At what altitude flying passenger planes?
Most Airliners today fly at an altitude of 10 to 12 thousand meters above the ground. To such a height, they come in about 20 minutes of flight. The choice of the height depends on the characteristics of the atmosphere. The air at altitude over 10 thousand feet very rarefied, allowing you to expend less fuel to overcome drag. But at the same time it has enough oxygen to support continuous combustion of aviation kerosene.
The height selection depends not on the willingness of the aircraft commander, and dispatch service is determined and depends on the weather, wind speed at earth and the direction of flight of the aircraft. The height at which a plane is called a flight level. Around the world single taken trains to fly and they can vary depending on country over which the plane flies. If the flight is from West to East, for it contains only odd levels (35, 37, 39 thousand feet). If the aircraft is flying West, the trains will, on the contrary, even (30, 36, 40 thousand feet).
Also the height of 10-12 thousand meters due to the lack of birds. Birds cause huge damage to aircraft, colliding with them on small heights. At high altitudes there is no such danger.
How high flying military aircraft?
Military aircraft fly with greater speed than passenger planes. For supersonic flight of military aircraft to fit the height of more than 13 thousand meters. At such heights is very low air density to reach the speed of sound is much easier than at the height less than 13 thousand kilometers. For certain missions, modern military aircraft can climb to a height of more than 25 thousand kilometers. A record maximum altitude belongs to domestic MiG-25 and is 37650 meters.
