Apologies yes - incorrect - the restrictions and stipulations for 4 engine planes wouldn't come from ETOPS classification (the 'T' standing for twin). Poorly / confusingly written on my part. Thanks for the correction. I was however labouring under the impression that the 180m number had some significance for 4 engine planes, so I wonder where I got that from.

The map above of a LHR-SFO is about as straight as you can get. However, regularly flying the route, there is considerable variance. My northern Canadian geography is hazy but, further east you generally fly south of Iceland but it's not too infrequent that you will fly directly over Iceland, and once we were north of Iceland. Likewise, further west, you generally enter the USA over Spokane but I have come down over Portland and also much further east, flying over SLC.

The OP raised a valid point and it's triggered off some interesting responses.

I remember being a bit surprised once when my wife and I had separate but concurrent flights from LHR to Tokyo, her flight (Virgin) was non-stop and I was Swiss flying via ZUR.
I expected a shorter leg from Zurich - ( it's east of London and nearer japan, I thought)
Turned out that the flight paths mileage LHR-NRT was about 10 kilometres different from that of ZUR-NRT

That only applies to twins.Maybe that is in the back of your mind.

Actually I think more like what's discussed in the link below must have been in my thoughts. Who knows. It's hard enough for me to remember my opinions without also having to remember the reason for them.... ;-)


https://aviation.stackexchange.com/q...uivalent-rules

Remember, unlike that map, the Earth is curved. So when it's put into a 2D format from a 3D reality, it will come out curved. That route is a shorter distance than what a straight line on the map would appear like. The Earth is not a sphere, but more of a squashed oval. Going on a more southerly route means more distance needs to be covered.

Now, generally airlines have logistics departments that work the routing. There are many factors which determine how a flight routes, and it is rarely straight. Even with GPS and the technological ability to fly direct point to point, most planes are still routed from beacon to beacon or nav point. Factors that influence this are airspace rules (restricted spaces around military zones, or countries that don't want foreign planes in their airspace like North Korea), safety factors like distance to diversion airports or radio dead zones, weather..either avoiding the bad stuff or taking advantage of the good weather.., and fuel savings. For example, avoiding a jet stream headwind for 12 hours by going north or south of it, or above it.