When using a poor flyer or average, I seem to have read somewhere that you must move every turn (hovering requires a feat or something like that). Does this imply taking a move action to move, every turn, mandatory (or else you fall)?
How does this affect full round actions ? Does 5-ft step count as your "movement"?
Can I cast, lets say summon monster, while flying with poor/average maneuverability?
Flight - Yes, you must move
Specifically you have to move your minimum forward speed, which is defined in the movement rules.
If a flying creature fails to maintain its minimum forward speed, it must land at the end of its movement. If it is too high above the ground to land, it falls straight down, descending 150 feet in the first round of falling. If this distance brings it to the ground, it takes falling damage. If the fall doesn’t bring the creature to the ground, it must spend its next turn recovering from the stall. It must succeed on a DC 20 Reflex save to recover. Otherwise it falls another 300 feet. If it hits the ground, it takes falling damage. Otherwise, it has another chance to recover on its next turn.
For poor and average, that is "half" of your movement speed. That means you have to use a move action to maintain that, a 5-foot step isn't far enough.
If you have Good/Perfect maneuverability, your minimum forward speed is "none", so you don't have to move.
The Hover feat also lets you stay still in the air without using a move action, so you could full round cast. You have to stop before you can use Hover in that way though, so if you want to start summoning and you're already moving, it doesn't help much.
Yes, you can cast Summon Monster
There are two ways to cast a full round spell. One of them is to use a full round action. The other one is to use two standard actions - Start and Complete Full Round Action.
The "start full-round action" standard action lets you start undertaking a full-round action, which you can complete in the following round by using another standard action. You can’t use this action to start or complete a full attack, charge, run, or withdraw.
On turn 1 you use your Move action to move and stay flying, and use Start Full Round Action to start summoning. On turn 2, use Complete Full Round Action to finish summoning, then use your Move action to stay flying once more.
Per the SRD, it simply states that they must fulfill a certain obligation of forward movement, not how they must do so.
Minimum Forward Speed
If a flying creature fails to maintain its minimum forward speed, it must land at the end
of its movement. If it is too high above the ground to land, it falls straight down,
descending 150 feet in the first round of falling. If this distance brings it to the ground,
it takes falling damage. If the fall doesn’t bring the creature to the ground, it must spend
its next turn recovering from the stall. It must succeed on a DC 20 Reflex save to recover.
Otherwise it falls another 300 feet. If it hits the ground, it takes falling damage.
Otherwise, it has another chance to recover on its next turn.
http://www.d20srd.org/srd/movement.htm#movingInThreeDimensions
So, for Poor or Average manueverability, they must include at least 1/2 their speed in their actions for the round or begin to fall. Whether this is simply via their move action for the round, or part of a charge (for example), or other form of movement — that is irrelevant. A 5-foot step is not enough.
Quoting from Flight rules in the DMG:
Minimum Forward Speed: If a flying creature fails to maintain its minimum forward speed, it must land at the end of its movement. If it is too high above the ground to land, it falls straight down, descending 150 feet in the first round of falling. If this distance brings it to the ground, it takes falling damage. If the fall doesn't bring the creature to the ground, it must spend its next turn recovering from the stall. It must succeed on a DC 20 Reflex save to recover. Otherwise it falls another 300 feet. If it hits the ground, it takes falling damage. Otherwise it has another chance to recover on its next turn.
So, plain and simple - you must move at least half your flight speed in order to stay airborne - so you need at least a move action to cover that. You can still decide to take the full round action if you are low enough to land or high enough to risk the reflex save after a 150 feet drop - but it's risky.
Note that in combination with spells such as featherfall or hover, you can allow yourself not to maintain your minimum forward speed, and haste will allow you to take a full action and a move action in the same round.
otherwise, land before taking that full round action
