High Altitude Black Out (Hypoxia)

If you are experiencing a dark screen at high altitudes, it is likely due to hypoxia, a lack of pressurization in the aircraft. In order to prevent this from occurring, you will need to carry out one of the following options:

• When you start your flight, pressurize the aircraft accordingly depending on the aircraft's instructions -OR-
• In the General Settings screen, turn off the feature to simulate blackout, redout, and hypoxia effects. 

If you are interesting in learning more about each aircraft's individual pressurization system, this information is present in the respective Pilots Operating Handbook found in the aircraft's main folder in X-Plane 11 -> Aircraft.