Dominick Reuter | AFP | Getty Images A Southwest Airlines jet sits on the runway at Philadelphia International Airport a...

Dominick Reuter | AFP | Getty Images A Southwest Airlines jet sits on the runway at Philadelphia International Airport a...

Robert Alexander | Getty Images Southwest Airlines passenger aircraft parked at their gates at McCarran International Ai...

A United Airlines airplane passes the skyline of lower Manhattan and One World Trade Center as it heads to a runway at N...

Jose Castillo | AP In this photo provided by passenger Jose Castillo, fellow passengers walk away from a burning America...