"Jay Honeck" wrote in message
news:4Oa_a.128203$o%2.55258@sccrnsc02...
In another thread, someone mentioned that only 13% (or something like
that)
of the possible 3-letter combinations are currently used as airport
identifiers. Given this fact, why in the world did they start mixing
numbers into the scene?

For example, why "C89" (for Sylvania Field, in WI) instead of some letter
combo?

Anyone know why the identifiers are a mix of letters and numbers the way
they are?

I had wondered this too. Doing a search, I found:
http://www2.faa.gov/atpubs/LID/ch1.htm
----
(Clipped from the doc):
c. Most one-number, two-letter identifiers have been assigned to aviation
weather reporting and observation stations and special-use locations. Some
of these identifiers may be assigned to public-use landing facilities within
the United States and its jurisdictions, which do not meet the requirements
for identifiers in the three-letter series. In this identifier series, the
number is always in the first position of the three-character combination.

d. Most one-letter, two-number identifiers are assigned to public-use
landing facilities within the United States and its jurisdictions, which do
not meet the requirements for identifiers in the three-letter series. Some
of these identifiers are also assigned to aviation weather reporting
stations.

1. One-letter, two-number identifiers are keyed by the alphabetical
letter. The letter may appear in the first, middle or last position in the
combination of three characters. When the letter signifies an Air Traffic
Control Center's area, the assignment will not change if the Center's
boundaries are realigned.
----

Some identifiers make some sense:
DSM, OKC, IOW, MSP, SIN...

Some don't:
PWA, MFE, EOK, CID, AXA, JYG, CVG...

It's "usually" smaller airports with the numbers:
Y58, 0K7...
Sometimes the Zero and "O" are difficult to distinguish between:

I know one airport that did want to change its identifier: SUX.
The English translation of Notams sometimes translates the Fort Dodge
airport identifier (FOD) as Foreign Object Damage.