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Modern passenger airliners, equipped with jet engines, spend most of their time in flight at an altitude of about 10 km, or more precisely, from 9 to 12 km. While some military aircraft can fly at altitudes of 20-30 km, passenger airliners have to deal with factors such as cost-effectiveness and flight safety.

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First of all, it is disadvantageous in terms of fuel consumption. The higher the altitude of the flight, the more fuel needs to be consumed to reach it. However, flying at a lower altitude will not result in savings on kerosene due to high frontal resistance. Air resistance is directly proportional to its density. At an altitude of 1 km, air density is 91% of its density at sea level. At an altitude of 5 km, it is 60%, and at 10 km, it is only 34%.

Secondly, at low altitude there is a risk of encountering birds, quadcopters, helicopters, and small aviation turboprop aircraft, which does not have a very favorable impact on safety. In addition, in the event of simultaneous failure of all engines, the airliner will be able to glide for a longer time the higher it was at the moment of discovering the malfunction. From a height of 10 km, modern airliners can glide for a distance of up to 150 – 200 km. Similar incidents can be read about here.

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Airbus A319 после попадания под град

Thirdly, the low flight altitude would not allow airplanes to avoid flying into thunderstorm clouds, and hail damage in aviation would have a much more widespread impact. 10 km is the boundary between the troposphere and the stratosphere, where aircraft are practically not threatened by weather phenomena, and turbulence is minimal.

At an altitude of 15 km, the air density is approximately half of that at an altitude of 10 km. It may seem possible to save even more fuel by reducing the drag, but the low air density at high altitudes leads to a critical decrease in lift. In order to compensate for this effect, modern airliners would have to develop significantly higher speeds and overcome the sound barrier, for which they are not structurally designed.

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Единственная фотография Конкорда, летящего на сверхзвуковой скорости, апрель 1985 г.

At an altitude of 15-18 km, flights could be performed by Concorde and Tu-144, but the high cost of operating these supersonic models prevented them from competing with traditional subsonic airliners. For example, a one-way ticket on Concorde from New York to London in 2000 cost about $10,000, which is equivalent to approximately $17,000 in todays money.

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The highest mountain settlements in the world are located at an altitude of about 5000 meters above sea level. People do not live higher due to the fact that the air becomes too thin: there is simply not enough oxygen in the atmosphere for long stays at higher altitudes.

Mountaineers start actively using oxygen cylinders during ascents starting from approximately 7000 meters. Modern passenger airplanes fly even higher, however, the people on board dont experience oxygen deficiency. Lets understand how the breathing air issue is solved in commercial airliners.

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So, most of the time during the flight, airplanes fly at an altitude of about 10 km, which is due to economic factors that can be read about here. At this altitude, the oxygen concentration is the same as at the Earths surface – about 21%. However, the atmospheric pressure is only about ~210 mm Hg compared to the average 750 mm Hg, which is considered normal on Earth. In other words, the air density here is insufficient for breathing, and in such conditions, a person can lose consciousness in just 2 minutes.

Aviation regulators, such as the FAA, require aircraft manufacturers to ensure that the cabin air pressure during flight corresponds to atmospheric pressure at an altitude no higher than 2400 meters, which is not less than 570 mmHg. It is considered that this is the threshold above which individuals with less robust health begin to experience discomfort.

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In modern aircraft models, the pressure on board can exceed 600 mm Hg, but the greater the difference between the pressure inside and outside the plane, the stronger, and therefore heavier, the aircraft structure should be. In order to maintain a minimum pressure of 580 mm Hg at an altitude of 10,000 meters, the cabins and cockpit in airliners were made airtight as early as the 1940s.

Of course, the oxygen on board during takeoff is not sufficient for the entire flight, so it was necessary to create a system for ventilating the cabin. It was too expensive to carry air cylinders, so it was necessary to find a way to turn the air outside into something suitable for breathing. And here the jet engines came to the rescue. With their rotating blades, they compress the incoming air (that is, increase the pressure), which allows for more efficient combustion in the combustion chambers.

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Part of the air is taken from the compressor, which is installed in front of the engine, for the cabin ventilation system. This air, after compression, has a temperature of over 200°C, so it is first cooled in a special heat exchanger. Then the air is supplied to the recirculation system and from there it enters the cabin. In order to fully ventilate, it is necessary not only to supply fresh air, but also to remove old air somewhere. This task was solved by installing an exhaust valve in the tail section of the fuselage.

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And yet, the air in the airplane cannot be called ideal, as its humidity is usually around 20%. For comparison, the average air humidity in the Sahara is 25%, and in the Atacama Desert in Chile, which is considered the driest on Earth, it is 17%. The low humidity in the aircraft cabin is explained by the fact that the sparse and very cold (around -50°C) air at an altitude of 10 km is unable to retain a large amount of moisture, and it further dries out after being heated in the compressor. However, this is only beneficial for the aircraft itself: low humidity prevents the formation of condensation and the subsequent corrosion.

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