Inverted flight is when your flying upside down. You need to do this for a whole hour but it does not need to be done in one go. You can check your progress for this achievement by heading to pilot profile; under flight time it will have a section for inverted. While this can be done in the Stearman, the RV-6A or P-51D Mustang are recommended as they are much easier to keep inverted. To invert your plane, I recommend you first set your Camera to Freelook with F11. This will make it easier to tell when you are fully inverted and also when the aircraft is stable. Then, be at least 1,000ft above the ground and make sure the throttle is at 100%. If you are using it, disable the mouse by hitting SPACE, then hit either left or right arrow until you are inverted. You may go too far and start to turn back around. If this happens, just keep tapping the left and right arrows until you are perfectly upside-down. Once you are there, just keep tapping the up arrow to keep yourself at the same height.