United uses a lot of different ways of assigning and utilizing available flight numbers in their available range, some practices which I've seen other airlines use as well. (I'm using flight numbers from today for example's sake)
Out-and-back numbers are quite common, but usually only used in United Express flight number ranges (ie UA6923 SFO
Also, "thru" flight numbers (ie UA930 SAN
and UA931 LHR
) save on separate flight numbers as well. In some cases they stretch even further, like UA839 which is assigned to ORD
. This sort of flight number assignment has seen widespread deployment in UAX operations recently, offering SAT
under UA7024, or YHZ
under UA7561, for example. While the allocated flight numbers on through flights or out-and-backs are always assigned to the same United Express carrier, change-of-gauges (COGs) are common (I've seen a SLC
flight number allocated to SkyWest with a CRJ700 for the first leg and a EMB-120 for the second leg).
You can also find a limited number of flight numbers which look as if they operate hub-to-hub, but stop in an intermediate location. UA6414, for example, operates DEN
On the codeshare side of things, airlines do have to be selective in which flights they would place codeshare numbers on if they are conserving their available numbers. A way to stretch these numbers is to place one codeshare number to match a "thru" flight number of the operating carrier. A current example would be CO6062, which overlays UA930 on the same routing SAN
It is probably technically possible to use a single codeshare number on an out-and-back even though the operating carrier utilizes different numbers on those legs, but in practice I haven't seen it myself.
Assigning duplicate IATA 2-letter codes for the same airline purely for expansion purposes poses a problem mainly because the majority of the codes are already assigned to airlines around the world. IATA designators identify an airline for ALL
commercial purposes, not just limited to flight numbers but also on tickets, airline tariff displays in ARC filings and GDS systems, and telecommunications (for example telex messaging). Overlapping in certain areas would prove costly. Plus it would be confusing for passengers.. In this case it would be smartest to just use the operating carrier rather than assigning a secondary identifier for codeshare purposes only.
Because all the combinations are for the most part completely assigned, IATA currently utilizes a "controlled duplicate" system where smaller airlines in different regions of the world are assigned the same two-letter code, to ensure that they cannot be mistaken for one another.
Airlines in alliances, JV
agreements, etc. don't always have to put their flight number on every single leg, as fare filings' "flight applications" usually permit travel on other alliance members with their own code (usually seen with something like "ANY LH
FLIGHT OPERATED BY LH
" under a UA
-filed transatlantic fare, for example), thus decreasing the need for additional codeshare number usage, while still providing the passenger benefit for a single filed through fare from origin to destination, along with airline loyalty program harmonization/cooperation.
Head in the clouds... yet feet planted firmly on the ground.