Callsign, flight number, registration and hex code: what each one is

Click on an aircraft in any flight tracker and you get a small pile of codes. They are not the same thing written four ways: each identifies something different, and knowing which is which tells you how to find a flight again.

The four identities at a glance

The examples are those of the Air Transat A330 in our go-around guide, as its transponder and the aircraft registers give them. The flight number is not among them: an aircraft broadcasts its callsign, not the number on your ticket, and trackers fill it in from schedules.

Flight number: what you book

The flight number is the airline's two-character IATA code followed by up to four digits: BA117, EK408, FR9647. IATA's codes are made for reservations, timetables and tickets, which is why your boarding pass, the departures board and airline apps all use them.

The same flight number usually runs every day on the same route, but not always with the same aircraft, and codeshare flights carry several numbers at once.

Callsign: what air traffic control calls it

The callsign is what the aircraft is called on the radio and in its flight plan. For an airline it is the three-letter ICAO designator plus a flight identification: BAW117 rather than BA117. On the radio the designator is spoken as a word of its own, the telephony designator, such as "Speedbird" for British Airways. ICAO notes that its three-letter designators are not to be confused with IATA's two-character codes.

Callsigns often differ from flight numbers on purpose. When two flights on the same frequency have similar numbers, crews can mishear an instruction meant for the other one, a problem known as call sign confusion that has led to aircraft climbing or descending when they should not. Many airlines in Europe and the Middle East therefore fly with alphanumeric callsigns. An ICAO paper gives real examples: Emirates EK408 flying as UAE58E, Qatar Airways QR672 as QTR71C, Ryanair FR9647 as RYR9QV. EUROCONTROL runs a service that checks airlines' schedules for similar callsigns before each season.

That is why a flight tracker may show an easyJet flight as EZY46AK: you cannot always work out the callsign from the flight number, and the reverse.

Aircraft that are not airline flights usually use their registration as their callsign, or a callsign agreed for the job, such as a military or air ambulance callsign.

Registration: what is painted on the aircraft

The registration identifies one aircraft, like a car's number plate, and is painted on its side. It starts with the nationality mark of the country where it is registered, taken from the international radio call sign series: G- for the United Kingdom, D- for Germany, C- for Canada, and N for the United States, where it is followed by up to five characters, never the letters I or O. In Germany, the first letter after the D shows the category of aircraft.

The registration stays with the aircraft for as long as it is registered in that country, whichever airline flies it and whichever flight number it carries.

24-bit address: the transponder's identity

Every transponder that can be addressed individually has a 24-bit ICAO address, usually written as six hexadecimal characters such as 4CA304. There are about 16.7 million possible addresses. Countries hand them out from blocks allocated in ICAO's rules, so the first part of an address shows which country registered the aircraft, but the address itself says nothing about the type or the operator.

Ground radar uses the address to question one aircraft at a time, which is what Mode S does, and every ADS-B message carries it. In the United States, each registration number comes with its own Mode S code, so the address follows the N-number. When an aircraft moves to another country's register, it gets a new registration and a new address.

For anyone following one aircraft, the address is the most reliable key: it is in every message the transponder sends, even when no callsign is.

Privacy: when the codes are hidden

Some owners do not want their aircraft followed. In the United States, the FAA's LADD programme filters chosen aircraft from the data it shares with tracking companies, which agree not to display them. Its Privacy ICAO Address programme goes further: eligible aircraft can use a temporary address that is not linked to the owner in the public register. The aircraft still broadcasts, so it can still be received; what is hidden is who it is.

How FlyScout shows them

FlyScout says who a flight is in words first, and puts the codes last. A finding reads "Go-around at London Gatwick", then "United Kingdom · Air Transat · Airbus A330-200", and only then the callsign and registration in small type. The words come from the codes: the airline from the callsign's designator, the aircraft type from aircraft databases kept by address, and the place from the position.