Why airplanes fly in arcs: great circles, the jet stream and North Atlantic tracks

If you follow a long-haul flight on a flight tracker for the first time, you wonder: the aircraft from Frankfurt to Los Angeles doesn’t head straight across the Atlantic but steeply northwest, over Iceland or Greenland and through Canada. On a world map, that looks like a huge detour. In truth, the arc is the shortest path – and the fact that it looks a little different every day comes down to the wind at ten kilometers altitude and a route system over the North Atlantic that is redrawn every day. Here you learn how the great circle, the jet stream and North Atlantic tracks work together and what that means for you at the fence, in the window seat and in front of the flight tracker.

The short answer

Airplanes don’t fly detours but roughly the shortest line on the globe. This line is called a great circle. Because every flat world map distorts the sphere, the great circle looks there like an arc bulging toward the pole. Two more things then bend the real route again:

• Wind: The jet stream at cruising altitude often makes the fastest route in terms of time a different one from the geometrically shortest.
• Rules and airspace: Over the North Atlantic there is a daily track system, over land a fixed air traffic control route network, plus closed airspace and requirements for twinjets.

The great circle: geometry that looks like a detour

Sphere versus map

Picture a globe and pull a string taut between Frankfurt and Los Angeles. The string does not lie along the 50th or 40th parallel but runs far to the north. That is exactly the great circle: the section of a circle whose plane passes through the center of the Earth. The equator is a great circle, and so is every meridian – parallels away from the equator are not. That is why following a line of latitude is almost never the shortest way.

On the familiar Mercator map used by many map services and flight tracker sites, areas near the poles are stretched strongly sideways. A line that looks straight on this map is a so-called rhumb line: a path with a constant compass heading. It is convenient to steer but longer. The true shortest path appears on the same map as an arc. Only on special map projections, the gnomonic maps, is the great circle a straight line.

How big is the difference?

How far the great circle and the “straight line on the map” are apart depends on two things: how far the route extends east–west and how far it lies from the equator. We calculated the following table with the airports’ coordinates on a sphere with the mean Earth radius. The values are rounded and show pure geometry, not flown distances.

Connection	Great circle	Constant heading (rhumb line)	Northernmost point of the great circle
Frankfurt – Dubai	approx. 4,840 km	approx. 4,900 km	approx. 50° N (starting point)
Frankfurt – New York JFK	approx. 6,190 km	approx. 6,500 km	approx. 54° N
London Heathrow – Los Angeles	approx. 8,760 km	approx. 9,730 km	approx. 62° N
Frankfurt – Los Angeles	approx. 9,320 km	approx. 10,560 km	approx. 65° N
Munich – San Francisco	approx. 9,440 km	approx. 10,930 km	approx. 68° N
Frankfurt – Tokyo Haneda	approx. 9,360 km	approx. 10,750 km	approx. 67° N

You can see the pattern: to Dubai, which is mostly southeast, the arc hardly matters. To the US West Coast or Japan, however, the great circle saves well over a thousand kilometers – and in return leads up to the latitudes of Iceland and southern Greenland. That is why many flights from central Europe to the west coast of North America cross Greenland and northern Canada.

The jet stream: tailwind at ten kilometers

What the jet stream is

The jet stream is a narrow band of very strong westerly wind in the upper troposphere. In the Northern Hemisphere there are mainly two: the polar jet stream, roughly between 30 and 60 degrees north, and the somewhat higher subtropical jet stream around 30 degrees north. The polar jet sits at about 9 kilometers altitude – exactly where airliners have their cruising altitude. Why airliners fly that high at all is explained in the article Why airplanes fly at around 10,000 meters.

Typical wind speeds in the polar jet are around 180 km/h, considerably more on some days; the highest measured values are around 400 km/h. The band is usually a few hundred kilometers wide but only a few kilometers thick, and it meanders: sometimes it runs almost straight from west to east, sometimes it makes wide waves to the north and south. It also shifts with the seasons – poleward in summer, toward the south in winter.

The stream was discovered in stages. The Japanese meteorologist Wasaburo Oishi detected it in the 1920s with pilot balloons near Mount Fuji but published in Esperanto, so his work remained little known outside Japan. The German term “Strahlströmung” (jet current) was coined by Heinrich Seilkopf in 1939. The strong high-altitude winds became widely known in World War II, when bomber and transport flights kept running into them.

What it does to the route

For airlines, the jet stream is money. A widebody jet flies through the air at roughly 900 km/h at cruising altitude. Add 150 km/h of tailwind, and it covers correspondingly more distance over the ground per hour – and burns less fuel for the same distance. That is why:

• Eastbound, the route is deliberately placed in the jet stream, even if that makes it somewhat longer than the great circle. The time gained from the tailwind outweighs the extra distance.
• Westbound, planning tries to avoid the core of the jet stream – for example, with a route farther north or south than the great circle.

That is the reason the same flight often draws visibly different lines on the flight tracker on the outbound and return legs, and why flight time westbound is noticeably longer. In situations with a particularly strong jet, ground speeds far above 1,000 km/h show up on flight tracker sites for eastbound jets – not because the aircraft flies faster, but because the air it flies in moves along. What the values on the map mean exactly is explained in Symbols, colors and data fields on flight tracker maps.

The flip side: at the edge of the jet stream, clear-air turbulence often occurs that you neither see nor detect on weather radar. That is also factored into route planning. The role played by flight dispatchers, who work out every route against weather, weight and fuel, is explained in the article on flight dispatchers.

The North Atlantic Tracks: highways rebuilt every day

Why they exist

Over the open Atlantic, there was long no radar. Air traffic control knew positions only from crew reports and therefore had to separate aircraft with large gaps. Because at the same time a large share of the traffic between Europe and North America travels at similar times, the Organized Track System emerged: a series of parallel routes that are laid out anew every day to match the current wind situation.

Mainly two control centers are responsible. Shanwick – the name combines Shannon and Prestwick – is operated by British air traffic control NATS at the Prestwick Centre in Scotland, with shortwave radio traffic running through a station in Ballygirreen near Shannon in Ireland. On the other side is Gander in Newfoundland, operated by Nav Canada. To the north, the Reykjavík control area borders it, and to the south that of Santa Maria in the Azores.

How the tracks work

• New every day: The tracks are set every day in coordination with airlines and neighboring control units and published via NOTAM.
• Two directions: The westbound tracks apply at 30 degrees west from roughly 11:30 to 19:00 UTC, the eastbound ones from roughly 01:00 to 08:00 UTC. So traffic toward North America flows during the day, and traffic toward Europe at night.
• Letters instead of numbers: The westbound tracks are called A, B, C and so on, with A the northernmost. The eastbound tracks count backward from Z, with Z the southernmost.
• Aligned with the wind: The eastbound tracks follow the strongest tailwind, the westbound tracks avoid the headwind. If the jet stream lies far to the south, the westbound tracks can swing far north toward Iceland and Greenland.

This rhythm, by the way, also explains why long-haul flights from North America mostly land in Europe in the morning and the flights in the opposite direction depart between late morning and afternoon. How to use this for your spotting day is in the article When long-haul flights land and depart in Europe.

What has changed

The system has become more flexible. In 2015, the minimum lateral spacing between tracks was halved from 60 to 30 nautical miles. Since 2017, flights in North Atlantic airspace may plan any flight level, not just the standard ones. In 2019, satellite-based ADS-B was added: receivers on satellites pick up the aircraft’s position reports even in the middle of the ocean. According to NATS, the required longitudinal separation dropped from about 40 to 14 nautical miles as a result, and more flights can choose their most economical route. In 2021, the track system was scaled down, and on some days there were no tracks at all. Using the tracks is no longer mandatory – many flights now plan free routes, but air traffic control can assign a track.

How the satellite receivers work and why flight tracker sites benefit from them too is covered in the article Space-based ADS-B.

More reasons for kinks in the route

The great circle and the wind explain the big arcs. The smaller corners and detours have other causes:

• Route network over land: Over Europe, jets fly along a network of waypoints and airways. After takeoff and before landing, they follow fixed departure and arrival routes – more in SID and STAR.
• Closed airspace: Since 2022 – as of October 2026 – Russian airspace has been closed to airlines from the EU, the United Kingdom and the US. Flights between Europe and East Asia have since detoured south over Central Asia or use polar routes again over Greenland, Canada and Alaska – with flight times that are in some cases considerably longer. For airlines like Finnair, whose network was built around the short path over Siberia, this was a deep cut.
• ETOPS: Twinjets may only move as far from suitable alternate airports as their certification allows. Over oceans and polar regions, that can influence the route. What is behind the rule is explained in ETOPS explained.
• Weather: Thunderstorm cells, volcanic ash or heavy turbulence force diversions that show up as zigzags on the flight tracker.
• Holding patterns before landing: The ovals in front of large airports have nothing to do with the long-haul route but with capacity at the destination – see Holding patterns explained.

Polar routes: the great circle in its most extreme form

How radically geometry can shorten routes is shown by the history of polar routes. In 1954, SAS was the first airline to open a scheduled service across the Arctic, still with DC-6B propeller aircraft between Los Angeles and Copenhagen, with stopovers in Greenland and Canada. During the Cold War, many flights from Europe to Japan went via Anchorage in Alaska because Soviet airspace was closed. From 1983, Finnair was the first airline to fly nonstop over the pole between Helsinki and Tokyo.

After the end of the Soviet Union, the route over Siberia became standard for Western airlines. Since 2022, many Europe–Asia flights have had to rely on detours again – part of it leads over Greenland and Alaska and thus once more close to the pole. You can find more about the longest nonstop connections in the world and why they are planned the way they are in Ultra-long-haul flights.

What the arcs mean for you as a spotter

On the ground: overflights and approaches

The great circles from Europe to North America lead over the British Isles, Iceland and the Atlantic; the routes to Asia and the Gulf run over eastern and southeastern Europe. That is why, over central Europe at certain times of day, you see whole chains of contrails in a similar direction. How these trails form and why they sometimes linger for a long time and sometimes vanish immediately is explained in Contrails. If you want to photograph the overflights at cruising altitude, the guide Photographing aircraft at cruising altitude helps.

For the airport itself, the route is mostly a question of timing: a strong tailwind over the Atlantic often brings the morning wave in Frankfurt and other hubs in earlier than scheduled. Widebody jets like the Airbus A350-900 or the Boeing 787-9 then show up on final approach, in some cases well ahead of the scheduled time.

In the window seat: choosing the right side

If you are flying to North America yourself, take a look at the route. On a northern track westbound, you see Greenland’s ice cap or the coast of Iceland in clear weather. How Keflavik plays a role as a connecting point for many transatlantic connections is shown by the airport page. Which side you choose for photos out of the window depends on route and sun position – tips in the article Choosing the best window seat for photos.

In front of the flight tracker: reading routes

Common misconceptions

• “The airline flies a detour to save fees.” Overflight charges and airspace structures play a role in planning, but the big arcs over the far north are explained simply by the spherical shape of the Earth.
• “Ground speed shows how fast the aircraft flies.” It shows how fast it moves over the ground. The speed through the air stays similar with tailwind and headwind.
• “The same route every day.” Over the Atlantic in particular, the route shifts with the wind situation, in some cases by hundreds of kilometers.
• “Polar routes go directly over the North Pole.” Mostly they only skim the Arctic. The great circle goes directly over the pole only for a few connections, for example between North America and parts of Asia.

Conclusion

The arcs on the world map are not a detour but the shortest line on a sphere. The great circle sets the basic shape, the jet stream bends it depending on direction and day, and over the North Atlantic the tracks set anew every day translate both into an orderly system. Closed airspace, ETOPS rules and the route network over land take care of the rest. Once you understand that, you read the flight tracker with different eyes – and you know why the heavies sometimes arrive earlier than expected in the morning.