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

Flight 108, carrying 230 people, was only moments into its ascent when its pilot reported the mid-air emergency to air traffic control.

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.