FS2004/FSX – Ascent Descent Calculator. This is a simple calculator built with ascent, descent and energy management in mind. It calculates when you need to start your descent and how quickly to descend/ascend. It uses the 3:1 rule. Version 1.2 is for Windows XP/Vista/7 (not tested with 8) and uses a new visual style; Windows 8, this makes it easier to use and generally just looks nicer. By Rupert Smith.