Generally speaking, the flight altitude of an airliner differs depending on the type and model of aircraft, its size, availability of certain equipment and functions. Small civil aircraft and some jets fly at altitudes of no more than 6,000 meters, while large and high-speed airliners fly at altitudes of 7,000 to 13,000 meters in the upper layers. Small light-engine airplanes usually do not fly above 2,000 meters.
Flight altitude is commonly referred to as the vertical distance to the hull of the aircraft. Depending on the level of origin one can differentiate between true (from the level of the point directly below the aircraft), relative (from some conventional level - runway threshold, airfield level, the highest point of relief, etc.) and absolute (from sea level).
Flight heights are divided into extremely low, low, medium and high. Extremely low differ depending on the type and speed of the aircraft, low - from extremely low to 1000 meters, medium - from 1000 to 5000 meters, high - over 5000 meters. The flight altitude should be distinguished from the echelon occupied by the aircraft. Echelons are counted by standard atmospheric pressure and have values defined in regulations.
But why do passenger airliners fly at an altitude of 10 kilometers? The fact is that the higher the flight speed, the lower the optimum air density. It is not economical to fly in dense air near the ground, and at an altitude of more than 12 kilometers, supersonic speeds would have to be developed in order to provide optimal conditions for the engines. The typical speed of 800-900 km/h for civil aviation is best achieved at an altitude of about 10,000 meters.