When a creature with non-magical flying speed is knocked prone and falls, how do I determine the falling damage?

Question

Xanathar's Guide to Everything provides new (optional) rules regarding Falling

Assuming I want "a flying creature to have a better chance of surviving a fall than a non-flying creature does" how does that work?

In the formula, I have to subtract the "creature's current flying speed", but I thought it is either reduced to 0 (e.g. by Earthbind) or disabled (by the prone condition) otherwise it wouldn't fall down.

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_{I am not sure whether I am allowed to quote the rules text, so I guess only those owning a copy of XGtE are able to answer this question.}

Answer 1

In this section, it says

This rule is helpful to a flier that is knocked prone but is still conscious and has a current flying speed that is greater than 0 feet. The rule is designed to simulate the creature flapping its wings furiously or taking similar measures to slow the velocity of its fall.

Meanwhile, the rules for flying say:

If a flying creature is knocked prone, has its speed reduced to 0, or is otherwise deprived of the ability to move, the creature falls, unless it has the ability to hover or it is being held aloft by magic, such as by the fly spell.

... so, there are different ways to knock a flying creature out of the air. Note that the prone condition doesn't reduce your speed — and it's clear that that's the case that the new optional rule really applies to. In that situation, the creature can't fly and falls, but can flap or take whatever measures. The rule notes that if the fall continues for more than one turn (that is, more than 500 feet), a "prone" creature with a fly speed can recover. Other more powerful means which reduce the target's flying speed to 0 still mean a dangerous fall.

But, also note that Earthbind is not one of these more dangerous situations, since it explicitly gives a soft landing:

An airborne creature affected by this spell safely descends at 60 feet per round until it reaches the ground or the spell ends.

(The text does not specify "no damage", but it's implied by "safely" — and note that this is the same rate that Feather Fall provides.)

So, to directly answer your question: being prone has no effect on the calculation. As the optional rule says, subtract the creature's flying speed from the distance fallen and then calculate damage as normal (1d6 bludgeoning damage per 10 feet) for the remaining distance.

Answer 2

I haven't seen the full optional rule but prone doesn't reduce your speed. So if the optional rule is 1d6 for each 10ft of falling after their base fly speed you would calculate it like 0d6 for a creature with 60ft fly speed who fell 60ft or less. Then a fall of 70ft would be 1d6 bludgeoning damage, 80ft 2d6, and so on.

Now if a spell or other ability reduces the fly speed to 0 then they would take fall damage as normal since they don't have a fly speed at the moment of their falling.