Actually, that term is not accurate. Been thinking about it for a while and I BELIEVE a wing can't be a self-equilibrated body since it wouldn't fly if it was. It's motion through the air MIGHT make it weightless but it wouldn't create lift since, to do so, it would have to go past equilibrium. A wing is either supported by the plane's structure and landing gear or it's supporting the structure. To be in equilibrium would change nothing since the wing is still being supported by the landing gear until it attains enough speed to lift the plane on it's own.