It's cool when watching the sunny weather and the clouds floating below to hear a message from the pilot that we are flying at an altitude of 10,000 meters and the temperature overboard is -50 degrees Celsius.
Why fly so high?
10 km altitude is an average. As a rule, we are talking about a range within 9-12 kilometers, where the courses of aircraft that carry passengers are laid. And it is not the pilot who chooses the altitude. The issue is decided by the dispatcher, it is he who calculates the height for each individual flight.
It is known that the air is thinner at high altitudes. This is due to a simple circumstance. The planet's atmosphere is held by its gravity. This force manifests itself most powerfully at the surface, holding the planet's air envelope, providing it with maximum density in the lower layers. The increase in atmospheric density is associated with the pressure of the overlying layers. The higher, the weaker the air pressure. The pressure increases closer to the surface from the weight of the upper layers of the air, as in the ocean the pressure increases due to the upper layers of water. The aircraft and its flight performance strongly depend on the air performance, primarily on its density.
Air is needed to provide lift, for normal operation of the engines. It is worth remembering that without oxygen, the combustion process does not occur, the engine stalls. If the density is low, this is bad, but too much is not needed either. Optimal conditions for civil aircraft are observed at an altitude of 10 km, in the air corridor from 9 to 12 km, depending on weather and other conditions. Too much density is not needed for the reason that it does not allow the required speed to be developed. Dense air masses slow down the movement of an aircraft in the same way that water slows down the movement of a swimmer.
In addition to the problems with the development of speed, flying at low altitude brings high fuel costs, while when traveling in more rarefied air masses, less fuel is spent. These are interrelated phenomena - in order to move in a denser space, more energy is required, and therefore more fuel.
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At the altitude recommended for civil aircraft, they can fly freely at their normal speed of 800-950 km per hour, without experiencing fuel costs, receiving enough oxygen.
Optimal heights
The air density within these limits remains sufficient to keep the aircraft flying at the indicated speed. At higher altitudes, a higher speed is required. So, when flying at an altitude of 12-15 km, a civil aircraft could only move at supersonic speeds, otherwise the air masses would not be able to keep it flying.
Modern design characteristics of civil aircraft make this height optimal for them. However, they may well fly at other heights, if necessary, slightly higher or much lower. But this is irrational, and may turn out to be dangerous for example due to weather conditions.