Prone is a condition to which a creature may be subject.
Conditions alter a creature's capabilities in a variety of ways … Most conditions … are impairments, but a few … can be advantageous.
A creature subjected to the prone condition gets a defense bonus against ranged attacks:
An attack roll against the creature has advantage if the attacker is
within 5 feet of the creature. Otherwise, the attack roll has
disadvantage.
at the cost of its own
disadvantage on Attack rolls.
and half of it's speed.
Now, you can elect to fall prone at any time during your turn, for free.
You can drop prone without using any of your speed.
And while
Combatants … lying on the ground … are prone.
The rules never specify that lying on the ground is a necessary condition. On the contrary, the designers have clarified that
You can almost always be knocked prone. About the only time it's
physically impossible for you to be knocked prone is when you're
affixed to something that keeps you upright.
and
If you're knocked prone underwater, you're subjected to the effects of
the prone condition as normal. One way to visualize it is that you're
floundering.
And even, specifically with regard to hovering
Q: If a creature is swimming, hovering or is otherwise unable to fall
(e.g. chained to a wall), does knocking them prone actually inflict
the prone condition?A: If it's physically impossible for you to be
prone, you're not subject to the prone condition.
Certainly, the ground couldn't be a necessary condition, as that would imply that you would be immune to the prone condition so long as you lie on a table or bed–or ad absurdum, a bedroll. It seems like we must conclude that the Prone condition is not literally an orientation to the ground, but rather is a mechanical game condition that takes on different flavors in different media.
Furthermore, it appears that, both RAW and RAI, you can be subject to the prone condition, regardless of the medium, be it suspended in a hammock, on top of a table, lying underground, floundering in water, hovering in the air, or the drifting through the void of space. And you can elect do this voluntarily–at no cost to your movement.
Specifically, with respect to flying, the rules address scenarios where a flying creature may be subject to the prone condition.
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 the fly spell.
Allowing for a flying creature to remain magically aloft or hover while prone.
XGTE further specifies that a creature,
by spending half its flying speed
may
counter the prone condition (as if it were standing up in midair).
(XGtE)
So, it seems to me that I can elect to go prone while flying (at least, so long as I am held magically aloft or can hover) AND gain a defense bonus from ranged attacks.
It seems generally accepted that this sort of strategy is acceptable on the ground, so, is this interpretation correct?
Best Answer
Yes, and there is nothing wrong with it, unlike what others seem to think
My archery characters spend most of their time in combat as far away as possible prone, they stand, shoot and drop again on their turns. This works because prone makes them harder to hit.
But why does it make them harder to hit? There are 2 types of answer. 1: that is what the rules say. 2: because being prone presents a smaller target and is therefore harder to aim at.
Either of these arguments works perfectly well in flight. You move position so you present a minimal cross section to those trying to shoot at you. Not only is it supported by the rules you outlined, but it makes perfect sense to try and fly like that, it is essentially using your movement to fly defensively rather than dodging, which is how real life works too.
If you were directly above someone I would likely rule otherwise because you are then technically giving them more to aim at when you lie flat, but as long as you are at a reasonable angle to say that prone does reduce your cross section I can't see a rules or logic reason to disallow this at all.
Nor do I think this is an edge case, this is simply tactical play which some tables really enjoy playing with. Both sides simply standing around throwing attacks at each other is unrealistic, doing whatever you can to minimise chances of being shot with an arrow is clever.