With so many aircraft designs flying around, why choose to develop a DC-9 for FSX? It’s a cool aircraft! What more do you need?! Well, actually the logic behind choosing to develop the DC-9 goes a bit deeper than that. I will try to explain.
Failure simulation. This is what in my opinion takes the DC-9 Classic, and all other addons that includes such a feature, to the next level. Usually when you simulate a system in an aircraft, you simulate ON or OFF in a normal state. With failure simulation a system can also be simulated ON or OFF in a failed state. While OFF is usually OFF regardless of state, ON and failed at the same time is where it can get really busy real fast on the flight deck.
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.