February 2, 2026
ICAO Codes Explained: The Four-Letter System Behind Every Flight Plan
ICAO codes are four-letter identifiers used by pilots, air traffic controllers, and airlines in flight operations. They cover every aerodrome on Earth — not just commercial airports.
While travelers know airports by their IATA codes, aviation professionals use a parallel system: ICAO codes. These four-letter identifiers are assigned by the International Civil Aviation Organization and cover every aerodrome in the world — from major international hubs to tiny grass strips in remote Alaska.
The structure of ICAO codes
ICAO codes always start with a letter that indicates the world region. The full structure is:
- K — contiguous United States
- E — Northern Europe (EG for UK, EK for Denmark, ES for Sweden)
- L — Southern Europe and Middle East
- Y — Australia
- Z — China and North Korea
The second letter narrows to a country or sub-region, and the remaining two letters identify the specific airport. So EGLL is London Heathrow (E = Europe, G = United Kingdom, LL = Heathrow) and KSEA is Seattle-Tacoma (K = contiguous US, SEA = the IATA code).
Why ICAO codes exist separately from IATA
IATA codes were designed for the commercial airline industry — booking, ticketing, baggage. ICAO codes were designed for safety-critical operational use: flight plans, air traffic control communications, NOTAMs (Notices to Airmen), and weather reporting. The two systems have different coverage requirements and different governing bodies.
Airports with only ICAO codes
Many airports have an ICAO code but no IATA code. These are typically small general aviation airstrips, heliports, and remote facilities that do not handle scheduled commercial traffic. You can find them in our small airports directory.