In a previous article, "Bill Denton" said:
As a quick rule of thumb, you append the "K" only to airports, and only to
those whose identifiers contain only letters (no numbers).

And only in the lower 48. Alaska and Hawaii (and US territories) don't
have an easy rule like that, but the ICAO ids for those airports start
with a P.

Theoretically only airports that produce weather reports or forecasts, or
that are airports of entry have ICAO ids[1], but in practice GPS
manufacturers (and my waypoint generators on http://navaid.com) do as you
say, and put a K on any airport id in the lower 48 that doesn't have
digits in it.

[1] A lot of airports had to change id to not have any numbers in the id
when we went to METAR/TAF for that reason, and also even now when an
airport gets an AWOS, if it had digits in the ID it will change id. Wings
Field near Philadelphia did that a few years ago.

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