Fly Allegiant into Phoenix-Mesa Gateway and your boarding pass says AZA. Ask a pilot for the same airport and you might hear KIWA. Look at an FAA chart and you will find IWA. Same runway, three different names. I ran into this once while comparing data sources and assumed one of them had a typo. None of them did.
The reason is simple once you see it: the three code systems are built by three different organizations, for three different jobs, on three different timelines. Most of the time they agree. When they don't, each one is right inside its own world.
The three systems, in one sentence each
IATA codes are three letters, assigned at the request of an airline, and they live on tickets, boarding passes, and baggage tags. FAA identifiers are the U.S. government's codes, used in charts, weather reports, and flight plans. ICAO codes are four letters with a regional prefix (K for the continental U.S., C for Canada), and they are what pilots and controllers actually say on the radio. We covered the IATA/ICAO split in detail in our IATA vs ICAO guide; the FAA is the third member of the family, and the one that causes the surprises.
The FAA's identifier system is older and entirely independent of IATA. In most of the country they coincide, because the FAA assigns the same three letters the airlines use. But neither organization is required to follow the other, and airports rebrand. That is where the disagreements come from.
Where IATA and FAA actually disagree
| Airport | IATA (your ticket) | FAA (the chart) | ICAO (the cockpit) |
|---|---|---|---|
| Phoenix-Mesa Gateway | AZA | IWA | KIWA |
| Saipan International | SPN | GSN | PGSN |
| Sawyer International (Marquette, MI) | MQT | SAW | KSAW |
Phoenix-Mesa Gateway is the cleanest example. The FAA still knows the field as IWA, from its history as Williams Air Force Base. When the airport rebranded for airline service, IATA issued AZA at the request of the carriers. The FAA kept IWA, ICAO built KIWA from the FAA code by prepending K, and every passenger flies on AZA. If you typed IWA into an airline booking site, you would get nowhere.
Marquette's Sawyer International tells the same story in reverse: the FAA gave it SAW, but SAW was already taken in the IATA system, so the airlines got MQT instead. And Saipan's split, SPN versus GSN, is a straight case of two organizations assigning two different identifiers to the same place.
Which code should you actually use?
It depends entirely on who you are talking to. If you are booking a flight, searching for a fare, or telling a friend where to meet you, use the IATA code. That is the only system the passenger-facing industry speaks.
If you are a student pilot filing a flight plan, reading a sectional chart, or listening to METAR weather reports, the FAA identifier is the one on the page, and the ICAO code is what you will hear on the radio. Mixing them up is more than a curiosity. Look up KIWA's weather and you get the right field; type AZA into a flight-planning tool and you might get nothing.
One more edge case worth knowing: because the systems are independent, a code can mean one airport to IATA and a completely different airport to the FAA. This is rare, but it is exactly why aviation professionals specify the system every time. Our database keeps all three identifiers for every U.S. airport, so you can search by whichever code you have in hand:
Search the free U.S. airport codes database
Search AZA, IWA, or KIWA and you land on the same airport. That is how it should work, and it is how the database is built. For the stories behind how the weird codes got assigned in the first place, see how airport codes are assigned, and for when codes move between airports entirely, can airport codes change?
Frequently asked questions
Can one airport really have two codes?
Yes. Phoenix-Mesa Gateway is AZA to IATA and IWA to the FAA; Saipan is SPN and GSN. The systems are independent, so neither side is wrong.
Why did Phoenix-Mesa get AZA?
IATA issues codes at the request of an airline, and AZA was the identifier chosen for airline service. The FAA's IWA predates the airport's civilian rebrand, from its days as Williams Air Force Base, and the FAA kept it.
What is the K in front of ICAO codes?
The first ICAO letter marks the region: K for the continental United States, C for Canada. Our Canadian Y-code story explains the other famous prefix.
Sources: IATA, "Fact Sheet: IATA Location Codes"; FAA airport identifier data; Wikipedia, "IATA airport code" and "Location identifier" (CC BY-SA). Figures verified October 2026.