If you don't maintain your aircraft well you might experience some failures during your flight on FSAirlines. At the beginning of a flight depending on the aircraft status the Client calculates if a failure will occur and when. If you have messages activated in the client options you will be notified when it happens (for those pilots who want to be sure where it comes from).
Currently the following failures are implemented but we plan to add more:
The gear is stuck and cannot be moved anymore (In FSX gear goes up automatically before it's stuck).
The flaps are stuck and cannot be moved anymore (In FSX flaps go up automatically before they're stuck).
One of the engines stops suddenly (In FSX it starts burning first).