There's only one real reference to flying, but it's a doozy of a reference from Nia as she slips into a coma. Here it is:
But by the time I was ten I wanted to be a balloonist, and just fly up high over everybody, and that's what I feel like I'm doing now.
I'm flying up high over everybody; way over the city and even myself. I'm flying over Bobby and my parents, and the park with all my friends in it. I guess this is what it must feel like to be dying. (27.3-4)
Usually, when we see birds or flight in literature, they symbolize freedom, or escape from physical or metaphorical bondage. In this case, though, Nia's flight is both a dream she's finally able to realize—she wanted to be a balloonist—and her slipping into death. In some ways, Nia is escaping, but her escape is from her life and her reality, and her loss is deeply felt by everyone, most especially Bobby. She isn't flying, then, so much as she's flying away. Safe travels, Nia.