Although primarily used in Second Life to designate airports, ICAO codes are also used to designate aircraft models and airlines. Aircraft utilize a four-digit alphanumeric code (for example the Embraer Phenom 300 has the ICAO code "E55P"), while airlines make use of a three-digit alphabetic code (for example, American Airlines has the code "AAL").
Airport ICAO codes
In real-life, ICAO codes are used by air traffic control and airline operations such as flight planning. They differ from IATA codes, which are generally used for airline timetables, reservations, and baggage tags. For example, the IATA code for London's Heathrow Airport is LHR and its ICAO code is EGLL. Most travelers usually see the IATA code on baggage tags and tickets and the ICAO code is used among other things by pilots and air traffic control. In general IATA codes are usually derived from the name of the airport or the city it serves, while ICAO codes are distributed by region and country.
In Second Life, the ICAO airport code system is an amalgamation of both the ICAO and IATA systems. The simplified system is universally used by air traffic control, airline operations, and passengers alike. Also, unlike the real-life ICAO, the code is usually derived from the name of the airport (with the added prefix of SL - derived from "Second Life"). For example, the ICAO code for Hollywood Airport is "SLHA".
Owners of new or existing airports requiring an ICAO code are advised to check the official Second Life ICAO list and choose an available four-digit code (listed in green). Applicants are required to register their airport with wiki staff (please contact AndyTGD or Ananda Ghost for details) to prevent duplicate codes being used by several airports simulataneously. Airports intending to add an ATC bot to their facility are advised to wait until confirmation.
While "SL--" is the generally accepted convention, there have been exceptions to the rule. For example, Wiener Neustadt Airport goes by the ICAO "LOWN" (based on its real-life counterpart).