Why planes fly curved routes instead of straight lines

On a flat map, many flight paths look like arcs bending toward the pole. They aren't detours: on a round Earth, the shortest route between two cities is a great circle, and flat maps can't show great circles as straight lines. Real routes then bend further to use winds, follow published airways, and, for twin-engine planes over oceans, stay within reach of airports.

The shortest route is a great circle

A great circle is the line you'd get by slicing the Earth through its center, like the equator. The shortest path between two points on a sphere always lies along one.

Flat maps have to stretch a round surface. On the familiar world maps, great-circle routes don't look like the most direct ones at all, except along the equator and the north–south meridians. That's why a flight between two cities at similar latitudes, such as North America and Europe, appears to arc north on the map: the route really is shorter, it just can't be drawn straight on flat paper. There is one kind of map, the gnomonic projection, on which every straight line is a great circle.

Try it: stretch a string between two cities on a globe. The string follows the great circle, and on a flat map it traces the same curve as the flight.

Winds bend routes too

The shortest distance isn't always the fastest trip. Jet streams, strong west-to-east winds near cruising altitude, can add or remove a lot of ground speed. NOAA notes that fuel is saved by staying in the jet stream when heading east and by avoiding it when heading west.

Over the North Atlantic, the effect is built into the system. Oceanic control centres publish organized tracks each day, placed close to the fastest route for that day's winds, mostly at flight levels 340 to 400. Because of the jet streams, the fastest eastbound and westbound routes are rarely the same, so there are separate track systems for each direction. The flight you take out and the flight home can follow quite different paths. Read can a delayed flight make up time?

Airways and air traffic control

Over land, airliners often fly along published routes. In the US, high-altitude jet routes serve aircraft from 18,000 feet up to flight level 450. Routes are also shaped by busy airspace, restricted areas, weather and instructions from air traffic control, so the flown path is rarely the ideal curve.

Twin-engine planes over oceans

US rules don't allow a two-engine airliner to fly a route more than 60 minutes' flying time (on one engine) from a suitable airport, unless the airline has approval for extended operations, known as ETOPS. With that approval, flights can go farther from land, but they still plan around alternate airports where they could divert. On long ocean crossings, the need for those alternate airports can shape the route.

See your real route, offline

FlyingOver shows your flight on a map along its route, with the countries, cities and coastlines below, even in airplane mode. It doesn't use GPS; it estimates your position from the route and schedule saved before takeoff. Read what state am I flying over?