I cant figure out which sub forum to post this in, but since this pertains to fsx (kind of) I guess I will ask here. Ive noticed how the polar routes change all the time. Sometimes you'll see aircraft fly way north over the Hudson Bay enroute to London, but on other days they will cross Northern Half of the USA and fly straight out into the Atlantic. This pertains to flights flying from the southwest portion of the USA. Is this based on wind? It always seems like it would be the smarter choice to always fly over northern canada to shorten the distance.

The idea is to minimize headwinds and maximize tailwinds. See NAT - North Atlantc Tracks (and PACOTS for the Pacific).Flight planners like FlightSim Commander let you download current NATs and PACOTS and display them on-screen. Cheers, - jahman.

Upper level winds are likely the main driving factor as stated above. If you want a general snapshot of the latest jet stream I have this site bookmarked: http://virga.sfsu.edu/crws/jetstream.html . The latest maps will give you a good general overview of current wind patterns at 300MB (about 32,000 feet). Another factor I think is worth mentioning and could be the reason why airliners sometimes take the southern routes is due to abnormal levels of radiation at the upper altitudes further north. I don't know how many airlines look at this information these days but there's a good possibility that their dispatch department may route them further south or fly them at a lower altitude to avoid high exposure areas. Like a say, I'm not sure if all the airlines consider this but I think some of the larger ones do. Someone can correct me if I'm wrong. Not that this would be at all a factor in FSX but for what it's worth.

Hi Spencer You may find this interesting - a track calculator by Michael Vone: http://www.flightsim.com/kdl.php?fid=32396 It enables you to calculate an optimum course based on prevailing winds, and answered a lot of questions I had about transatlantic routes. Mike