Why is an aircraft allowed to fly in the stratosphere?

Aircraft are allowed to fly in the stratosphere because it is not forbidden, and because there is no reason to forbid it. Nobody had to ask for permission to do this. As soon as aircraft were physically capable of flight at such a high altitude, they began doing so. read more

Absolutely. Although the altitude of the tropopause (the lower boundary of the stratosphere) varies depending on latitude, jet airliners, cargo jets and military jets often fly in the stratosphere, since the thinner air causes less drag than the thicker air down in the troposphere. read more

Commercial airlines often cruise in the stratosphere, albeit at its lower reaches. However, airplanes fly due to the lift created by air flowing over/under their wings. An airplane's engines provide thrust, which move its wings through the air (i.e., increases the flow of air over/under the wings). read more