The U.S. plan has been implemented once ( other than tests ) since its inception , and even then , it was a modified version . On September 11 , 2001 , the codeword was broadcast ordering that all U.S. air traffic be grounded , after the September 11 attacks . Even in that instance , the emergency plan was only partially implemented as the Defense Department left command and control of the air traffic system with the FAA and intentionally allowed all radio navigational aids to remain in operation to aid in the process of controlling and landing the thousands of planes which were aloft in domestic airspace
September 11 attacks
September 11 attacks
Security Control of Air Traffic and Air Navigation Aids