Why do commercial jets fly at 9000 meters altitude?

Answer 1

Commercial Jets fly the very high for several reasons. First, at 30,000 ft (9000m) the air is thinner, so the aircraft has to push less out of the way to fly the same speed at a lower altitude where the air is more dense. Because the aircraft has to push more air to achieve the desired speed, the engines don't work as hard, also called "cruising."

The second reason is that up high, the winds are much much faster, especially in the jet-stream, and a good pilot can use the winds to his advantage in a tail wind and fly faster and farther with less fuel. Conversely he can avoid the disadvantage of headwinds as much as possible by varying his altitude accordingly.

Thirdly, though the air is much thinner, it usually flows uniformly and thus the risk of heavy turbulence, often caused by rising warm air meeting cool air, can be avoided. Pilots who encounter adverse conditions often issue en-route radio reports to weather service stations, called "pireps," that give the exact position, time, and severity of bad conditions like icing or turbulence.

Lastly, commercial jets travel for long distances, and because it's efficient to travel at high altitudes, the use "great arcs" instead of straight lines, to navigate across the (spherical) surface of the earth. Furthermore, while there are mountains like the Himalaya's that reach 30,000ft, and there a few species of birds that fly that high, there really isn't much else up there, so the danger of hitting something is slim.

Hope I've answered your question.