Actually, this is only true if the aircraft that the United States President is in is a USAF craft. If it is operated by the Army then it is called Army One, if by the Marines, Marine One, etc. If it is a private or commercial aircraft, it is Executive One. And the aircraft's designation becomes x One as soon as the President is aboard. For instance, when Lyndon Johnson took the oath of office aboard a plane from Dallas to Washington, the designation was switched in-flight.