The planes have arrived, fraught with heavy cargoes and passengers, circling above. New arrivals are added to the top of the stack, and aircraft at the bottom should be taken out and allowed to make an approach first, letting each aircraft above move down one level. There are limits to both fuel and vertical airspace. Laden aircraft can’t just zoom off into some weightless outer orbital, or magically appear on the ground without some orderly intervening process. What goes up, must come down, and all that. That’s what gravity is all about.
Jan 30 2010