- 3-letter airport codes that end in “X” (LAX, PHX, PDX, etc.) are usually airports that existed with a 2-letter code before the 3-letter system began in the 1930s
- The National Park Service maintains a tiny airstrip at Kitty Hawk, NC, with the code FFA for First Flight Airport
- If an airport name or city begins with a K, N, Q, W, Y, or Z is cannot be used as the first letter in the 3-letter airport code as these are reserved for special uses – N denotes a Naval air field, K and W are radio stations, Y means the location is in Canada, Z that it is in cyberspace, and Q that it is an international telecommunications station
- Airports within 200 nautical miles may not duplicate 1st and 2nd, 2nd and 3rd, or 1st and 3rd letters of their 3-letter codes
- Since Dulles International Airport could not be called DIA because it was too close to DCA (Reagan National), the letters were reversed to become IAD
- London and New York City are recognized as being served by the most commercial airports, with 6 each
- Los Angeles and Vancouver are right behind, with 5 commercial airports each.
