Mayday warnings happen regularly, say aviation experts—and both pilots and air traffic controllers are trained to know what to do.

On July 25, United Airlines Flight 108 made an emergency return to its departure site, Washington Dulles International Airport, after experiencing a suspected engine failure.

The Boeing 787, flying from the Washington, D.C., area to Munich, had to make a hasty return to the airport "to address a mechanical issue," United said.