Mutants and Masterminds 2E uses the standard d20 system definition of Prone, as follows.
Prone: The character is lying on the ground. He suffers a –4 penalty on
melee attack rolls. Opponents receive a +4 bonus on melee attacks against
him but a –4 penalty on ranged attacks. Standing up is a move action.
3E and DCA use a similar definition:
A prone character is lying on the ground, receiving a –5 circumstance penalty on close attack checks. Opponents receive a +5 circumstance bonus to close attack checks but a –5 penalty to ranged attack checks (effectively giving the prone character total cover against ranged attacks). Prone characters are hindered [1/2 movement]. Standing up from a prone position is a move action.
Both systems include a feat called Prone Fighting which removes the melee penalties to attack and defense, 2E also adding the ability to move at half speed while crawling instead of 5' per round, something standard in 3E Prone with or without the feat.
So, as can be seen in the description, there is nothing that prevents a flying character from being Prone, or having the Prone condition applied to them (in fact, in 3E, the Affliction power includes Prone as a 2nd tier effect). What should happen then? I know D&D 4E clarified that a flying creature rendered Prone is defined as crashing into the ground. And apparently, the 3.5 D&D Rules Compendium stated that "A winged creature can be tripped, and if it is, it falls as if it didn't maintain its minimum forward speed". I don't think either automatically applies to Mutants and Masterminds.
Related, should a Flying creature still have to spend a Move action to remove Prone (or use Acrobatics or Instant Up to do so as a Free action)? Does the answer change for Burrow or Teleport?
Best Answer
I do not have the books on hand, and I will assume you are the Games Master of your game in this answer.
Yes, flying creatures can be made "Prone". Even though they are not lying flat on the ground (since there is no ground), you can think in the line of the 3.5 rule:
A flying creature with the status of prone has lost control of flight. If it is not buyoant, it will begin falling to the ground. There is no rule on how fast it falls (if it is some parachute effect from its wings/wirlwind/etc). It can spend a move action to recover control (stop falling and losing the bonuses).
On the buyoant thing, Air elementals, beholders, any levitating monster, djinn and the like can be considered buyoant, even though they are heavier than air. For spells and effects, its a bit trickier, check the specific description. Also, if the creature is not buyoant but do not have a minimum forward speed, it can still fall, but won't fall as fast as a rock.
This creature ALSO changes position involuntarily. Melee opponents will have to spend move in order to keep melee range, BUT they probably can't keep up with the speed of a falling creature unless they are crazy enough to allow themselves to plummet also. I don't have the books here, so I am unsure if it should provoke an AoO.
Ranged attacks still suffer the penalty. The flying creature is flailing around trying to recover control of flight, so its harder to hit.
The prone/falling creature could use a feat or skill to recover the flight faster. If I was a flying character, I would like to regain my standing as a free action.
It is like the pixie in "Dust: an Elysian tail" said:
I am unsure what you meant by burrow or teleport. If the creature can teleport while flying? probably yes, but there is the concentration issue. I think that for a falling landlubber (elf wizard had flight dispelled), I would call some sort of check to see if he keeps the cool enough to be able to cast.
Not related to D20 or M&M, but in an Earthdawn campaing, our windling warrior only fought either on the ground or at a height greater than one round's worth of falling. Better safe than with broken bones, I guess.
Finally, for M&M, I would go for what is more dramatic. Yes, you can apply the rules from D20-ish on that system, no problem.