it for all errors, tepmerature and pressure, crompressibility...etc...so the short of it is...if it comes up with a number like 200kts...then its going to say that you need a T/o Alternate within 200 NM of your departure airport. Then once an airport is selected it will do its fuel calculations and tell you how much fuel will be burned on the way to the alternate. Thats about all i remember about it