Aang happened to be walking by with Korra in search of Sokka. What better expert to solve their boomerang debate? He idly talked to her before noticing the scene, "I'm telling you, Korra, I'm pretty sure boomerangs work just fine underwater even without using bending."

And then he freezes for a moment as he sees the scene going on before him. Ed, surrounded by shadows. Sokka, bleeding. And Winry, standing up to him.

Aang was rushing forward in a panic. "Sokka! Winry! Ed! What's going on?"