Why Planes Do Not Fly in Straight Lines on Flat Maps
When passengers watch their in-flight entertainment flight map, routes between North America and Europe or Asia frequently curve high over Greenland, Alaska, or the Arctic Ocean. While flat Mercator maps make these paths look like dramatic detours, they actually represent the absolute shortest path over the curved surface of the Earth.
In spherical geometry, the intersection of a sphere with a plane passing through its center is known as a Great Circle. Every meridian of longitude is a great semi-circle, while the Equator is the only parallel of latitude that forms a great circle. All other straight lines drawn on flat charts (known as rhumb lines or loxodromes) maintain a constant compass heading but cover significantly more nautical miles.
Jet Streams and Flight Planning
Commercial airlines rarely fly pure great-circle arcs in real-time operations. Airline dispatchers optimize daily flight trajectories using organized track systems (such as the North Atlantic Tracks and Pacific Organised Track System) to ride powerful jet streams traveling west-to-east, or detour around severe weather and restricted military airspace.