When does a flying creature start and end falling?

Question

Situation:

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:

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?

Answer 1

Xanather's Guide to Everything now clarifies this in two ways. Firstly, it provides an explicit explanation of the RAW and provides an optional rule for long falls.

RAW - Immediate fall & immediate damage

The rule for falling assumes that a creature immediately drops the entire distance when it falls. (XGtE)

So by standard rules the answer to your questions are:

1. Alice starts falling on Bob's turn immediately after the trigger that causes her to fall
2. She immediately falls the entire distance and takes the damage (barring some intervention that halts her fall).

As for your follow up questions:

• Is a reaction (e.g. casting Feather Fall) on Bob's turn the only hope?
  • That or a readied action are about the only ones I can think of right now
• Can Alice's ally Charlie swoop in on his subsequent turn within the same round to catch her?
  • No, she has already fallen
• Can Alice spend half her movement on her next turn to "stand from prone" and pull out of her involuntary dive?
  • No, she has already fallen
• 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?
  • Yes, you can prepare an action this way to use half your movement to stand up from prone.

Optional RAW - 500ft/turn & End of her next turn

However, XGtE also provides an optional rule for long falls that says:

When you fall from a great height, you instantly descend up to 500 feet. If you’re still falling on your next turn, you descend up to 500 feet at the end of that turn. This process continues until the fall ends, either because you hit the ground or the fall is otherwise halted. (XGtE)

In this case, the answers to your questions are:

1. Alice starts falling on Bob's turn immediately after the trigger that causes her to fall (same as in the RAW case).
2. She falls 500 feet instantly which still leaves her 105 feet in the air. On the end of her next turn she will fall the rest of the distance if she does nothing, but allows her to presumably use her turn to take preventative action if she so desired and was able.

As for your follow up questions:

• Is a reaction (e.g. casting Feather Fall) on Bob's turn the only hope?
  • No since she will take her next turn (as would anybody else who was there) before hitting the ground
• Can Alice's ally Charlie swoop in on his subsequent turn within the same round to catch her?
  • Yes! Though the mechanics of how this would actually work would be up to the DM as catching a falling person is going to not be an easy task!
• Can Alice spend half her movement on her next turn to "stand from prone" and pull out of her involuntary dive?
  • Yes! At the beginning of Alice's turn per:

    But if that creature starts any of its later turns still falling and is prone, it can halt the fall on its turn by spending half its flying speed to counter the prone condition (as if it were standing up in midair). (XGtE)

• 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?
  • Yes you can. Same as with the standard RAW.

Which method is best for your group will be up to your DM, but personally, I find the optional rule to be much more intuitive and reasonable an answer at my table.

Answer 2

As you've alluded to in your question, the RAW as far as they go are:

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 is being held aloft by magic, such as the fly spell (PHB 191)

At the end of the fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell... (PHB 183)

The gap in the RAW is what happens between it falling (ie the start of the fall), and the end of the fall. The DMG does not help - the rules on flying on DMG 119-120 say nothing about this situation.

As the RAW are silent we're in the realm of DM's discretion. I think if the creature was at 10 or 20 feet, this question would not arise, and the creature would take bludgeoning damage immediately after falling prone. But at 600 feet it falls to the DM to decide if the creature has a round or more of falling during which it could recover from being prone.

In any case, the prone condition is gained on the attacker's turn, as is the case in any combat, and so the creature falls (starts to fall) on the attacker's turn.

Answer 3

From the rules:

At the end of a fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell, to a maximum of 20d6. The creature lands prone, unless it avoids taking damage from the fall. (PHB p.183, "Falling")

and

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. (PHB p.190, "Flying Movement")

As I read this, where specific beats general, knocking a flying creature prone does not give it the Prone condition; it causes it to fall instead. If that fall results in impact with the ground then the creature is no longer falling and, if it takes damage, has the Prone condition.

There is nothing in this that indicates how long it takes to fall so this is up to the DM.

For guidance, some elementary physics, the distance an object initially at rest falls in a vacuum is one half the acceleration due to gravity times the time squared.

For earth g=9.8m/s/s or 32 ft/s/s.

So after 1 second it has fallen 16 ft, after 2: 64, 3: 114, 4: 256, 5: 400 and 6 (or 1 round) 546.

Given that standing from prone consumes half your movement (about one quarter of your activity for the round) you will fall about 50 feet even if you can commence recovery instantaneously; but you cant commence recovery until your turn which is clearly sometime (but not more than 6 seconds) after the event that caused you to fall. Feather fall works because it is magic.

However, damage caps out after a 200 foot fall in D&D physics. If we assume that this represents terminal (possibly literally) velocity then, in round figures, you reach this speed after 3.5 seconds and are then travelling at about 110ft per second. Adopting this for really long falls means that you fall 475 ft in the first round and 660 feet per round thereafter.

Answer 4

You start falling when you go prone. The only part of the RAW that covers this is a very brief section on page 183 of the PHB, from it we can assert this:

1. She starts falling immediately, on Bob's turn.

2. She hits the ground immediately as well, on Bob's turn.

There's not a lot to go on here, other than that, falling happens, and that there is no time so it's assumed to happen immediately.

Unlike in previous editions (at least in 4e), 5e contains no provisions for multiple turn falls (though you may rule that certain falls do take place over multiple turns, see addendum).

So basically, you, in an instant, fall all of the distance between where you are and the ground. The only thing that can save you is (As you say) a reaction cast of feather fall or similar.

Addendum, since this is attracting downvotes:

99% of the time, multi-round falling is completely and utterly irrelevant to D&D. For instance, in the example presented in the OP, the Aarokockra is already nearly out of combat effective range, and as part of the example, completely removes herself from it. That's not a useful strategy unless you're running away (you're out of range of a lot of spells, and you're definitely out of effective range for weapons). So for 99% of situations, you can say that the fall is immediate.

The times where this actually does come up is more for mid air battles; something the rules already don't cover well. Thus if you're running a mid air combat, you're already going to need some written house rules in order to run it effectively. Depending on what you expect to happen, you may need to specify how many feet you fall per round. For this you could do the math (remember to factor the proper gravity coefficient for your setting), or borrow a number from a previous edition (4e uses 500' per round). Whatever you choose, it should be more than 60' per round as this is what feather fall uses, and it should likely also be more than 200' per round as this is what you take for falling damage when you hit.

All this to say, if multi-round falling comes up in your game, you're already probably into the domain of house rules with in air combat, so rule accordingly.

Answer 5

As other people have pointed out, per RAW, there are no set rules for actual fall speed outside of certain spells which are meant to slow your fall speed.

Also, something I don't think anyone else has picked up on yet:

At the end of the fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell... (PHB 183)

This actually would imply that it's not the fall that kills you, it's stopping it. So if a player stops their fall instantaneously, you'd better believe that the damage rules still apply to them. Anyway, since the actual falling bit is what's missing, I'm going to derive some rules from real-world physics.

A few simple mathematics (through the SUVAT equations and assuming gravity is roughly on par with that of the Earth's surface) lead to some helpful results (calculations are using metric, actual rule options are given at the end:

In 6 seconds, a you fall ~180m (580ft.), assuming a gravitational constant of 9.8m/s/s. You also reach a speed of 58.8m/s, which is convenient because it is approximately terminal velocity (actually 56m/s.) Having reached 58.8m/s, you then fall at ~360m (1160ft.) every 6 seconds. However we're not quite done here:

Let us assume that the start of any falling will happen, on average, in the middle of a round, giving the players 3 seconds of falling until they can start taking normal actions. In 3 seconds, you only fall 44.1m (145ft.) and reach a speed of 29.4m/s. Then in the subsequent 6 seconds, you fall 264m (865ft.), reaching terminal velocity 3 seconds in. From then on, you fall 360m (1160ft.) per round.

Now then! Let's round out those numbers and tie them into some meaningful rules (we'll round down the later ones since we're actually slightly above terminal velocity):

1. In the first round of falling, a falling creature descends 150 feet.
2. In the second round, they descend 850 feet.
3. In each round after the second, they descend 1150 feet.

These rules assume that if a creature starts falling, it will normally happen roughly in the middle of a round, so they only fall for 3 seconds before they or another creature will be able to use their action to help, rather than assuming that they fall for an entire round. (The downside is that the 20d6 fall damage cap doesn't make sense then, as you fall ~580 feet before reaching terminal velocity, not 200 feet.)

BONUS ROUND:

I also did some maths and worked out some physics based off the 20d6 fall damage cap, treating it as if D&D worlds have thicker atmospheres. Works out to a terminal velocity of 34.6m/s, significantly lower than the real world. Ends up a bit simpler, because you actually pretty much reach terminal velocity within 3 seconds.

1. In the first round of falling, a falling creature descends 60 feet.
2. In each subsequent round, they descend 680 feet.

Answer 6

Rate of Falling (Xanathar's Guide to Everything p.77)

When you fall from a great height, you instantly descend 500'. If you're still falling on your next turn you descend another 500' at the end of that turn. The process continues until the fall ends

Your case is defined as: you are flying at a height of 605', cannot hover and fall due to being knocked prone.

1. You fall 500' at the point you are knocked prone.
2. Your fall does not end as you are still 105' above the ground
3. Other people take their turn (if any)
4. On your initiative order you take your normal turn, and can:
   • "stand up" (taking half your movement) and your fall ends at 105' above the ground (you don't have to but this is a good idea unless you are immune to falling damage!)
   • have half your movement and the rest of your normal actions