The hang glider just sucks. It did in the last far cry too. In truth, they suck in real life. So the game is very accurate. Snake in Escape from L.A. just doesn't happen.
To make the glider work easily you would have to edit a lot of game files, core physics and stuff. Not an amateur job.
Well it's just that I have flown hangliders and enjoyed it. I also fly Ultralight Aircraft and am a Basic Flight Instructor, so I like the illusion of flying as well as the reality.