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Official definition states that an ICAO designator consists of 4 letters (A-Z). But there seems to be quite a lot of airports in the U.S. that don't follow this guideline. Of those, about 300 broadcast an official METAR, which leads me to believe the ID's are genuine.

Does anyone know what the status of these airport designators is, are these actual and valid and official airport codes or just made up and somehow got into the METAR lists?

Couple of examples:

KU42    South Valley Regional Airport
KF70    French Valley Airport
KM01    General Dewitt Spain Airport
K5C1    Boerne Stage Field
K1R8    Bay Minette Municipal Airport
K8D3    Sisseton Municipal Airport
K21D    Lake Elmo Airport
K06C    Schaumburg Regional Airport
KO69    Petaluma Municipal Airport
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  • $\begingroup$ About the CCCC identifier for the reporting station of TAF/METAR/SPECI: "The TAF code uses the ICAO four-letter location identifiers. In the conterminous United States, the three-letter identifier is prefixed with a K. For example SEA (Seattle) becomes KSEA". SEA, 5C1, 069, etc are FAA local identifiers. Note ICAO only deals with international matters, not with municipal airfields. $\endgroup$
    – mins
    Apr 20 at 17:33

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No, those are not ICAO codes. ICAO airport identifiers consist of four letters, full stop. But that does not mean that any code which is other-than-four-letters is "invalid" or "unofficial!" There are more standards for identifying airports than just the ICAO code.

As explained in this Av.SE answer, FAA Location Identifiers for airports have the following formats:

  • Three letters
  • One number and two letters
  • One letter and two numbers
  • Two letters and two numbers

At first blush, some of those K-codes appear to be valid FAA LIDs of that last format... but the four-character FAA LID uses two consecutive letters which are based on the USPS abbreviation for the state where they are located, for example MA or SD. When they run out they can get creative, like IS for Illinois, but KM and KF from your examples do not correspond with any US state.

So as it happens, the identifiers you posted are not valid airport identifiers—not under the ICAO system and not under the FAA system either! Those four-character identifiers beginning with K are National Weather Service identifiers for the weather stations located at various airports. Even though the airports themselves have only an FAA LID, not an ICAO airport code, the NWS identifies all CONUS weather stations using a four-character code prefixed with K. (Note that there does not need to be an airport for there to be a weather station—Mount Washington Observatory is identified as KMWN.) If you strip the leading K from your example identifiers you will discover valid and current FAA LIDs which refer to active airports.

It is true that having an advanced on-airport weather station (and a 5,000' runway) is grounds for assigning a three-letter, rather than letter-number, FAA LID. But if the weather station was added after the fact, the general preference is to keep the LID the same.

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  • $\begingroup$ Another FAA format is one digit, one letter, one digit -- such as 0S9 (leading zero) for Jefferson County International Airport in Port Townsend, WA. The LID includes a large number of examples in this format. faa.gov/air_traffic/flight_info/aeronav/aero_data/loc_id_search/… $\endgroup$
    – user2070305
    Apr 26 at 19:49
  • $\begingroup$ Yes, that falls under the "one letter and two numbers" format I mentioned. $\endgroup$
    – randomhead
    Apr 27 at 13:10

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