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.

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.

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