It really depends on what the size of the jets. The Airlines fly from 32,000 to 38,000 feet.
Normal Jet Planes fly at about 30,000 feet
Military Jet planes fly anywhere from 100 feet all the way up to 80,000 (Some do go higher but those are for special occasions)
Planes fly high to get above the weather, and to avoid air traffic. For jet planes, certain altitudes, all quite high, afford more efficient flight conditions.
Jet planes fly with the help of Thrust and Lift produced by the jet engines.
Jet airplanes fly at high altitudes because it is more fuel efficient. Flying at higher altitudes uses less fuel than flying at lower altitudes.
its engines
Most commercial jet liners fly in the stratosphere. Some military aircrafts fly higher than that. Small planes fly lower than the jet liners.
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stratusphere
Small planes fly at 100 to 200 mph. Airliners fly around 350 to 500 mph. Jet fighters fly faster.
Not as a rule, although many are pilots.
because of weather phenomena
A large jet can get over 12 Kilometers