Questions about flying and switching between modes of movement when flying [closed]

This is a compound question and could probably use being broken up in to separate questions:

1. A monster is flying along at a speed of 120 ft with a maneuverability of average in order to maintain its forward speed it must fly 60 ft forward. Rather than fly straight down (which is a form of forward speed) it instead chooses to fly horizontally for 60 ft and then fly down, because it's flying down its fly speed is now doubled to 240 ft. So therefore the total fly down movement it can achieve in one move action is 240 - 60 = 180 ft. Is this correct?

Then instead of flying the full 180 ft down it instead flies 30 ft down and levels off at angle of no more than 60 degrees (its maximum up angle). Note I believe this is not considered between down and up because it is not flying up it is simply leveling off.

So it's now traveled a total distance of 90 ft (60 + 30) and because it has now 'switched' back to its regular flying speed it can now travel a further 30 ft forward (120 - 90 = 30) before its move action must come to an end. Is this correct?

2. Is it legal by RAW to switch modes of movement within one move action? For example from flying to ground movement. Examining the monster manual page 309:

Flight (Ex or Su): A creature with this ability can cease or resume flight as a free action. If the ability is supernatural, it becomes ineffective in an antimagic field, and the creature loses its ability to fly for as long as the antimagic effect persists.

3. What happens if the monster has already flown 60 ft. It wishes and can land to continue moving along the ground but its land speed is 60 ft? Does its move action end there because it has already traveled 60 ft flying?
4. What happens when it has only flown 30 ft but wishes to land and can land and wishes to continue movement? Is it permitted to move on the ground an extra 30 ft for a total of 60 ft its total land speed movement thereby ending its move action? Page 20 of the Dungeon Master's Guide is silent on this and only refers to it flailing to maintain its minimum forward speed and thereby end its movement.
• Hi there! After reading your question I would agree with your first remark that it would probably be best to split this post up into multiples. It seems as if the core of your question(s) really is about how to properly calculate movement. If you were able to split up or boil down your questions into something more easily answered, it would help us provide said answer! I'd edit it myself but I want to make sure we really capture what you're after here. Thanks! Jun 18, 2019 at 13:25
• Yeah if you could edit it in a format acceptable to stack exchange that would be appreciated I'm relatively new to stack exchange so don't really know about the conventions used here. I will trust your discretion with the split, you seem to have a good understanding of the component parts of this question. Thanks! Jun 18, 2019 at 13:29
• I'd state B as an assumption, and keep the main question. Take C and D out completely. That leaves one stated assumption that can be challenged or run with, and a single question that has a definitive answer. E.g. "Assuming switching modes of movement within one move action, is the above calculation correct?
– GcL
Jun 18, 2019 at 13:32
• There is no need to split this up—if there were actual answers available to these questions, it might be best to have each one get its own question, but as it turns out, each of these questions has the exact same answer: there is no answer. The rules don’t cover any of this. Asking these questions separately is just going to require putting the same answer—that the rules don’t cover it—in more places. Jun 18, 2019 at 13:32
• @KRyan The post could, at minimum, use some cleanup. There are several distinct scenarios posed here, and even if they have no answer, they are distinct questions. However, they ARE quite similar and they address the same issues at their core, so I believe a clarifying edit is possible that can present this post as a single question while still addressing OP's concern. I'm not personally familiar enough with these particular issues that I could do this myself and be confident that I got the intent right, though Jun 18, 2019 at 13:37

1 Answer

There are no rules for how to measure your movement when how much movement you get changes in the middle of your movement. So when you change from horizontal flight to a dive, when you change from flying to walking or vice versa, and so on, are all completely undefined. I am as certain as you can be about this. I have looked high and low for these rules, I have checked every book I could think of—Player’s Handbook, Dungeon Master’s Guide, Monster Manual, Rules Compendium, Draconomicon, Dragon Magic, Races of the Dragon, I even perused the Rules of the Game articles—nothing. I have mentioned that complete absence in the rules several times on this very site, in fact.

I searched because it came up while I was writing alternate movement rules, and I felt that I needed to understand how the existing worked in every detail before I changed them. This particular detail very simply isn’t explained anywhere.

The suggestion I have seen the most people consider to be the most “fair” is to pro-rate your movement based on how much movement, proportionally, you have used. So if you have a 60-ft. flight speed and a 30-ft. land speed, and you fly 40 feet before landing, you have used ⅔ of your flight speed, meaning you should have ⅓ of your movement left—10 feet now that you’re on the ground.

The problem is that this is non-trivial arithmetic that you could potentially have to do every round. Every time you shift to a different speed, you have to calculate

$$d_\text{remaining} = \left(1 - \frac{d_\text{used}}{d_\text{old speed}}\right) \times d_\text{new speed}$$

That is just begging for mistakes to be made. Not every value for $$\d_\text{used}\$$ is going to be a small, neat fraction of $$\d_\text{old speed}\$$ like 40 is to 60, nor is the resulting fraction necessarily going to evenly divide $$\d_\text{new speed}\$$ as ⅓ does to 30. And if you shift speeds more than once it just gets more complicated.

So while this is the “most fair” ruling, it is a problematic one for gameplay. Maybe it’s OK if it comes up once or twice, but it is not going to work well if it’s happening several times a combat. In fact, if that were to happen, I would probably program a little web app to perform exactly that calculation so that we weren’t forced to try to do it in our heads or break out pencil and paper to do it by hand—but I’m a web developer who could do something like that in about five minutes, which isn’t true for everyone, obviously.

Failing that, you have to resort to something “less fair,” probably something like “if you have used less than half of your movement in one speed when you switch speeds, you can use half of the new speed.” Halving, rather than $$\1 - \frac{d_\text{used}}{d_\text{old speed}}\$$, is something you can probably reliably perform—and it doesn’t change every time, so you only have to calculate it once.

That, or you just ban it altogether, and say you cannot change speeds mid-movement. If you want to move before take-off, you have to move into position and then take-off next turn. If you land, you cannot move on the ground until next turn. And so on. Very simple to run—no math—and the complete lack of rules for this might imply it is “intended,” but it’s also very limiting in ways that seem very strange and don’t fit the narrative well at all.

I suppose you could go the other way and allow someone to just use each of their movement modes in full, but I have never seen anyone suggest that is fair—the fly spell already doubles most character’s movement speed, for it to instead potentially triple it is a bit, well, preposterous. I wouldn’t run things that way, certainly.

All of that said, as it turns out, I have literally never had this come up in any game I have run or played in. It seems like this thing that really ought to be covered and have rules, and that like those rules would come up, but they just... don’t. Flying is so good that there is just no reason to ever stop doing it, so once you can, you largely forget about your land speed. And swimming comes up almost never. My groups do tend to have a gentlemen’s agreement not to use burrow speeds, which would cause it to come up more, so that could be a source of problems here if you use them—but again, I suggest you don’t because they cause a lot of logistical headaches for the DM and can wind up ignoring large portions of the adventure.

• It seems like you have researched this question thoroughly in the past, so thanks for that. I'm going to give it a while before marking this question answered just to see if there are any further contributions. Jun 18, 2019 at 13:59