Salve!...ho un dubbio regolistico: se un effetto o un incantesimo rende prona ha effetto su una creatura che sta volando cosa succede a questa?...inizia a precipitare? o non succede nulla?

Se non ricordo male, il manuale del master proponeva di farle stallare per 1 round.

stallare vuol dire stordito? In caso vuol dire che inizia anche a precipitare?

E' spiegato tutto molto meglio nei vari articoli sul movimento apparsi nella sezione Rules of the Game del sito ufficiale... Ecco l'estratto che interessa:

Trip

Most creature using wings or other appendages to fly can be tripped. Incorporeal creatures with perfect maneuverability, and creatures that don't rely on their limbs to fly cannot be tripped when in flight.

Resolving the Trip Attempt: The attacker makes a Strength check. The defender can oppose the attempt with a Strength check or a Dexterity check. Each creature gets a bonus based on its maneuverability rating, as follows: perfect +12, good maneuverability +8, average +4, poor +0, clumsy -4.

Stability bonuses do not apply in aerial overruns.

Trip Results: A successful trip forces the defender to stall (even if the tripped creature doesn't have a minimum forward speed) rather than knocking the defender prone.

E questa è la parte sullo stallo:

Stalling and Freefalling

Stalling represents the failure of a flying creature's wings (or other motive agent) to keep the creature aloft. The rules are a little sketchy when it comes to what happened during a stall, so here are some unofficial suggestions.

A stalling creature falls, but it wings provide considerable drag and tend to slow the creature's fall. As noted earlier, a creature falls 150 feet during the first round spent stalling, and it falls 300 feet each round thereafter. Wingless flyers that stall still have some residual lift and fall more slowly than non-flyers.

A flying creature that cannot maintain its minimum forward speed because it has been rendered unconscious, has become paralyzed, has become magically held, or becomes unable to move for some other reason stalls at the beginning of its first turn after the debilitating effect occurs.

A stalling creature can take no actions, except to recover from the stall. It loses its Dexterity bonus to Armor Class (if any) while stalling. As noted earlier, recovering from a stall requires a Reflex save (DC 20).

A stalling creature falls more or less straight down, but it also tumbles and spins erratically. Melee or ranged attacks made against a stalling creature have a 20% miss chance.

A nonflyer (or flyer falling through the air) freefalls rather than stalls. A creature in freefall drops 500 feet the first round and 1,000 feet each round thereafter. While in freefall, a creature can attempt a single action each round. It must make a Dexterity or Strength check (creature's choice, DC 15) to avoid dropping any item it tries to use. Spellcasting is possible, but doing so requires a Concentration check (DC 15 + spell level) and if the spell has a material component, the creature must first check to see if it drops the component.