I tested that new build for Hover and Stable Hover and both now work as expected.

I've now moved on to testing Pop-Up.

Question: with Pop-Up engaged, should Alt-J/K adjust the altitude? They do at the moment.

The problem with allowing the height to be adjusted once Pop-Up is engaged, is I can adjust it down. So when I disengage Pop-Up (or Pop-Down again), it hits the ground if it's lower than the 20 metres that it moves by. Either guard against this, or don't allow Alt-J/K to adjust the height. I prefer guarding against it though, as I could pop-up too high and want to adjust it down slightly.

Another issue I found, when in stable hover, and I engage pop-up. If I then move the cyclic to disengage stable hover I can fly around and all is fine. But if I go back into stable hover, and hit Shift-P to Pop-Up, it goes down instead of Up as I was already "popped-up" before. This could end in a collision with the ground. Maybe reset the pop-up variable when hover disengages?