I wasn’t really planning on making this a three part series, but based on the response to the two first parts I feel the need to address some of the comments I have received and tidy up a few loose ends, because the big elephant is still in the room.
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.
Being an old design, the MD-80 cockpit has what seems like knobs and switches everywhere. This can be a bit confusing and overwhelming for the pilots just starting to fly this bird. One of the most frequently asked questions we get on the support forum is "how do you start it up?". Well, this video tutorial seeks to answer that very question.