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    Re: Allowing for current. was: Re: Noon sun fix example
    From: Gary LaPook
    Date: 2009 Oct 02, 13:32 -0700

    This basic procedure is used on long range trans oceanic flights which
    allows shorter flight times with savings in fuel by the use of "pressure
    pattern navigation." An illustration will make this clear. Say you are
    planning a three thousand mile flight directly across a low pressure
    area to a destination located exactly on the opposite side of the
    pressure system. If the plane wants to stay exactly on course it will
    have a left cross wind for the first half of the flight and a right
    cross wind for the second half of the flight. The pilot will have to
    adjust his heading to put in a cross wind correction angle, first to the
    left and then to the right,  and this will result in a lower ground
    speed on each of these legs. But if he just takes up the heading
    straight for his destination the left crosswind will blow him off course
    to the right for the first half of the flight and then the right
    crosswind will blow him back to the left the same amount during the
    second half and he will arrive at his destination without changing
    heading and in less time than if he had tried to maintain his exact
    course line. He will also get a tailwind. Since it would be rare for the
    destination to be exactly on the opposite side of the pressure system,
    you modify this procedure to calculate the net drift over the whole
    flight by simply looking at the difference in the air pressure at the
    departure and at the destination by use of the latitude scale on your
    E-6B. (There, you pilots never had any idea what that scale was for, now
    did you?) You then aim for a point to the upwind side of the destination
    to account for the net drift. Then you can maintain that heading and you
    will end up at your destination.
    George Huxtable wrote:
    > This was triggered by Jeremy's reference to the effect of current, but it's
    > about a rather different situation, of tidal currents.
    > It's arisen here before, a long time ago. I can illustrate it best by a
    > little problem that faces many small-boat navigators crossing the English
    > Channel, between Anvil Point, a headland South of Poole, and Cherbourg, to
    > its South by about 50 miles. I will simplify the details, but for many small
    > craft that passage will take about 12 hours, or just two tides' worth. Say
    > the tide flows at about 2.5 knots, a total displacement of about 11 miles
    > East, over the first 6-hour period; then 11 miles West, over the next 6
    > hours..
    > Many navigators will set the destination of Cherbourg as an intended
    > waypoint, then religiously adjust their heading to keep their ground-track
    > along that intended line, angling against the current to keep it so. They
    > are, of course, wasting time and energy. Because, over 12 hours, the net
    > tidal displacement, East then West, will sum up to be close to zero, there
    > will be no overall effect of tidal current. In which case, the correct and
    > easy procedure is to head due South, allowing your craft to be swept
    > up-channel first, then back again later. By ignoring the instantaneous tide,
    > and the resulting cross-track error, the vessel is heading exactly
    > Southwards all the time; the best that can be done.
    > Indeed, set out like that, it seems pretty obvious, but it's hard to
    > convince many navigators that they can, and should, ignore those warnings of
    > cross-track error.
    > George.
    > contact George Huxtable, at  george@hux.me.uk
    > or at +44 1865 820222 (from UK, 01865 820222)
    > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    > >
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