Hi, been flying on VATSIM for a while and observing on Euroscope and FR24, how do airlines assign callsigns to their flight?
In the most simple cases, most airlines (American, Cathay etc) just use the assigned flight number as the callsign, e.g. flight CX524 has the callsign CPA524.
Some airlines (British Airways Shuttle, Iberia, Lufthansa etc) will alter the flight number, adding a letter or two to form the callsign, e.g. IB723 has the callsign IBE07ML, or BA1488 has the callsign SHT6F.
Virgin Atlantic is the only airline I’ve noticed that uses their flight numbers as their callsigns, but adds a letter at the end on most flights, e.g. VS9 has the callsign VIR9J, or VS127 has the callsign VIR127C.
A lot of airlines use a mixture of the callsign formats above, like British Airways or Finnair.
To my recollection, some of the airlines from Europe may have static flight numbers associated with the airport pairs being used. For example, When they flew it, Virgin flew VIR43 from EGKK-KLAS, and VIR44 from KLAS-EGKK. Since those flights were daily, and you can not have two of the same callsign in the air at the same time, they would append a letter (sometimes two) to the callsign to differentiate between the two flights in the air at the same time.
By contrast, we could have multiple United or Jetblue flights from KLAX-KJFK in the air at the same time, each with a different callsign. In short, sometimes the flight number is only used on that given route.
It’s often done to prevent confusion when similar callsigns either will or might be in the same airspace at the same time, e.g. if ABC9 and DEF9 from different airlines in the same airspace they might use ABC9J and DEF9Q. Or one might operate with a completely different callsign such as DEF41TH.
Another reason is if a daily international flight is so late that the next one with the same flight number might be in the same airspace, so one might operate as ABC12 and the next one as ABC12D.
You can read up on how it works, and the rules associated, it also works to eliminate letters and numbers that look similar like O and 0, or I and 1.
Airlines that subscribe to the service send in their summer or winter flight numbers and the system works out if there is going to be conflicting flights.