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

A disaster was seemingly averted when a United Airlines flight bound for Munich, Germany, was forced to make an emergency landing shortly after taking off from Washington Dulles International Airport.

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.