Like Cit says, you can fly it any way you like. Top to bottom or vice versa, count deaths, sorties, or not, its all up to you.
The challenge is to climb OUT of your ussual bird & into something different. Get 5 kills in it & move on to another. Want to make it tougher? Then try for 5 kills without dieing.
Get stuck in a plane? Skip it & come back to it later.