I'm not math and science-y, but I can take a crack at it.

DS7's plan won't work because you're moving at the same velocity as the plane when you jump. So unless you can jump in the opposite direction at the same speed the plane is going at when it's crashing, you're still hitting the ground HARD. Humans can't jump that well!

BTW, "Mythbusters" tried this, only with a falling elevator. (Same principles in action, though.) I think they found that jumping didn't even help with that, and the speed would be a lot slower because you wouldn't have time to reach maximum velocity, as you would presumably do with a plane.

Kriston