A decade of reorganizations and merger activity in U.S. airlines has rationalized the business for air carriers here, bolstering balance sheets, lessening competition and making domestic flying more lucrative than it's been in years. Air carriers share prices have responded affirmatively, rising nearly 50% in the last year. However, a new cloud is developing on the horizon, as global carriers eyeball the lucrative U.S. market, and amp up their appetite for U.S. destinations. This week, Emirates Airline of Dubai confirmed that it's placed a huge order for Boeing's 777X aircraft, with an eye on those domestic opportunities. It adds up to more congestion in U.S. airspace.