Posted by on May 15, 2017 in Tell Me Why Numerous Questions and Answers |

# Why Do Planes Fly in a Curve and Not a Straight Line to Their Destination?

A Boeing 747 burns over 10 tonnes of fuel every hour and much more when it is climbing up to altitude. That equates to about 4 litres per second. So you would think that they would fly in perfectly straight lines from the departure airport to the destination airport in order to minimize fuel burn. But if you have ever looked at one of the map displays on board, you may have seen that the route is not straight. Why is that? Well, as is often the case, there is no one simple answer. Here are some of the typical reasons:

The Earth isn’t flat

Your map is a two-dimensional representation of the surface of the Earth – which is not flat but rather an ‘oblate spheroid.’ The shortest route is not a straight line on the map – it’s a part of a ‘great circle.’ So the shortest route between Vancouver and Paris (both 49 degrees north) is via Greenland (68 degrees north).

The flight paths of airplanes seem strange when we see them on a flat map for a couple of reasons. Contrary to what explorers believed hundreds of years ago, Earth is not flat. Instead, it’s a sphere that’s slightly flattened at the poles. When traditional flat maps are made, distortions occur when the sphere is made flat.

Those flight paths that don’t appear straight on a flat map will look quite straight if you look at them on a globe. To test it for yourself, use a string to connect two cities halfway across the world. You’ll see that the flight path, as represented by the string, is not what you would expect it to be if you were looking at a flat map.

Instead of relying on flat maps, airplanes follow “great circle” routes that account for the curvature of the Earth. What looks like a long detour on a flat map actually turns out to be the shortest route across the globe through the air.

The next time you think about traveling from one place to another via air, don’t forget to think in three dimensions! Just follow the curve of the Earth and you’ll get there in no time!

Route network

Aircraft used to fly by reference to ground features – rivers, roads, railways, or coastlines. With the advent of radio beacons, aircraft then flew between these beacons and a system of ‘highways in the sky’ was developed. Even though most aircraft can now navigate remarkably accurately without having to track between radio beacons, this route network persists, even though most of the ‘waypoints’ – the nodes of the route network – are not associated with physical beacons.

Weather – avoiding or making use of winds

Even in the air, the shortest route is not always the quickest. Strong winds can add or subtract a lot of time. This is most clear to see in crossing the Atlantic, where the Jet Stream is typically at least 100 knots (185 km/h) and can be twice as much as that. So the best westbound route can be hundreds of kilometres south or north of the best eastbound route.

Other forms of weather can also affect the route selected. Cumulonimbus clouds (the ones that generate thunderstorms) can be dangerous for aircraft with vertical air currents of as much as 50 knots (1,500 metres per minute) as well as icing, wind shear, lightning, hail etc. Unsurprisingly, pilots tend to avoid them.

Military exercises

The military need to run exercises and test fire missiles from ships etc. and this means that the affected airspace is closed to civilian flights, which have to divert around the area.

Conflict zones

Safety is always a priority and the tragedy of MH-17 highlighted the dangers of flying over conflict zones.

Cost

Each country charges for the use of its air navigation services, depending on the length of the flight in that country and the size of the aircraft. On average, a flight will pay about \$900 but this figure will be much less for a small aircraft flying locally and greater for a large aircraft flying across the entire continent.

The charges are ultimately based on the cost of providing the service and some countries cost more than others. For example, Germany costs twice as much as Poland and this means that an aircraft flying from, say, Stockholm to Pisa may choose to fly a slightly longer route than necessary in order to avoid German airspace.

This choice is of course influenced by the cost of the additional fuel needed to fly a longer route.

Strikes

Sometimes there are air traffic control strikes and these may affect not just flights to or from the affected country but also over flights. So an aircraft flying from Spain to Germany may encounter problems if there is industrial action in France. The airline then has a choice – to cancel the flight, to wait until it can get a slot for a flight across France or to go around France.

Congestion

Even without strikes there can be congestion in the skies, if there are too many flights expected in a particular part of airspace at a certain time – more than the local air navigation service provider has declared as being safe to handle – then aircraft is hold on the ground to smooth out the dangerous peak. However, airlines do have the option to choose an alternative routing in order to avoid the busy part of the airspace. This might be a longer route but the additional cost is balanced by saving the costs associated with delays.

At airports, such congestion used to be managed by use of ‘stacks,’ aircraft circling near the airport and gradually descending until it is their turn to land.

Near airports

In any case, it is common for aircraft near airports to fly a slightly longer route than the theoretical minimum. This may be because the aircraft has to fly around the airport so that it can land into wind. Aircraft may also have to avoid certain areas (such as the city centre) because of noise restrictions. In a complex piece of airspace such as near Paris with multiple airports, there are specific three dimensional arrival and departure routes in order to maintain separation, for example between aircraft departing Orly to the north and aircraft arriving at Charles de Gaulle from the south. With all these reasons for deviations, it seems unlikely that any flight actually manages to fly a direct route.

Content for this question contributed by Kristen Allison, resident of Myricks, Berkley, Bristol County, Massachusetts, USA