The rules state that "At the beginning of the next turn, you can move in a different direction than you did the previous turn without making a check." If at the start of the turn the flier chooses to turn 180 degrees, would that still cost the 10 movement? And is that 10 feet of movement just turning 180 degrees, or does that include the 5 feet of movement back in the direction it started from? (I'm assuming turning a full 180 degrees uses the full 10 move before actual movement, but want to make sure.)
No movement cost or Fly check needed if changing flying direction at the beginning of the character's turn, in same manner as a character moving on the ground. The movement costs and Fly checks only apply when the character attempts to make a move other in a straight line during his movement.
For example, a flying with flying 30 ft can move forward 60 feet as a full round move in a straight line, and turn 180 degrees at the start of its next turn to return to its previous position, all with no fly checks.
However, if the same flying wants to turn 180 degrees during its turn while moving, let's say after 30 feet (move action) to return to his starting position with its other move action, that character would be required to speed the movement cost and make the Fly check. If it fails, it continues to move forward instead of making the turn.