The airports have the threefold classification in demand today: domestic, longhaul, shorthaul. Should this be kept as a baseline in the future too?
Meaning that you cannot fly longhaul from an airport that has in real life only 100% domestic demand for example? I have done some number crunching and this would reduce the need of calculations (number of possible route pairs) quite heavily
When speaking of airports, airport needs to have some facilities for international flights, such as customs office, passport/visa/immigration control facility. If the airport does not have it, no international flights should be allowed.
Did some checking again, and if we use the present-day demand allocation (DOM, SH, LH) we end up with about 950 000 route combinations that can have demand more than 0 pax (don't say they have, but with settings and demand allocation in mind, it would be possible). This figure is bearable taking into account the required update interval.
If we take into account only the country relations (where for example demand between two countries is set to zero by master settings), and the route minimum length rules (ie. domestic route must be at least 40 NM etc), we end up with 7,5 million potential route combinations. Which is too much to handle if the demand system would become "live" and changeable with the squares method.
So would need to come up with the mentioned classification for airports to eliminate most of the 0-demand routes right from the beginning, by setting certain airports 'domestic only' and so forth.
Probably a combination of current demand allocation (if DOM=100%) and low overall pax level at the airport (< 50 000 pax / year?) and low airport size classification (sizeclass < 3?) would do the trick... With the ability to set the 'domestic only' flag manually too for any exceptions to this, and perhaps even hardcode it to events module for some historical changes (ie. airport turning from intl to domestic only).
As far as combinations to consider, the system should perhaps calculate only the ones that are "connected" by scheduled flights.
Due to the increased complexity of the square demand model and base demand has to be pre-calculated in my mind, and it also enables us to make smooth "over time" changes to the demand instead of rapid overnight drops. It could be for those flights that have schedules but then route planning would become quite slow, as it would need to calculate the demand data on the fly for non-served route you are checking out. But this is the another option - currently I will focus on seeing if it's feasible to make the base data of all possible combinations in the background...
As far as international airports not allowed domestic flights - my guess is that there are only handful of those, and could be flagged manually. But it would put some airports at a disadvantage (for example Narita).
Any other airports that are international, but do not allow domestic flights? Cannot think of any really, so in that sense I would leave this "intl only" flag out alltogether.
edit: here's a list of all airports that have 0% domestic demand set in database, I guess most of these are in small countries that have no other domestic airports or they are too close - or have only LCC operating to them, like EBCI and EKSN?VHHH, VHHK, WSSS, WSAP, EBBR, OTBD, LHBP, OKBK, OBBI, VMMC, OLBA, PGUM, TBPB, ESKN, TFFF, ELLX, EBCI, FMEE, LJLJ, WBSB, EHRD, TIST, LCEN, TXKF, LATI, FYWH, WPDL, LUKK, TLPL, GUCY, SOCA, TLPC, TISX, LXGB, TUPJ, DBBB, SMJP, NCRG, EBLG, PLCH, ANYN, EHGG, TVSV, TQPF, GGOV, FDMS, EBOS, EBAW, PTRO, FXMM, EIWF, LUBL, TVSU, VNVT, LJMB, VQPR, WSSL, LHSM, HKEL, LHDC, LHPR, YPXM, GMMH, LFBU, EERU, EGMD, KHKS, KCXY
I'm not sure if this helps, but most domestic airports cannot support the largest planes (with exceptions). You can fly a 747 into a small airport with a short runway if you only have a handful of passengers, but this would be pointless. The easiest way to overcome this, in my opinion, would be to have a standardized max distance avail based on max runway length.
Indeed .. This sounds smart too, with some alterations. There is the airport size classification vs. airplane size limit already in place, so this is the easiest way to determine this perhaps:
- if airport can accept only sizeclass "small" planes
- check generally what is the longest range "small" plane (the 'ferry range')
- if the route length is over that (^), then demand is assumed 0.
- this does not take into account possible techstops, but generally speaking should be enough as the ferry range (=full fuel, no pax) is used.
Also, while JumboShrimp has a good point about international airports needing customs facilities etc, you essentially have a chicken/egg problem. If a major carrier like Delta came into a smaller airport and said they wanted to fly daily flights to an international destination, you'd see a customs office pop up overnight.
But would see this quite unlikely..? And perhaps we should set some realistic limits anyway to avoid people making "ridiculous" hubs and transfer airports?