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.
The failure simulation in the DC-9 Classic is not just isolated to the one system or component set to fail. It goes much deeper than that. Certain failures can actually trigger a series of failures if you do not properly handle the initial problem. In many real accidents, there is a clear chain of events leading up to the accident. Break that chain by properly dealing with one of the events potentially leading up to an accident and you save the day. 
The failure simulation system in the DC-9 Classic allows you to practice how to respond to different types of situations that can, and often do, arise at the worst possible moment. This is exactly why professional pilots regularly spend hours in the simulator practicing and maintaining their skills so that when one day the unthinkable does happen, they can properly deal with it and bring everybody down safely.
The DC-9 Classic simulates 140+ individual system failures and situations in 3 categories of emergencies and 14 categories of abnormal situations. You can set and activate as many of them as you like at any time. For each failure item you first set a trigger situation. For example, you want the failure to occur at lift off, climbing above a set altitude, passing a marker, when selecting gear down, etc. You can then set a delay parameter for each failure item. For example, you want the failure to trigger exactly 5 minutes after climbing through 1500 feet. The delay can also be set to randomly trigger within the timeframe given. For example, the failure should be trigger within 15 minutes after climbing through FL240. To further mix it up, you can also set an overall probability for the failure to trigger at all.
You can simultaneously set a number of different systems to fail at different times. You can set them up as a chain of events, thus creating more complex failure scenarios. However, some failures will automatically lead to more failures if not handled properly. For example, you have a simple malfunction of an instrument. You find that the circuit breaker for that instrument has popped. You reset the circuit breaker. It pops again. You reset it again. All seems fine for a while. Suddenly you have smoke in the cockpit! Electrical fire!
The DC-9 Classic comes with a number of ready-to-fly failure scenarios, but you can also create and save your own scenarios. Once you have setup all the individual failure items, including how and when to trigger them, save the scenario for later use – just in case you don’t nail it the first time around. Practice makes perfect. The failure scenarios files can also be shared with other DC-9 Classic pilots. Maybe you want to create an “exciting” scenario for your friends to fly. Hey Bob, see if you can land this one! 
Using the random delay and an overall probability on the trigger for the failure, you can create scenarios for yourself, and others, where you don’t know exactly what is going to happen before taking off. Create a scenario with 10 or 20 failure items set at low probability and you might only get one or two during your flight – but you don’t know which two, or when, until the heat is on.
The DC-9 Classic Aircraft Operating Manual has detailed descriptions of the procedures required to troubleshoot and handle all the simulated failure situations. You will find the appropriate sections of the Emergency and Abnormal chapters indispensable when faced with some of the more challenging failure scenarios the DC-9 Classic failure system can offer you.
