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.
Today I went plane spotting. I haven't done that in ages, so it was time. Plane spotting is a great pastime. It's fun and relaxing. Besides flight simulation and aviation, I also have a passion for photography. Plane spotting is a great way to combine those two interests!
Let's take a look at the overhead panel! "Hell yeah!", I hear you say. It is after all the nerve center of the aircraft. Nothing goes in the aircraft until you consult the overhead panel first.