Well, the hardest thing is landing, followed by taking off. Although I guess certain in-flight emergencies would really be the hardest - mainly during landing or take off. The hardest thing they had to do was fly them straight and low enough to hit the buildings.
Relative to the air it's flying through, yes. Relative to the ground? Not so much. And that's what matters most when transitioning from or to being a ground vehicle (takeoff/landing).
