Why airport codes look nothing like the city

Airport codes look arbitrary because they mostly are not descriptions of airports at all. They are inherited identifiers — from weather stations, railway telegraph offices, military bases and towns that no longer exist — and the system has never allowed itself to renumber.

They started as weather station codes

In the 1930s American airlines used the two-letter identifiers of the National Weather Service stations at each field. When two letters ran out, a third was added. Airports whose two-letter code already worked simply put an X on the end, which is the entire explanation for LAX, PDX and PHX. The X does not stand for anything.

Some letters were already spoken for

The US Navy holds every radio call sign beginning with N, which is why no American airport code starts with one — Newark could not be NEW and became EWR instead. W and K went to broadcast stations east and west of the Mississippi, and Q was reserved for international telegraph codes. Between them those reservations pushed a great many American airports into codes that look like nothing.

The code outlives the name

• ORD is Orchard Field, the name Chicago O'Hare had until 1949.
• MCO is McCoy Air Force Base, which left Orlando in 1975.
• BNA is Berry Field Nashville, named for a man who ran a works programme in 1937.
• BOM is Bombay, PEK is Peking, MAA is Madras, CCU is Calcutta and BLR is Bangalore — five cities that changed their names and could not change their codes.
• MSY is Moisant Stock Yards, after an aviator who died on the site in 1910.

Changing an IATA code means touching every reservation system, timetable, baggage system and flight plan in the world simultaneously. It is done perhaps once a generation. New York's Idlewild became JFK in 1963 and Baltimore's BAL became BWI in 1973, and those are the famous cases precisely because they are so rare.

Why Canadian codes start with Y

Almost every Canadian airport code begins with Y, and the two letters after it usually come from a pre-existing railway telegraph identifier rather than the city name — which is why Toronto is YYZ and Montreal is YUL. The usual explanation for the Y itself is that it marked a station with a co-located weather or radio facility. Transport Canada has never published a definitive account, several competing versions circulate, and anyone who tells you the answer confidently is repeating one of them.

The four-letter codes are a different system

ICAO codes are four letters and assigned geographically, so they group by region rather than by name: K covers the contiguous United States, C covers Canada, E covers northern Europe, L southern Europe, R north-east Asia, Y Australia. That is why Heathrow is EGLL and Los Angeles is KLAX. IATA codes are for passengers and commerce; ICAO codes are for pilots and controllers. Neither was designed to be guessable from the other.

City codes are not airport codes

LON, NYC, TYO, PAR, MIL, MOW, SAO and BUE are metropolitan codes covering every airport serving a city. Searching LON returns Heathrow, Gatwick, Stansted, Luton, City and Southend together. They are useful for finding a fare and useless for finding a terminal, and booking one by mistake is how people end up 40 miles from where they meant to be.