March 22, 2026
How Airport Codes Are Used in Flight Planning
When pilots file a flight plan, they use ICAO codes, not IATA codes. Here is what a flight plan looks like and how airport codes fit into the larger system of aviation identifiers.
Every instrument flight (and many VFR flights) requires a flight plan — a formal document submitted to air traffic control that specifies the aircraft, route, altitude, fuel, and destination. Airport codes are central to flight planning, but pilots use ICAO codes, not the IATA codes passengers see on boarding passes.
The ICAO flight plan format
An ICAO flight plan (ICAO Doc 4444) includes, among other fields:
- ADEP — Aerodrome of Departure (4-letter ICAO code, e.g. KSEA for Seattle)
- ADES — Aerodrome of Destination (e.g. KLAX for Los Angeles)
- ALTN — Alternate aerodrome, where the flight will divert if the destination is unavailable
- ROUTE — A string of waypoints, airways, and transitions connecting departure to destination
Why ICAO codes in flight plans?
ICAO codes have global, unambiguous coverage. Every aerodrome — including small strips, heliports, and seaplane bases — has an ICAO identifier. IATA codes only cover commercial airports with scheduled service. For flight planning, you need a system that covers every possible departure and destination, not just the ones that sell tickets.
IATA vs ICAO in practice
A passenger buying a ticket to Seattle uses SEA. The pilot flying them there files a flight plan using KSEA. The baggage handlers at the destination see SEA on the bag tag. Both systems operate simultaneously — one for commercial passenger operations, one for flight operations. See our full comparison in IATA vs ICAO Explained.