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.

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.