Airport Codes: The ABC's A History and Explanation

From ABE (Allentown/Bethlehem/Easton, Pennsylvania) to ZRH (Zurich, Switzerland), airports around the world are universally known by a unique three-letter code: the "International Air Transport Association (IATA) Location Identifier" in aviation-speak. It's obviously much easier for pilots, controllers, travel agents, frequent flyers, computers and baggage handlers to say and write ORD than the O'Hare International Airport in Chicago, Illinois-but how did this practice start, and why are some airport codes easy to understand (ABE and ZRH) while others seem to make absolutely no sense (ORD)?

When the Wright brothers first took to the air in 1903, there was no need for coding airports since an airport was literally any convenient field with a strong wind. However, the National Weather Service did tabulate data from cities around the country using a two-letter identification system. Early airlines simply copied this system, but as airline service exploded in the 1930's, towns without weather station codes needed identification. Some bureaucrat had a brainstorm and the three-letter system was born, giving a seemingly endless 17,576 different combinations. To ease the transition, existing airports placed an X after the weather station code. The Los Angeles tag became LAX, Portland became PDX, Phoenix became PHX and so on. Incidentally at the historic sand dune in Kitty Hawk where the first flight occurred the U.S. National Parks Service maintains a tiny airstrip called FFA-First Flight Airport.

Many station codes are simply the first three letters of the city name: ATL is Atlanta, BOS is Boston, MIA is Miami, SIN is Singapore, and SYD is Sydney, Australia. The first letter(s) of multiple cities served forms other codes: DFW for Dallas Fort Worth, MSP for Minneapolis/St. Paul, and GSP for Greenville/Spartanburg, South Carolina. Sometimes the city name lends itself to one letter for each word, such as Salt Lake City (SLC), Port of Spain in Trinidad & Tobago (POS), or even Port au Prince, Haiti (PAP).

Most of the "hard to decipher" identifiers become obvious if one knows the name of the airport rather than the city served. A Louisiana example is ESF, for Esler Field in Alexandra. Orly airport (ORY) and Charles De Gaulle airport (CDG) serve Paris, France, while Tokyo, Japan has the Narita airport (NRT). When you know what the code represents, some curious acronyms become obvious: MSY is the former Moisant Stock Yards in New Orleans, CMH is Columbus Municipal Hangar, BWI is Baltimore Washington International, LGW is London Gatwick, and LHR is London Heathrow!

This system of identifying airports caught on quickly and soon expanded to include all radio navigation aids used by pilots. The VOR on the field at ORD sends out the Morse code for ORD. Recently some VORs not located at the airport of the same name changed identifiers to prevent possible confusion. The clearance "cross 10 miles south of Chattanooga" was confusing when the airport and VOR were five miles apart. FAA surprisingly didn't try to change the name of the city but changed the VOR, resulting in the Chattanooga Airport (CHA) and the Choo Choo VOR (GCO)!

All localizer identifiers are prefaced with an "I." Compass locators are assigned a two-letter identifier, normally using the localizer as a base. For example, at ABC the localizer might be IABC, the locator outer marker, AB, and the locator inner marker, BC. (Note, outside the US radio navigation aid naming may be much less formal.)

Some special interest groups successfully lobbied the government to obtain their own special letters. The Navy saved all the new 'N' codes. Naval aviators learn to fly at NPA in Pensacola, Florida and then dream of going to "Top Gun" in Miramar, California (NKX). The Federal Communications Committee set aside the 'W' and 'K' codes for radio stations east and west of the Mississippi respectively. 'Q' was designated for international telecommunications. 'Z' was reserved for special uses. The Canadians made off with all the remaining 'Y codes which helps explain YUL for Montreal, YYC for Calgary, etc. (The start of the the song YYZ by the band Rush is the Morse code for the letters Y Y Z. Rush is from Toronto.) One of the special uses for 'Z' is identifying locations in cyberspace. What am I talking about? Well, an example is ZCX the computer address of the FAA's air traffic control headquarters central flow control facility. ZCX is not an airport but a command center just outside Washington D.C., that controls the airline traffic into major terminals.

The lack of these letters puts a crimp in the logic of some codes: if the city starts with a 'N,' 'W,' or 'K,' it's time to get creative! Norfolk, Virginia, ignored the 'N' to get ORF; Newark, New Jersey, is EWR, Newport News, Virginia, chose to use the name of the airport to get PHF -Patrick Henry Field. Both Wilmington, North Carolina and Key West, Florida followed Norfolk's lead to obtain ILM and EYW. West Palm Beach in Florida did some rearranging to get PBI -Palm Beach International; Kansas City, Missouri became MKC and more recently the 'new' Kansas City airport chose MCI. (The code for Kansas City International Airport, MCI, was assigned during the early design phase of the airport when the name was going to be Mid-Continent International. Shortly before it opened, Kansas City officials decided to change the name so people would know what city it was in. It was too late to change the code.)

The continued growth of aviation world-wide meant that three letter combinations were insufficient to identify every airport. Eventually the system expanded, allowing numbers and four digit combinations; however, an airport served by scheduled route air-carrier or military airlift aircraft always has a code comprising of only three letters. Raleigh, North Carolina, not only has RDU (Raleigh/Durham International), but also the much smaller 5W5 (South Raleigh Airport), W17 (Raleigh East Airport), 2NC3 (Sky-5 helipad), and ONC4 (Wake Medical Center Heliport). The two-letter, two-number identifiers use the two-letter Post Office or supplemental abbreviation of the State for the two letters: 2ND9 is in North Dakota, 85FL is in Florida, etc. Hilton Head Island, South Carolina, known as 49J when it was a general aviation airport, now has airline service and therefore a new airport identifier-HHH.

Lacking both 'W' and 'N' Washington National has a code of DCA for District of Columbia Airport. The newer Dulles airport just outside D.C. was DIA (from Dulles International Airport); however, the DIA and DCA were easy to confuse, especially when hastily written in chalk on a baggage cart, scribbled on a tag or a handwritten air traffic control strip, so we are stuck with the backwards IAD. Now one of the rules of the game is "the first and second letters or second and third letters of an identifier may not be duplicated with less than 200 nautical miles separation."

Houston has HOU for the William B. Hobby airport. The 200 mile rule lead to the airport label of IAH, for the new Intercontinental Airport Houston. Louisville, Kentucky, already had an airport with the logical code of LOU; therefore, the letters for the new airport had to be something radically different: SDF stands for Standiford Field.

The airport ciphers sometimes don't originate with the city or airfield name but with the county in which it resides. Longview/Kilgore in Texas is GGG, from Gregg county airport. The 'W' in Detroit's DTW comes from Wayne county; the 'P' in Greenville's PGV comes from its location in Pitt County, North Carolina. However the John Wayne Airport serving Orange County takes its call letters from the less recognized Santa Ana (SNA).

A little more geography cracks the code for CVG, MDT and GTR. Cincinnati, Ohio, has its airport located in northern Kentucky (look at a map if you don't believe me!); therefore, Cincinnati's ID actually comes from the town of Covington - CVG. Harrisburg International is physically located in Middletown, Pennsylvania (MDT). Any Mississippi State Bulldogs' fan can tell you that Columbus, Starkville, and West Point form the Golden Triangle of Mississippi, with airline service at the Golden Triangle Regional Airport (GTR).

History, rather than geography, solves the puzzle of BNA, TYS, GEG, OGG and MCO. The main airport in Nashville, Tennessee, was named in honor of Col. Harry Berry who helped build it: BNA. Knoxville, also in Tennessee, doesn't have a single letter in common with its tag of TYS; however, a historian would know that the Tyson family donated the land in honor of their son killed in World War I. The current Orlando International Airport stands on the land that used to be McCoy Air Force Base (MCO). Spokane International Airport is coded as GEG in honor of Major Harold C. Geiger, a pioneer in Army aviation and ballooning. Geiger field was renamed in 1960 but the code was not changed. Kahului Airport, Maui, was designated as OGG in honor of aviation legend, and Lihue native, Capt. Bertram J. Hogg (pronounced Hoag).

One of the world's largest airports, JFK, is also one of the very few that changed call letters. A change is rare because an identifier becomes so well known to airline staff that changes are not normally permitted. Interestingly the John F. Kennedy airport's former code also came from the name of the field - IDL for Idlewild airport. If you knew that Fort Myers used to be called SouthWest Florida Regional, the RSW moniker starts to make sense. A code used by American Airlines but never seen by the traveling public is GSW. Pilots spend months at GSW, but no planes land or take-off there. The mystery is solved when you discover that Americans' Flight Academy, with its many simulators and classrooms, is in Ft. Worth on the former site of the Greater SouthWest Airport. A airport that has worked hard to change its given code is Sioux City's Sioux Gateway Airport-SUX. Mayor Craig Berenstein described the SUX code as an "embarrassment". City leaders petitioned the FAA to change the code in 1998 and again in 2002. At one point the FAA offered the city five alternatives-GWU, GYO, GYT, SGV and GAY-but airport trustees didn't like any of them enough to change. In 2007 the airport made the best of their sucky cipher and started promoting the airport with the slogan "FLY SUX."

Years ago, entire metropolitan areas were given a code to include many airports; NYC covered New York City and LON signified London. Unfortunately there are no new metropolitan area codes due to the scarcity of letters. These codes provide the ability to quickly look up in a computerized reservation system all the flights to a certain city without using separate codes for each airport. Entering WAS as a designation will give me the next few flights to BWI, IAD and DCA-the Washington, D.C. area. In fact, three letter codes are so scarce that after a year they can be recycled: when Idlewild Airport became JFK, the old IDL tag was retired then reused for Indianola, Mississippi.

Airlines use the three-letter codes internationally in their own network, Sita, for messages such as passenger loads and departure times. World ATC and weather agencies use a separate teleprinter network, the Aeronautical Fixed Telecommunications Network (AFTN), which uses a four-letter "location indicator." Going from large area to actual airport, the first letter relates to the part of the world and the second letter the country. The third letter is a group of airports within that country. Most countries who use this particular convention use a letter to denote the FIR in which the airport is located. So F is Frankfurt FIR in Germany, M is Munich; P is Paris FIR, M is Marseilles. Other ways to use the third letter include identifying a group of airports with a common factor. For example, A was used in Germany for all Canadian and American air force bases. The last letter positively identifies a specific airport.

Thus Aberdeen, Scotland, has the International Civil Aviation Organization (ICAO) location indicator of EGPD-E for Northern Europe, G for United Kingdom, P for Scottish region, and D for Dyce field. Want to figure out LFPG? It's L for southern Europe, F for France, P for Paris FIR, and G for Charles de Gaulle airport. Easy! Another example to decode is EDMM. E for northern Europe, D for Deutchland (Germany), M for Munchen (Munich) FIR, and M again for the Munich airport. Some are more easy to remember. The Dominican Republic codes all start with M (Central America) and D (Dominican) and then, well, see if you spot the code for international airports in Santo Domingo (MDSD), La Romana (MDLR), Puerto Plata (MDPP), Punta Cana (MDPC) and Santiago (MDST).

So if London Heathrow has two codes - and it does, LHR and EGLL - how come I've heard Chicago O'Hare only called ORD? The answer is unique to the United States. In the 48 contiguous States the ICAO code is formed simply by adding a "K" to the FAA code. This explains why international flight plans refer to KORD, KMIA, KJFK, etc. A meeting of two rules is Key West, the FAA code is EYW (lose the 'K') and the IATA code is KEYW (add a 'K') which works great for KEY West.

Other airports within FAA jurisdiction have ICAO codes usually formed by taking two letters from the FAA name and prefixing them with PA for Alaska, PH for Hawaii, or PG for Guam. PT appears to be Pacific Trust Territories (Pohnpei, Yap, Chuuk, etc.), and there are various random other P codes like PWAK for Wake Island.

Instrument pilots are familiar with another type of identifier, not for an airport or navigation aid but simply for a point in space. Every airway intersection is a five letter, supposedly pronounceable, combination, from AADCO to ZZARP. Just like airports, some fixes are named after towns: close to Raleigh, N.C., are DUHAM (over Durham) and CHAPL (over Chapel Hill) intersections. Some are named after people or events, and others are named just for fun: BORED, BUILT, BUTCH. The excellent book Chasing The Glory, by Michael Parfit, introduces us to the woman who names intersections, Macho Irene. She dreams of some pompous B-747 captain saying, "Roger, will report MUMMI."

From LA to DUHAM, identifiers have grown in complexity and are now used as computer codes in vast reservations systems and flight management computers. The latest trend in bag-tagging technology is laser printed bar codes for automated luggage systems. Will # ## # ## ever completely replace DFW? The answer may be found at DIA, the new Denver International Airport which used identifier DVX when the runways opened for testing, but inherited the familiar DEN code when Denver's Stapleton airport closed down.

Oh, still wondering about the world's busiest airport, O'Hare International, and its ORD code? Well once upon a time, before the editor and publisher of the Chicago Tribune, Colonel Robert McCormick suggested a name change as tribute to pilot Lt. Cmdr. Edward "Butch" O'Hare, United States Navy, there was an airstrip well to the northwest of Chicago with a quaint, peaceful name-Orchard Field.

-- Dave English

This article was published in the journal of the Air Line Pilots Association, Air Line Pilot, in December of 1994. I wrote it, but ALPA holds the copyright and reserves all rights other then you reading it directly from this Internet page. This online version is kept updated and revised.

ICAO airport codes entry at Wikipedia
IATA airport codes entry at Wikipedia
Official IATA Airline Coding Directory