You fire as if one hex closer,(being above or below increase target size) but take the deflection modifier if firing from directly behind or ahead.
I like this one! But who will be at an advantage, the one at higher or at lower altitude?
Only the firing a/c has the advantage in either situation. The target a/c is presenting more surface area to the firer than if they were in the same plane. Mostly more wing and tail area are visible, but any shot out of plane is automatically deflection since coming down or going up, the target a/c is moving across the plane of movement of the firing a/c.