On one morning in August, a British Airways Boeing 777-300 crossed the radar on my desk labelled BAW28F. Earlier that morning another BA 777-300 had come through as BAW29C. Later on, a Virgin A330 showed up as VIR106F, and a Ryanair 737-800 as RYR19EN.
None of those look like flight numbers you could book. BA flights are sold as BA-something, not BAW-something, and nobody sells a ticket on flight 28F. So what is the screen actually showing?
Two different codes
Every airline has two identifiers. The two-letter IATA code, BA for British Airways, is the one on tickets, departure boards and booking sites. The three-letter ICAO code, BAW, is the one used by air traffic control, and it is the one aircraft broadcast.
The code a flight tracker shows is the callsign, and a callsign is built from the three-letter ICAO code followed by one to four characters. For a long time those characters were simply the commercial flight number. On a quiet radio frequency that works fine.
The problem with numbers
Flight numbers tend to be handed out in batches of sequential, very similar numbers. Put several of them on one busy frequency and they start to sound alike. A crew hears an instruction meant for a different aircraft with a nearly identical callsign, and accepts it.
The UK Civil Aviation Authority's circular on callsign confusion gives the breakdown. Of the callsign confusion occurrences it records, 84% involved purely numeric callsigns. Only 10% involved purely alphanumeric ones. That is why so many airlines have moved to callsigns with letters on the end, and why the callsign stopped matching the flight number.
The rules
EUROCONTROL's Call Sign Similarity rules set out what airlines should avoid when choosing callsigns within their own schedule. Some of them are obvious once you read them:
- Identical final digits. ABC 234 and ABC 534 are too alike.
- Anagrams. ABC 1368, 1386, 1638 and 1836 are easy to scramble.
- Triple repetition. ABC 111 or ABC 444, because a digit is easily dropped.
- Letters that look like digits. An "O" next to a "0", or an "I" next to a "1".
And two are wonderfully specific:
- Anything that sounds like a flight level. ABC 330 could be heard as "flight level three-three-zero". ICAO's own guidance is that, wherever practicable, 0 and 5 should not be the final figure of a callsign.
- Anything that looks like a runway. ABC 27L is a callsign that could be confused with runway two-seven left. Two digits from 01 to 36 followed by L or R are best avoided.
The allowed formats are tidy: pure numbers of one to four digits, numbers ending in one letter, or numbers ending in two letters. So BAW28F is digits plus one letter. BAW21ML and RYR19EN are digits plus two. Every one of them was chosen to be hard to mistake for something else.
What it means when you are watching the screen
BAW28F is not a glitch or a garbled flight number. It is a British Airways flight whose callsign was deliberately picked so that nobody on the frequency could confuse it with another aircraft, a flight level or a runway.
The same goes for the easyJet A319 that crossed the screen as EZY397Q. Next time a tracker shows you a callsign with letters on the end, it is a flight wearing a code someone thought carefully about.