Here is a puzzle we will use to introduce an essential navigation concept for solving the most famous mystery in aviation history (see Which Way, Amelia?)

Depicted in the diagram is the schematic of the plan for a flight by dead reckoning from an airport in the south across a large body water for a landing at an airport located close to the coastline.  A point-to-point course line has been drawn.  Diverging lines indicating the maximum expected navigational error attributable to unknown winds aloft and to steering errors.

Also shown are the limits of visibility expected as the aircraft reaches the shoreline.  If the intended course is maintained, all will be well and good.  Near the extremes in navigational errors, however, the airport may not be visible.  The pilot must decide which way to turn.  A wrong decision will cost extra flying time and fuel -- if indeed there is enough fuel remaining after the crossing for traipsing up and down the coast looking for the airport.