Answering Questions: Flying and Grappling

Grappling in Midair

Question: My understanding is that one of the rules that can be used is roughly " a creature with a fly speed and not a hover speed if knocked prone falls 500 feet instantly" or something like that.

I am curious if the same thing would happen if the same creature were to be instead grappled, reducing its speed to zero, and then let go, or would they immediately regain their fly speed and not fall because they are no longer under the grapple condition?

First if you are simply looking to knock someone out of the sky you are WAY better off using the Shove action instead of attempting to Grapple. Here is why.. The shove action gives the target the prone condition and thus they fall per the rules.  

Lets set up the scenario here.

Two creatures (A & B) are flying and A decides it will be funny to watch B fall to its demise. A will have to make an unarmed attack to either grapple or shove B. 

If A succeeds its unarmed attack it can inflict the Prone condition on B and causes B to fall. 

Flying - A variety of effects allow a creature to fly. While flying, you fall if you have the Incapacitated or Prone condition or your Fly Speed is reduced to 0. You can stay aloft in those circumstances if you can hover. See also “Falling” and “Fly Speed.”

Unless B has a reaction it can take it falls and takes 1d6 per 10 ft up to a maximum of 20d6. (Ouch!)

Now lets set up that same scenario where A opts to Grapple B instead of knocking them prone. 

A swoops in and succeeds in making the grappling attack. B has the grappled condition (PG 367 of the 2024 PHB) and has the following imposed on it as a result.

Speed 0. Your Speed is 0 and can’t increase.

Attacks Affected. You have Disadvantage on attack rolls against any target other than the grappler.

Movable. The grappler can drag or carry you when it moves, but every foot of movement costs it 1 extra foot unless you are Tiny or two or more sizes smaller than it.

Unless A has a second action, it can't let go until its next turn. So A is effectively holding B in the air until its next turn. (GM Might want to check to see if A is strong enough to do this. But we will assume A is.) If it does have a second attack action it can end the grapple and B drops like a rock. No movement, no flight.

If it doesn't have an action B gets to take a turn and has a few actions it can take. 

1. B can try to break free, if it succeeds the grappled condition is ended and flight speed is restored so B doesn't fall.
2. B might suspect what A is attempting to do and can spend its action to ready a movement. The triggering condition will be "When A lets go, I fly X number of feet away from A." (PHB PG 372)
3. B might decide, that if they are going down, they will take A with them. B makes an unarmed attack to grapple A and if B wins, A sees their movement reduced to zero and both A and B fall.