✦ Knocked Prone: A flying creature that is knocked prone crashes.
Crashing:
✦ Safe Distance: A flying creature that crashes immediately drops a distance equal to its fly speed. If it reaches the ground, it lands safely.
✦ Falling: If the flier has not yet reached the ground, it crashes.
✦ Crashes: A creature that crashes falls all the way to the ground and takes falling damage.
What I don't know is if the falling damage is based on the total distance or the total distance minus the fly speed of the flier.