When you look up a flight, you usually search a flight number — the code on your boarding pass, like BA249 or UA100. But the pilot never says that on the radio, and it is not quite what a controller sees on screen. That job belongs to the callsign. The two point to the same flight, but they are built differently and don’t always carry the same number.
Flight number: the commercial label
A flight number is a marketing and scheduling identifier. It pairs a two-letter IATA airline code with a number — BA for British Airways, UA for United, JL for Japan Airlines. So BA249 is British Airways flight 249. This is the code your ticket, the airport departure board, and booking sites all use, because it is short and easy to read.
Flight numbers also carry hidden meaning. Even numbers often mean eastbound or northbound legs, odd numbers the return; low numbers tend to be flagship long-haul routes. Airlines reuse the same number daily for a scheduled route, so BA249 is the same London–São Paulo service every day.
Callsign: the radio and radar identity
A callsign is what the crew speaks to air traffic control and what the aircraft broadcasts electronically. It uses the airline’s ICAO telephony designator — a distinctive spoken name — plus a flight number. British Airways’ designator is Speedbird, so BA249 becomes “Speedbird 249” on the radio. United is United, Japan Airlines is Japan Air, Emirates is Emirates.
Written out for radar and ADS-B, the callsign uses the three-letter ICAO airline code instead: BAW249, UAL100, JAL and so on. That is the string a flight tracker actually receives from the aircraft. It is deliberately unambiguous over a crackly radio — no two airlines share a telephony designator, which matters when a controller is talking to dozens of aircraft at once.
Why they don’t always match
You might expect BA249 to always fly as Speedbird 249 — but often it doesn’t. Airlines change the callsign number when two flights with similar-sounding numbers would be on frequency at the same time. If Speedbird 249 and Speedbird 429 risk being confused, one may be reassigned to something like Speedbird 27AB. This is a flight number/callsign mismatch, and it is common on busy routes.
Codeshares add another twist: a single aircraft might be sold under several flight numbers by different airlines, but it flies under just one callsign — the one belonging to the airline actually operating the metal.
What you’ll see on the radar
On the FlyOverhead radar, the label attached to each aircraft comes from its broadcast callsign — the BAW249 form, not the ticket’s BA249. FlyOverhead maps that back to the airline and, where it can, the recognisable flight number. If a plane overhead shows a callsign that doesn’t match any scheduled flight number you can find, you’ve likely spotted a deliberate mismatch — the same flight, wearing its radio name instead of its ticket name. Pair this with the squawk code and the ICAO type code and you can read almost everything a controller sees.