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

A United Airlines pilot declared mayday late last month, suspecting possible engine failure after takeoff from Dulles International Airport near Washington, D.C. “Engine failure, left engine, United 108 declaring an emergency.

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.