Part of the answer to your question is going to be found in the particular requirements of your "box", and what is loaded in your database.

But you should understand that while there are some similarities between U.S. airports with IATA identifiers and their ICAO and FAA identifier cousins, they are not all created equal. All three coding systems have their own conventions and quirks.

To answer the first question, you need to know a little bit about the FAA identifiers, which are 3 or 4 digits, may be 1 letter and 2 numbers, or 1 number and 2 letters, or 2 letters and 2 numbers, or 3 letters.

1 letter and 2 numbers - public-use airports that don't meet the requirements for a 3-letter identifier -- letter can be 1st, 2nd, or 3rd digit

1 number and 2 letters - special-use locations and some public-use airports -- number is always 1st digit

2 numbers and 2 letters - private-use airports -- 2 letters are always adjacent (1st/2nd, 2nd/3rd, or 3rd/4th digits) and represent the state where the airport is located.

3 letters - have manned ATC facilities, receive scheduled airline service, or designated ports of entry -- IF the location also has an IATA code, the FAA code will usually be identical. It is also possible that the FAA 3-letter code is the same as the IATA code for an airport somewhere else.



All IATA codes are 3 letters, and there's a long list of guidelines for determining which airports get them, and what they are.


ICAO codes are 4 letters, where the first 2 letters represent the region and the country of the airport, with the exception of C (Canada) and K (USA) where the 2nd letter does NOT represent a region. For codes starting with K, the remaining 3 letters are the FAA code.


Granby, Colorado:

FAA Code: GNB
IATA Code: GBY
ICAO Code: KGNB


If your system requires the K, it is obviously looking for the ICAO identifier. If it also accepts 2H2, it uses FAA identifiers for those airports without ICAO identifiers.

More here: Airport Location Identifiers Deciphered







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