NavList:

I keep a watch on air traffic across the Southern Ocean from Austalia/NZ to Santiago. The most planes I have seen (on FlightRadar24) is five. Sometimes zero. Quite a difference from the north Atlantic. Notes I have kept on my observations and calculations were on scrap paper and unfortunately have been thrown away. The following comments are therefore off the top of my head.

I suggest that there are three reasons that determine routing.

(a)  ETOPS -  some aircraft cross from, say, Sydney to Santiago at latitude 33°S which is approximately the rumb line. I am guessing that this is for ETOPS reasons particular to that flight because at the same time other flights travel further south.

(b) Many planes travel on a what looks like the GC. Having thrown my hotes away I cannot remember the latitude of vertex of the GC. Maybe this route is for minimum distance?

(c) Finally the most interesting flights (to me) travel as high as 75°S. This is probably because of strong westerly winds. If I wasn't a scrooge and paid for a flightradar24 subcription I would know what the wind speed was. Such flights can be predicted becauase after departing Santago they travel more or less parallel to the west coast of South American for an hour or two (almost to opposite Cape Horn). Then they turn west.

While researching ETOPS I discovered that flight dispatching is a profession. Various organisations run courses and issue qualifications.

Currently there is a Santiago - Sydney flight off the southern tip of NZ and a Saab 340 on a Wellington - Chatham Islands flight. The latter is 774 km (have not converted to nm) so is probably on a rumb line.

David C