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As an Air Cadet, we go gliding very frequently. However, I've noticed something about the airport that we go to. The airport that we go to, Markham Airport, has a different airport code. CNU8.

Meanwhile, other airports, such as Toronto-Buttonville, Toronto-City-Centre, and Toronto-Pearson all have the following codes respectively: CYKZ, CYTZ, and CYYZ.

Do the numbers in airport codes signify something? If they don't, why is there a mix of both letters and numbers?

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The code for Markham Airport, CNU8, is a Transport Canada identifier.

The codes for the other airports are ICAO airport codes. ICAO airport codes always consist of 4 letters. Markham Airport does not have an ICAO airport code.

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  • $\begingroup$ Interesting. I didn't catch that. Is this just a Canadian thing, or do similar things apply worldwide? $\endgroup$ – Zizouz212 May 5 '15 at 23:10
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    $\begingroup$ In many countries there are small airfields that do not have an ICAO or IATA identifier. Mostly these are glider or helicopter fields. Often there is a local identification scheme for coding these airports. $\endgroup$ – DeltaLima May 5 '15 at 23:16
  • $\begingroup$ @DeltaLima -- in the US at least, many of them service fixed-wing GA birds as well $\endgroup$ – UnrecognizedFallingObject May 5 '15 at 23:50
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    $\begingroup$ Yes, lots of GA airports in the U.S. don't have ICAO codes. However, that Wiki article seems to suggest that ICAO codes can contain numbers: "The ICAO airport code or location indicator is a four-character alphanumeric code." $\endgroup$ – reirab May 6 '15 at 1:47
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There are numerous coding schemes for airport identifiers... they're often broken down into 3 main categories. ICAO, IATA, and country of origin. Sometimes these are all the same, other times, each one is completely different from the other. Each of these schemes evolved for different purposes(or diverged from each other at various times, aka ICAO added a fourth character from the IATA code bank and then diverged from there).

  • ICAO - International air traffic
  • IATA - Ticketing information for the airlines.
  • Country of origin - A way to identify all the airports in a specific country.

For example, in the US, there are some 15,000 airports. Most are small grass strips or similar, but each often needs its own identifier. There aren't enough 3 letter or 4 letter only combinations, so they start adding numbers. 4V4, 20GE, etc.

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  • $\begingroup$ "sometimes these are all the same"—ICAO and IATA codes can't ever be the same, because ICAO codes are 4-letter and IATA ones are 3-letter. $\endgroup$ – Jan Hudec May 6 '15 at 5:06
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    $\begingroup$ @JanHudec I think by "the same" he means the last three letters are the same, which is often true in the USA and Canada. $\endgroup$ – Random832 May 6 '15 at 5:10
  • $\begingroup$ @Random832: Yes, in US and Canada the ICAO code is often the IATA code with K (in US) or C (in Canada) prefix. In the rest of the world it is rare, because most ICAO location prefixes are 2-letter. $\endgroup$ – Jan Hudec May 6 '15 at 5:42
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The FAA's rules are in their Location Identifiers paper, which explains how the numbers are used, at least for the US. It also mentions Transport Canada, by the way, and I guess that Canada uses similar rules.

There are specific requirements for an airport to qualify for a three-letter identifier:

a. Three-letter identifiers are assigned as radio call signs to aeronautical navigation aids; to airports with a manned air traffic control facility or navigational aid within airport boundary; to airports that receive scheduled route air carrier or military airlift service, and to airports designated by the U.S. Customs Service as Airports of Entry. Some of these identifiers are assigned to certain staffed aviation weather reporting stations or for airports commissioning Automated Weather Observation Systems, level III (AWOS-III) or higher that have paved runways 5,000 ft or longer.

That three-letter code is then prefixed with 'K' for the continental US, to give the four-letter international code:

An international location indicator is a four-letter code used in international telecommunications. The location indicator for airports in the contiguous United States is the three-letter identifier preceded by "K". For other non-contiguous United States airports, the following two letter prefix will be used:

Alaska - PA, PF, PO, PP
Hawaii - PH
Puerto Rico - TJ
Virgin Islands - TI
(See ICAO Document 7910 for listings.)

The one-letter two-number format is usually used for public airports that don't qualify for a three-letter identifier:

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.

The two-letter, two-number format is usually used for private airports that don't qualify for a three-letter identifier:

e. Two-letter, two-number identifiers are assigned to private-use landing facilities in the United States and its jurisdictions which do not meet the requirements for three-character assignments. They are keyed by the two-letter Post Office or supplemental abbreviation (listed below) of the state with which they are associated. The two-letter code appears in the first two, middle, or last two positions of the four-character code.

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