There are actually two separate coding systems in play here.
IATA (International Air Transport Association) codes are 3 letters and are used for commercial bookings etc.
They are assigned only to commercial airports throughout the world.
ORD is the IATA code for Chicago O'Hare.
Pilots and ATC use the four letter ICAO (International Civil Aviation Authority) codes, which are assigned to every public-accessible airfield in the world.
These are systematically allocated with the first one or two letters indicating the country.
K is used as the first letter of airfields in the continental US (with separate codes for Alaska, Hawaii & territories in the Pacific/Caribbean matching regional standards).
Commercial airports in the continental US tend to have the two codes in matching pairs such as ORD/KORD, LAX/KLAX etc
but this is not the case in other countries.
Manchester, UK has IATA code MAN but ICAO code EGCC.
Meanwhile smaller airfields in the US may only have an ICAO code, where the corresponding IATA code (minus the K) could belong to an international airport elsewhere in the world (for instance KPRG is Edgar County Airport, Illinois and PRG is Prague Václav Havel Airport, Czech Rep.).
Finally, the very smallest airfields in the US (typically private/restricted access fields, in any case not large enough to warrant an ICAO code) are given a 3- or 4-character code by the FAA which extends the ICAO system by allowing digits as well as letters.
Small public fields have a 3 character code (eg 9S2 or C97) while private fields have 4 characters which typically include the state abbreviation eg TX05.
For more details of the exact determination of which code type a US airfield is assigned by the FAA, see reirab's excellent self-answered question: How does the FAA determine which format of location identifier to assign to an airport?
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The other answers have lots of excellent context & background info, but to give the direct answer to the direct question, *no, you don't use the "K" * with ATC under normal circumstances.
The (rare) exception will be when it is necessary for some reason, but that is almost never.
In voice communication, "direct Midland VOR" or "direct Midland airport" are used, rather than "direct MAF" or "direct KMAF".
Obviously, printed NOTAMS and flight planning work differently.
If you were to be cleared "direct MAF" (which is both the 3-letter FAA ID for Midland, and also the non-colocated VOR), it would be appropriate to query "is that the airport or the VOR?
" if you aren't sure.
(If you are navigating VOR to VOR to MAF & beyond, you could safely assume the VOR; if your destination is Midland, asking the controller would be appropriate.)
But when asking you to "say destination," it's understood you're landing at the airport rather than the navaid out in the field!
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The "K prefixed" and the "non-K prefixed" codes are actually different designation systems.
The former is used by ICAO and the latter by IATA , and each have specific use cases by different authorities.
Not all airports in the world have their ICAO and IATA codes differentiated by leading K (an airport might not even have both).
This is usually holds true only to the larger airports and only in the US.
In case of airports in the rest of the world this is not true: some airports' ICAO and IATA codes are totally different from each other, while others' do bear some varying degree of similarity.
You can get an idea by looking at these lists: https://en.wikipedia.org/wiki/List_of_airports_by_IATA_and_ICAO_code http://www.flugzeuginfo.net/table_airportcodes_country-location_en.php
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Usually, yes, and in particular when using the airport for position reporting.
Sometimes there will be a VOR or VORTAC which uses the same three letter identifier nearby but collocated on the airport so it can cause confusion for a controller.
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When programming your GPS or FMS you need to distinguish between airports and VORs e.g. KLAX is an airport and LAX is a VOR on the airport.
But controllers rarely say the letters and instead use the names for airports and VORs.
It is usually clear from the context and the destination that the pilot has filed.
A typical clearance from my airport might be Crepe Three departure, Morro Bay transition, San Marcus then as filed.
The pilot knows to follow the CREPE3 departure which terminates at MQO and then direct to FLW and from there whatever was filed.
When asking for VFR flight following, pilots usually just give the name of the destination airport unless there are multiple airports in the same city.
For example, Bakersfield has two airports so to make sure the controller knows where you want to go you would give the identifier, either BFL or L45.
Also, if you are going to a small airport outside of the local area the controller may not have heard of it and will ask for the identifier.
I don’t recall hearing anyone say the identifier with a K when talking on the radio.
But that’s not necessarily something I’d
notice.
You might want to listen to some of the clearance delivery channels on LiveATC to get a feel for how pilots and ATC communicate routes.
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In my experience, you usually use the NAME of the airport (or navaid).
Center controllers are well familiar with their AOR (and usually adjacent ones just by experience).
Now for longer VFR flights where you may be crossing multiple centers, they may not recognize the named airport, so giving them the identifier can be helpful.
This is not an issue for IFR where they already have your full plan.
Finally, a 'little secret' is that the name an airport is called locally is often different than the name in your AFD.
For example, Lewis A Jackson Regional Airport (I19) in Xenia Ohio is "Greene County" (not "Jackson Regional").
Calling the FBO on the phone before flight will uncover that, or listening on the radio as you approach.
If unsure, go ahead and append the identifier to the name you guess it should be called.
Yeah, you're chewing bandwidth, but that's more efficient than the back and forth and better than a mistaken ID.
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I'm a new student, and I see the US airport codes on charts with the K prefix, e.g. KORD for Chicago O'Hare.  I have always seen on commercial flight booking sites, only the last three, letters such as "ORD" are displayed.  Are pilots meant to pronounce the full code or just the letters after the prefix, assuming the K prefix, when talking with ATC?
Do you say prefix K for airport codes in the US when talking with ATC?
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<air-traffic-control><airport><general-aviation>
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Apparently there is some debate about whether K is used whentalking to ATC in the US. It is, nevertheless, considered acceptable to use it. When talking on the radio, the "K" is not used under normal circumstances. However, some would argue that you should usually use it when reporting position. Another opinion is that most often, the name of the airport is used. In any case, if in doubt, append the identifier to the name of the airport.
When talking on the radio, the "K" is not used under normal circumstances. You usually use it when reporting position. Most often the name of the airport is used. If in doubt, append the identifier to the name of the airport.