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

A United Airlines flight traveling from Washington Dulles International Airport made an emergency landing last month after the pilot declared a mayday call shortly after takeoff due to engine failure.

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