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    Re: Flying to Antarctica
    From: Antoine Couëtte
    Date: 2019 Jan 21, 16:41 -0800

    RE : Flying-Antarctica-Morris-jan-2019-g44044

    Hello Brad,

    " I am not quite sure what they mean by "true direction".  Is that the instantaneous course on the standard latitude/longitude globe?   I think so. ": Yes, True direction is a direction referenced to the geographical North.

    My reference states that the grid system simplifies in that the wind direction remains constant. " : This statement needs clarification. As long as the high altitude winds follow a great circle track - which may often be [approximately] the case on rather short distances (e.g. up to 1,000 NM), but not always since jetstreams sometimes sharply "turn" generating [strong] Clear Air Turbulence - then the "grid wind direction" remains constant since on the Aircharts used for transpolar navigation any Great Circle is very close from a straight line. Since straight lines cross the airgrid at the same angles, then - yes - the grid wind direction remains [approximately] constant.

    Best Regards,


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