NavList:

In my experience you had to use whichever projection you were given or whichever you could you could find.  Therefore, it was advantageous to be aware of the advantages and disadvantages of all the projections you were likely to come across and be aware of how best to deal with the disadvantages.  This is probably why the RAF found it took about a year to train a Navigator.  As I recall the standard RAF plotting charts for Navigators were the PM1 (Plotting Mercator 1: 1,000,000) for slower aircraft and the PM2 (Plotting Mercator 1: 2,000,000) for faster aircraft.  I never did work out whether this was because the RAF still had millions of them left from days of yore; to give us more to do to keep us busy; or because Navigators always did like banging their heads against brick walls.

John Howard you said: You cannot plot a radar bearing on a Mercator chart.  Any bearing from an electronic device ie. RDF, or VOR ( an aircraft navaid ) can't be plotted because they are great circle.  Even a visual sight is great circle but is short distance so any projection will do.

Not so,  without the distraction of flying the aircraft at the same time, a Navigator could apply conversion angle.  The first day of our 12 months Navigator training they taught us how to fit our brand new 21” nav-rule into our brand new new 15 inch wide nav-bag.   On the second day they showed us how to stick a red ‘day-glow’ triangle onto the north point of our brand new Douglas protractor, so we always used it the right way up.  Then on the third day they showed us how to cut the conversion angle scale off the bottom of a PM2 chart and stick it with sticky backed transparent plastic alongside the little diagram of how to apply it onto the top of our brand new pencil box.

As I recall, suppose you were heading 270°T in the Northern Hemisphere and Beacon X bore 210° relative and conversion angle from the scale happened to be 2 degrees, X would bear 270°T+210°R-360°=120°T on the great circle.  Therefore you would plot 120°T+180°+2°CA = 302° from X, but I could be wrong.  It was a long time ago.  DaveP