Why Aircraft Don’t Fly In A Straight Line From Origin To Destination

Have you ever wondered why aircraft don’t fly in a straight line from their origin to their destination? There are several reasons why, but the big two are to do with the curvature of the earth and the jet streams.

Guessing that it was a safety precaution wouldn’t be entirely wrong either, as when planning a route many pilots prefer to maximize the number of airports along their flight path. While emergencies are extremely rare when you consider how many planes take to the skies each day, knowing where you can land in an emergency is important nevertheless.

Emirates airliner taking off
The earth revolves on its axis forcing the middle to bulge out slightly. Photo: Pxhere

Having said that, it is hard to think how stressful it would be if you lost power over the middle of an ocean with nowhere to land. Equally, if the plane had a medical emergency on board that meant it needed you to land ASAP, being on a well thought out route will pay dividends.

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Flight paths are all about money and time

Ultimately, the flight path an airliner takes comes down to saving time and fuel. It is easy to forget that airlines are a business whose profits depend on how quickly it can move passengers and cargo between destinations.

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Knowing where you can land in an emergency is important. Photo: Ryanair

In the end, it all comes down to speed being the primary factor in determining an airliner’s flight path. If you were to ask a classroom of students studying mathematics what is the quickest way to get from A to B the answer would, of course, be a straight line.

This may be true on a two-dimensional map, but the earth is not flat (despite what some apparently believe). To understand this properly, you need to look at a globe to appreciate the curvature of the earth in order to understand flight paths better.

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Curving towards the poles is quicker than flying in a straight line

Even using a globe of the earth is not entirely right, as the distance when making a complete circle of the earth from north to south is shorter than making a circle of the earth around the equator.

Flying closer to the poles is quicker than a straight line. Photo: Pixabay

The reason for this is that the earth revolves on its axis, forcing the middle to bulge out slightly. The curvature of the earth and its extra equatorial width mean that curving towards the poles is a shorter distance than flying in a straight line.

The jet stream

Another criteria airlines use for determining flight paths are air currents known as the jet stream. These high altitude air currents exist near the top of the troposphere, which is the lowest layer of the earth’s atmosphere and the place where most weather occurs.

tropopause
The jet stream can reach speeds of over 200 miles per hour. Photo: Bredk Wikipedia

The border between the troposphere and the next layer, the stratosphere, is known as the tropopause, an altitude that fluctuates between four and twelve miles above the earth’s surface. This fluctuation results in rapid shifts in air temperature and pressure, which creates a wind tunnel that can reach speeds of over 200 miles per hour.

These extreme speeds are most common during the winter months when the temperature difference is greatest, but regular wind speeds of 80 to 140 miles per hour are quite common.

The earth has four main jet streams, two in each hemisphere, and thanks to the earth’s rotation they mostly flow west to east. The two most important for airlines are the polar jet stream, which forms near the Arctic Circle, and the subtropical jet stream near the equator.

This means that flying with the jet stream can shave time off a flight. However, flying into the jet stream will slow you down.

Turbulence

While pilots do their best to avoid turbulence, sometimes it is unavoidable. Turbulence is caused by hot air rising from the earth and is why gliders can stay in the air so long without the power of a motor. There is less turbulence over water than over land, because the primary cause of turbulence is hot air rising from the ground. Water distributes heat a lot better than soil, making flights over the ocean much smoother.

swiss turbulence
Clear-air turbulence can be dangerous. Photo: Pixabay

While flying with the jet stream is a big advantage it does have a downside called clear air turbulence (CAT) which occurs along the edges of the stream. Far more intense than normal turbulence and almost impossible to predict, clear-air turbulence is a good reason as to why you should always keep your seatbelt fastened while flying.

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Martin Dellow

Explain to me how an aeroplane can take off or land if the Earth could spin at 1000mph?
als…why do Aviators use a flat Earth map and flight charts?
Curve ! Really

Dennis

Well explained – You pass with credit – 76% 😀

Jose

Nice explanation of why planes don’t fly in a straight line but what about waypoints? I thought commercial aircraft follow a series of waypoints to get to their destination. And waypoints are never in a straight line.

Atul Jain

but the earth is not flat (despite what some apparently believe).
Haha. Nice dig towards those stupid enough to believe that the earth is flat.

GERALD WINNINGHAM

I haven’t been in the cockpit for a couple of years, I was Aircraft Maintenance and took advantage of those jumpseats every chance I could. Our crews were always asking ATC for clearance for GPS direct clearance and I am not sure about the International trips but here in the… Read more »