Three letter Airline Designator - The International Civil Aviation Organisation (ICAO) switched to three letter designators on 25th October 1987 due to the depletion of the ICAO two letter designator system. The ICAO designators are used primarily for air traffic control communications and the aeronautical fixed telecommunications network (AFTN). Three letter designators are being assigned in cooperation with ICAO, the United States Federal Aviation Administration (FAA) and Transport Canada. when IATA converts to three letter designators, the same designators assigned by ICAO will be assigned by IATA.