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Why runways aren’t built on hills? Going downhill planes would take off easier!

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Posted November 5, 2018

You know that airplanes are able to fly because air quickly moves around the wings, creating a difference in pressure, which represents itself as lift – a force driving the airplane upwards. That‘s a perfect combination of physics and nature in action, but in order to reach that airflow around the wings, airplane has to reach a certain speed. It would be easier if it was going downhill, so why runways are not built on small hills?

Lukla’s runway is on an incline, but in other places it wouldn’t work. Image credit: Superikonoskop via Wikimedia(CC BY-SA 3.0)

When people are looking for a new place suitable for a new airport, they are always looking for a few things. First of all, the location has to be strategically convenient – it has to be close to some city. Then it also has to be free of any kind of obstacles – buildings, villages, rivers, forests and so on. Finally, it has to be flat. Of course, nothing is ever flat in nature, but as flat as you might expect is good enough.

Here, take a look at the runway of the famous Tenzing–Hillary Airport, better known as Lukla airport, in eastern Nepal. It is very small and yet very well-known for many things. First of all, it is at an altitude of 2,845 m, which is quite a lot. Then it has a runway, which is 527 m in length, but also at an 11.7% gradient. Combine that with a weird approach procedure due to the difficult terrain and you will understand why it is always on the list of the world’s most dangerous airports. But you also kind of have to go there if you are ever planning of climbing up the Mount Everest.

But let’s come back to that gradient – 11.7% is quite noticeable. However, this slight downhill helps airplanes reach higher speed quicker when they are taking off and to break more efficiently when they are landing. So why not many other airports have angled runways?

The main reason is that an inclined runway would be one-directional. It means that planes could land only flying towards a particular direction (so that they could land uphill) and they could also only take off going downhill. That is bad news, because most airports are busy and they need traffic coming and going quickly. Airplanes also typically take off and land against the wind – an option once again made unavailable by angled runways. Finally, transition from an inclined runway to a level terminal would be tricky and it would just be too difficult to do in most situations.

Lukla airport is unique. They had to build it that way in order to fit the airport in that mountainous area. It is also not terribly busy and one-direction runway is not a big issue. But, of course, it is regarded as one of the most dangerous airports in the world and no one wants to follow its example.

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