Next month I'm flying to Okinawa via Amsterdam and Taipei by China Airlines.
I've been looking at the outward and inward routes AMS-TPE and was interested in seeing the difference.
I guess choice of routes is both prevailing winds (as per transatlantic) and political.
KLM flies the AMS-TPE route over Russia in both directions though of course East-West is more northerly than West-East
China Airlines AMS inward;
As you say, winds play a big part in choice of route especially if you consider high level winds are very strong with jet streams blowing at up to 200 knots. If you consider a typical long haul aircraft cruises at about 520 knots in still air, the difference can be sizeable if you aim for tailwinds and avoid headwinds although you’re not going to be stuck in a jet stream the whole time of course.
Politics doesn’t play a big part in it really although there are some bits of airspace to avoid, usually because it’s kicking off on the ground so Syria and Ukraine are out right now. Also, you need to think about diversions should the need arise e.g. if you’re planned destination is in India, don’t divert to Karachi unless it’s essential although overflight is ok. We routinely fly over Afghanistan and ATC is brilliant although for some reason all the controllers have Texas accents......
Most airlines will go for MTT, Minimum Time Track, ie quickest as fuel is the biggest cost. Other things come into it too like Navigation Charges. In broad terms, every country you fly over charges for using its airspace, Nav Aids and of course, ATC. Fly over lots of countries and this can add up especially if the difference between various MTT options is small, it can tip the balance. Most Flight Planning now is highly automated and has all these parameters built in and teams of top blokes who keep it all updated. A typical flight plan for a 10 hour flight is usually accurate to within a couple of minutes and a couple of hundred kg of fuel.
Another factor is ETOPs. This is Extendend Twin (engine) operations, sometimes referred to as EROPs (Range) or Engines Turn Or Passengers Swim. Complicated but basically unless you’ve got >2 engines you mustn’t be more than typically 180 minutes flying time from a suitable diversion en route. Using southerly routes across the Atlantic to eastern Caribbean destinations like Barbados this can be an issue and across the polar routes if weather is bad (you can’t plan to use an airport under 20’ Of snow) and across the Pacific it can be a big factor.