The DC-9 is equipped with the Sperry SP-50 Automatic Flight Control System (AFCS), which is a gyroscopically controlled electromechanical system. The DC-9 is an old school aircraft and the autopilot installed in this aircraft is very much old school too. If you are used to flying with modern autopilots, you will find that this autopilot works a bit differently than the other autopilots you may be familiar with.
The Automatic Aircraft Configuration feature, or Auto Config for short, has become one of the signature features of my aircraft. The idea for it came as the answer to the question which has spawned a lot of the features in my aircraft; what would I like to see in a product that I would buy?
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.