In post 16985, I would take exception to the assertion that planes do not fly in a straight line. A great circle route drawn on a Mercator projection will often exhibit a curve because of the limitations of the medium. However if you take a string and use it to connect two points on a globe, what you see is a perfect depiction of a great circle route.
Now to achieve this, given our method of charting our earth the airplane may have to constantly change headings, but no turns must be made.
Best
Les
