Why do you have to open the pneumatic crossfeed valves to de-ice the wings? Exactly what does the AC bus cross-tie switch do? And why do the engines only draw fuel from the center tank when all the fuel booster pumps are on? The key to understanding these and many other aspects of your aircraft is studying the schematic drawings for the various aircraft systems. That may sound a little bit dry, but to help you out I have included a Schematics section in the DC-9 Classic Flight Center that gives you live interactive schematic drawings.
I was recently ask why Coolsky doesn't develop for and support Lockheed Martin's Prepar3D flight simulator. The discussion quickly turned into one about the future of flight simming since P3D is often held up as the future of flight simming. I will try to explain here why I don't develop for or support P3D, and why I don't think it is the future of flight simming.
Radio navigation is the name of the game when you are flying a DC-9. Radio navigation, like all new things in life, can be difficult to understand at first. But once you have mastered it, it becomes second nature and you will have a much better mental picture of where you are, where you are going, and how to get there.