What Is ICAO Code? Definition & Meaning in Aviation
ICAO codes are the codes used by pilots, air traffic control and weather reports: three letters for airlines (PIA) and four letters for airports (OPKC).
In plain language
Airport ICAO codes follow a regional pattern: airports in Pakistan start with OP. Airline ICAO codes are what you see at the start of a radar callsign.
Example
OPKC is the ICAO code of Karachi’s airport, and its METAR is reported under that code.
Why it matters
ICAO codes are what weather reports, flight plans and trackers use. If you look up a METAR or a callsign, you will need the ICAO code, not the IATA one.