irdeggman said:
Pg 322 of the MM (Primary source rule) covers what a Fly Speed is.
And it states that:
“A creature with a fly speed can move through the air at the indicated speed if carrying no more than a light load.”
So yes they do conflict with a primary source and hence apply to the conditions specified (MoP applies the the planes).
You quote is accurate. However nothing in that quote addresses what happens when you try to fly through something other than air.
"Air" is "A colorless, odorless, tasteless, gaseous mixture, mainly nitrogen (approximately 78 percent) and oxygen (approximately 21 percent) with lesser amounts of argon, carbon dioxide, hydrogen, neon, helium, and other gases."
Can you fly thorugh pure oxygen?
Can you fly through a poisonous gas?
What about in a vacuum (assuming you can otherwise survive)?
None of those are "air," that's for certain.
The rule is a persmissive, not restrictive rule. In other words, it tells you that with a fly speed you can move though the air. It does not say what happens in any other conditions, nor does it necessarily restrict flying from applying to other environments.
It does not tell you, for example, whether you can fly underwater at some penalty to you flying ability.
It say you CAN fly in the air. It does not say you CANNOT fly underwater, or in pure oxygen, etc.
The core rules simply do not address unusual applications of flying, such as underwater.