DMG 47: A flying creature that is knocked prone crashes.
DMG 48: Crashing:
1: A flying creature that crashes immediately drops a distance equal to its fly speed. If it reaches the ground, it lands safely.
2: If it has not yet reached the ground, it crashes
3: A creature that crashes falls all the way to the ground and takes falling damage.
[Personal note: this is a really bad choice of words here. Some pedantic rules lawyer is going to argue that step 2 allows the creature to resume from step 1 of "crashing."]
Well I'll be. Thanks for the quick response!