Structure
Unlike the IATA codes, the ICAO codes generally have a regional structure, are not duplicated, and are comprehensive. In general, the first letter is allocated by continent and represents a country or group of countries within that continent. The second letter generally represents a country within that region, and the remaining two are used to identify each airport. The exception to this rule is larger countries that have single-letter country codes, where the remaining three letters identify the airport. In either case, ICAO codes generally provide geographical context unlike IATA codes. For example, if one knows that the ICAO code for Heathrow is EGLL, then one can deduce that the airport EGNH is somewhere in the UK (it is Blackpool International Airport). On the other hand, knowing that the IATA code for Heathrow is LHR does not enable one to deduce the location of the airport LHV with any greater certainty (it is William T. Piper Memorial Airport in Lock Haven, Pennsylvania in the United States).
There are a few exceptions to the regional structure of the ICAO code made for political or administrative reasons. For example, the RAF Mount Pleasant air base in the Falkland Islands is assigned the ICAO code EGYP as though it were in the United Kingdom, but a nearby civilian airport such as Port Stanley Airport is assigned SFAL, consistent with South America. Similarly Saint Pierre and Miquelon is controlled by France, and airports there are assigned LFxx as though they were in Europe. Further, in region L (Southern Europe), all available 2-letter prefixes have been exhausted and thus no additional countries can be added. Thus when Kosovo declared independence, there was no space in the Lxxx codes to accommodate it, so airports in Kosovo were assigned BKxx, grouping Kosovo with Greenland and Iceland.
The letters I, J and X are not currently used as the first letter of any ICAO identifier. In Russia and CIS, Latin letter X (or its Morse/Baudot Cyrillic equivalent Ь) is used to designate government, military and experimental aviation airfields in internal airfield codes similar in structure and purpose to ICAO codes but not used internationally. Q is reserved for international radiocommunications and other non-geographical special uses (see Q code).
In the contiguous United States, Canada, and some airports in Mexico, most, but not all, airports have been assigned three-letter IATA codes which are the same as their ICAO code without the leading K, W, or C. e.g., YYC and CYYC (Calgary International Airport, Calgary, Alberta), IAD and KIAD (Washington Dulles International Airport, Chantilly, Virginia). These codes are not to be confused with radio or television call signs, even though both countries use four-letter call signs starting with those letters. However, because Alaska, Hawaii and United States territories have their own 2-letter ICAO prefix, the situation there is similar to other smaller countries and the ICAO code of their airports is typically different from its corresponding 3-letter FAA/IATA identifier. For example, Hilo International Airport (PHTO vs ITO) and Juneau International Airport (PAJN vs JNU).
ZZZZ is a special code which is used when no ICAO code exists for the airport and is normally used in flight plans.
A list of airports, sorted by ICAO code, is available below.
Read more about this topic: International Civil Aviation Organization Airport Code
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