Of course it allows curves!  It&#x27;s just that they can&#x27;t have curvature less than a great circle....~<p>Due to the way the military produces software specifications, it is very likely that the unbroken chain of subject-matter-experts from the days of ochre pictograms on cave walls to today have ensured that the software program functions <i>exactly the same</i> as drawing lines on charts by hand.<p>Since human navigators did not initially possess the capability to continuously recalculate courses, bearings, and headings, as would be required in a curved flight path, the ground-based and flight computers don&#x27;t do it now.  So planes still keep a fixed heading between a set of points, and any possible roundness is encoded into the turn data or a specialized, predefined pattern.