The United Airlines terminal is viewed at Newark Liberty Airport on July 8, 2015 in Newark, N.J. The airline was criticized on Sunday for enforcing its buddy pass dress code, barring from the plane two girls in their early teens who were wearing leggings. Spencer Platt/Getty Images hide caption
Crowds gather to watch the Fourth of July fireworks show last year at Kailua Beach on the Hawaiian island of Oahu. Julie Thurston Photography via Getty Images hide caption
Passengers at O'Hare International Airport wait in line to be screened at a Transportation Security Administration (TSA) checkpoint. Scott Olson/Getty Images hide caption
Passengers wait in line to be screened at a Transportation Security Administration checkpoint at Chicago's O'Hare International Airport on Monday. Scott Olson/Getty Images hide caption
People walk away from Brussels airport after Tuesday's terrorist attack. Analysts say the violence may reduce travel for a while but the industry should bounce back. Geert Vanden Wijngaert/AP hide caption
Lines of travelers snake past TSA agents at the Seattle-Tacoma International Airport last fall. Elaine Thompson/AP hide caption
Passengers walk through the terminal as they head to their flights at Reagan National Airport in Arlington, Va., on Dec. 23, 2015. Saul Loeb/AFP/Getty Images hide caption
Fuel prices have plummeted this year, but airfares have remained high. The International Air Transport Association estimates that the world's airlines will rake in nearly $20 billion in profits this year. Brennan Linsley/AP hide caption
A Eurocontrol map showing the air traffic situation over Europe on Friday. Eurocontrol hide caption
A plane takes off over a departure board at Hartsfield-Jackson Airport in Atlanta last November. Airlines say they expect an uptick in Thanksgiving travel this year. David Goldman/AP hide caption
Air traffic connections from West Africa to the rest of the world: While Guinea, Liberia and Sierra Leone don't have many flights outside the region, Nigeria is well-connected to Europe and the U.S. PLOS Currents: Outbreaks hide caption
Passengers packed the waiting hall Tuesday at Hongqiao Railway Station, which services a terminal at Shanghai Hongqiao International Airport. Johannes Eisele/AFP/Getty Images hide caption
Emirates executive Thierry Antinori poses for the camera in a first-class seat aboard an Airbus A380 in Munich in 2011. Alexander Hassenstein/Getty Images hide caption
Coming in December: the eTrack and the eTag, from Air France and KLM. The innovations let you drop off your bags without checking them, and track them throughout your journey. Courtesy of FastTrack Co. hide caption