PPRuNe Forums - Jeppesen Air Charts

Source: http://www.jeppesen.com/download/aopa/oct-aopa.pdf

"Because Jeppesen uses the Lambert Conformal Conic projection for the enroute charts, a straight line is as close as possible to a geodesic line (better than a great circle route.) The closer that your route is to the two standard parallels of 33° and 45° on the chart, the better your straight line There are cautions, however. Placing our round earth on a flat piece of paper will cause distortions, particularly on long east-west routes. If your route is 180° or 360°, here is virtually no distortion in the course line."

A geodesic line = G.C. (just doesn't go all the way around the globe, so is technically more accurate description). Anyway, a straight line is a very close approximation of the G.C. route... good enough for government work :}