When Does Aang Possess The Avatar State Fully?

The way I see it, there are two different ways to interpret "when Aang possesses the Avatar State fully," and I like to separate them: one is when the Avatar State physically overwhelms him (Aang is possessed by the power and faces of past Avatars), and the other is when Aang actually masters that state and can call it without losing himself. Those are related but not the same, and the show teases both arcs across Book Two and Book Three.

If you’re asking when the Avatar State takes over him in its most complete visual/powerful form, the biggest moment is during the finale of 'Sozin's Comet' — that scene where the past Avatars appear behind him and he explodes with raw bending is the clearest example of a full Avatar-state possession display. Earlier big showings happen in 'The Siege of the North' and in bits across Book Two (the episode 'The Avatar State' and the clash in 'The Crossroads of Destiny'), but those are more fragmentary or triggered by trauma. If, instead, you mean when Aang finally has real control—when he can decide how to use that power without being consumed—that arc is trickier. He almost reaches emotional mastery in 'The Guru' when Pathik helps him open chakras, but Azula interrupts. The real turning point is the lion turtle scene during the 'Sozin's Comet' run: he learns 'energybending' and makes a conscious moral choice to remove Ozai's bending rather than kill him. That choice is the clearest sign of matured control: he can access Avatar-level power and still remain himself.

So, the short-but-nuanced takeaway I keep coming back to: full possession (faces and raw force) visibly occurs in the 'Sozin's Comet' climax, but true personal mastery and ethical agency over the Avatar State is completed only once he integrates his spirituality and the lion turtle’s lesson — he never becomes a permanent Avatar-State automaton, he becomes a responsible Avatar instead.