Why aircraft don’t always fly in straight lines

14 September 2016
As is often the case, there is no one simple answer. Here are some of the typical reasons.

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 minimise 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).

Map showing the curved trajectory of an airplane travelling between Vancouver and Paris over Greenland.

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.

Part of the route network showing the Free Route Airspace in Hungary.

Download the ERC - 06/H Upper Chart (Central Europe - Ukraine) here.

We are now working with our partners across Europe to move to ‘Free Route Airspace’, doing away with the old network. As with anything in aviation, such a significant change needs to be done safely but we are making real progress, with FRA in operation for many parts of Europe at night or at weekends and, in several countries, it is now 24/7. The proportion of FRA airspace is expected to increase considerably over the next few years. One of the roles of EUROCONTROL, as Network Manager, is to help make this happen through the European Route Network Design Function.

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.

An aircraft flying through dark clouds.

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. However, we already work closely with our colleagues to make sure that the airspace is opened as soon as possible once the military don’t need it anymore and we are currently developing new procedures, tools, data exchange systems and services to improve this process – all part of the ‘Advanced Flexible Use of Airspace’ programme.

Conflict zones

Safety is always a priority and the tragedy of MH-17 highlighted the dangers of flying over conflict zones. ICAO, the International Civil Aviation Organization, have urged everyone to share information on such risks and EUROCONTROL as Network Manager is playing its part by making available to the aviation community any information it receives about the airspace over and near Europe.

Cost

Each country in Europe 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 €800 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.

Global Unit Rates 2016

This choice is of course influenced by the cost of the additional fuel needed to fly a longer route. At present, the cost of fuel is relatively low and thus we are seeing more flights avoiding more expensive airspace. There are calls for a harmonised charging system but there are difficulties with this. For example, an airline based in a low charging country might well find that its overall annual bill would increase.

Strikes

Sometimes there are air traffic control strikes and these may affect not just flights to or from the affected country but also overflights. 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. An important role of EUROCONTROL as Network Manager is to manage the flow of traffic across Europe. So if we can see that 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 we will hold aircraft 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.

Busy areas in the sky.

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. However, we rarely see this any more except at airports handing a lot of aircraft from outside Europe – aircraft that are not subject to the flow management procedures.

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.

So how efficient are we?

With all these reasons for deviations, it seems unlikely that any flight actually manages to fly a direct route. However, the efficiency of flights is actually quite remarkable. In 2015, despite major challenges as a result of, for example, the closure of airspace in Ukraine, flight efficiency actually improved with the average flight being just 2.8% longer than the great circle route. 

It is difficult to determine exactly what the optimum figure should be (it will be greater than zero because of the effect of the winds) but it is clear that, while there is still work to do to reduce this figure further (and also to make the vertical flight profile of the flight more efficient), aircraft do, in general, fly fairly direct routes.

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