F/A-18 Hornets flying toward each other have an extremely fast closing speed.
Air show photography tip: Simply tracking one of these planes with a telephoto lens is challenging. Shooting a crossing in one-shot mode is much more difficult. Using a high-frame-rate DSLR in high speed burst mode and focusing on tracking one aircraft makes this challenge more easily accomplishable.
I also use AI Servo focus mode with all AF points active if the sky is the only background. If the planes are flying in front of other objects, a specific focus point may be needed to prevent the background from catching AF's attention. Using a specific AF point (or single AF with AF point expansion turned on) makes accurate tracking of the planes more critical.
In this example, I should have been tracking the front-most plane to have more sharp image.