impossible angles for a full pitch and roll sim

[phaseform]

I'm in the process of building a flight sim with full roll and pitch, and I'm wondering what simulation software will do if I ask it to do something impossible. For example from flying level, pitch straight down, then roll 90 degrees, what will happen?! This is possible of course, this simply represents rotating both rotation axes 90 degrees, but if I were to pull up, that would be impossible, as my pitch axis is perpendicular to the orientation to the required pitch of the plane at this time!
Or the same problem, I roll 90 degrees from level flight, and while maintaining wings at 90 degrees to ground level, I yaw the plane to be facing straight down, same situation, I can't pull up in the sim from here?!
Obviously there is a solution to this in existing flight sims, but what?!