Why doesn't a plane fly in a straight line?

The main reason has nothing to do with aviation: it's geometry. On a spherical surface, the shortest distance between two points is the arc of the great circle connecting them — the circle whose center coincides with the Earth's center. When that curve is drawn on a Mercator projection — the one used by almost all maps and onboard screens — it appears arched toward the nearest pole.

The classic example is Madrid–New York: the optimal route climbs toward the North Atlantic instead of crossing horizontally, making anyone looking at the map think the plane is taking a detour. The opposite is true. A flight between Europe and East Asia passes close to the Arctic for the same reason. On a globe with a taut string, the supposed detour disappears.

The second cause is meteorological. At cruising altitudes, jet streams flow — ribbons of wind several hundred kilometers wide where speed can exceed 200 km/h (about 110 kn) and in extreme cases double that figure. Flying within a favorable stream shortens flight time and fuel burn; flying against it increases them sharply. That's why transatlantic routes aren't symmetrical: eastbound flights seek out the jet stream, while westbound ones avoid it, and the optimal route is recalculated daily based on the forecast.

Over the North Atlantic this is organized through an oceanic route structure published twice a day and adjusted to the jet stream's position. Dispatchers also choose a cruising altitude and speed that minimize total cost, not distance, and avoid areas of turbulence, convective storms or volcanic ash, which force detours of hundreds of kilometers.

Over land the aircraft doesn't choose freely: it flies along published airways, defined by reporting points and radio aids, with flight levels assigned according to direction of travel. Each airport's standard departure and arrival procedures also impose curved paths for separation and noise reasons over populated areas. Air traffic control introduces additional deviations to maintain separation between aircraft.

Added to that are no-fly zones: airspace closed due to armed conflicts, military restrictions or overflight denials under agreements between states. Over the past decade several closures have completely redrawn entire corridors between Europe and Asia.

Finally, twin-engine flights over ocean or desert operate under ETOPS rules, which limit the maximum distance to a suitable alternate airport, expressed in minutes of flight with one engine inoperative. This requires the route to stay within certain arcs around available diversion airports, which can slightly separate it from the theoretical great circle. Modern performance-based navigation allows more direct paths than the old corridors between radio aids, but even the most direct flight possible will still appear curved on the map.