Situation:
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Alice the Aarakokra sees Bob the Battlemaster coming. She ascends to 605', just out of the 600' range of Bob's longbow.

Bob (Str 14) performs a running high jump 5' into the air and successfully hits Alice with his longbow, despite firing at disadvantage for long range. He spends one of his maneuver dice to perform the Trip Attack maneuver, and succeeds on the contested roll to knock Alice prone.

Alice is now flying and prone, so she falls. Falling does 1d6 bludgeoning damage per 10' fallen, to a maximum of 20d6.

Questions:
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1. When does Alice start falling? It is on Bob's turn as soon as she's knocked prone, or some time else such as the beginning of her own next turn?
2. When does Alice hit the ground? On Bob's turn as soon as she's knocked prone, or some time else?

These answers determine whether anyone can save Alice from her 20d6 of certain doom.

* Is a reaction (e.g. casting *Feather Fall*) on Bob's turn the only
hope?
* Can Alice's ally Charlie swoop in on his subsequent turn within the
same round to catch her?
* Can Alice spend half her movement on her next turn to "stand from
prone" and pull out of her involuntary dive?
* If Alice had expected to be shot down, could she have spent an action
to be Ready to "stand from prone" while moving through Bob's
airspace?