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Most commercial airplanes cruise at an altitude of nearly 35,000 feet – around 6.62 miles in the air! If you think about it, that’s quite high, isn’t it?
What’s wrong with flying just a couple miles above the ground, as long as the plane clears all ground structures (e.g., towers and skyscrapers)?
Well, first of all, the height at which most planes fly is not some arbitrarily chosen number. There are very good reasons why planes fly so high in the sky.
Air resistance and fuel efficiency
One of the main reasons commercial airplanes fly so high is air resistance. You see, the higher you go above the ground, the thinner the atmosphere becomes, and therefore, the less resistance there is on the flight of the plane.
It’s pretty straightforward actually – the more air molecules the plane must move through, the more energy it will need, the more fuel it will need and consequently, the operating costs will be higher.
Due to lower resistance at higher altitudes, commercial airplanes can keep going with minimal fuel expenditure. That’s why 35,000 feet is referred to as ‘cruising altitude’.
A balance between operating costs and fuel efficiency is achieved somewhere around 35,000 feet, which is why commercial airplanes usually fly at that attitude. Note that not all aircraft engines are equally fuel efficient when surrounded by thin air at that altitude; some engines actually perform better when they have thicker air around them.
Avoidance of weather-related events
Another important reason why commercial airlines fly so high up is that, at that height, they get more ‘stable’ air and don’t usually have to worry about clouds and weather-related events (e.g., thunderstorms).
Although planes can still fly through clouds or storms, they will experience a lot of turbulence. This can cause discomfort in the passengers and even create a panic on the plane.
As an airline company, you don’t want that; what you want is to give your passengers as smooth a flight as possible.
This one is sort of obvious. As a pilot, you don’t want to duck and dart through towers, buildings, and other ground structures while flying a plane. Regardless of how cool that may sound to some of you, it’s simply too unsafe, not to mention impossible!
Terrain is marked by sea level, so some terrains may be much higher above sea level than the runway/airstrip. That’s why planes ‘climb’ at a good enough altitude to stay away from all sorts of ground structures.
This is not just about ground structures; cruising at 35,000 feet also ensures that the plane is well above where most birds fly. This is crucial, as bird hits can be much more than a mere hiccup or nuisance.
Say that you’re flying a commercial jet just a mile above the ground and something goes wrong. The plane starts to plummet. You know that the problem causing the plane to descend rapidly can be fixed, but the plane is falling too fast and you simply don’t have enough time to fix the problem. At that moment, you would think, “If only I had more time…”
That’s another reason why commercial planes fly at an altitude of around 6.6 miles; all that height acts as a ‘safety cushion’ and gives the pilots some time to fix things if anything goes wrong!