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

United Airlines released a statement Monday after Flight 108, bound for Munich, Germany, had to turn back. The Boeing 787 with 219 passengers and 11 crew members had to retreat to Dulles International Airport after the left engine failed.

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.