How High Do Planes Fly? It’s Simple Science

Today, we have complete access to that highway up above. The descendants of the flying machine―the modern-day aircraft―have far surpassed the birds that we once used to envy. With them, we can now fly at great speeds, cruising along the highway of the sky, covering hundreds to thousands of miles in only a matter of hours.

Our ability to fly however, depends upon the adherence to certain rules of nature. An important one among them is the choice of the correct flying-height, or altitude. Effectively, depending upon several factors, we are required to pick the right lane in the highway of the sky. In the following sections, we shall find out what these important factors are, and how they affect the choice of altitude for different aircraft.

Thinner air provides much less air resistance to movement, allowing an airplane to fly at a particular speed with minimal forward thrust, thus conserving a lot of on-board fuel. This is the reason why planes fly so high above the Earth.

Firstly, to climb up above the Earth’s surface, one has to overcome the Earth’s gravitational force. To do this, the aircraft needs to burn a lot of fuel. The higher the aircraft intends go, the more fuel it will have to burn. To completely escape the air cover and enter outer space, a large amount of fuel will be needed, making such a flight most uneconomical.

Secondly, and much more importantly, is the fact that airplanes require air in order to fly. An airplane flies by exploiting the four aerodynamic forces – lift, weight, thrust, and drag. Of these, lift and thrust are positive forces that an airplane has to generate, which must be greater than the negative ones, namely weight and drag, in order for the airplane to rise up and fly forward. Both these require the presence of air of at least a certain amount of thickness.

In jet powered airplanes, to create a forward thrust, air is required to be sucked in through the inlet, and then compressed and combusted to create a pressurized jet, which thrusts the plane in the forward direction. Air is also required to create the lift in the wings to make the airplane rise. At very high altitudes, the thin air is insufficient for both, thrust and lift generation, and so the aircraft must be allowed to rise to only a certain range of safe altitudes, known as cruising altitudes.

Hence, a pilot needs to be constantly aware of the terrain height he/she is flying over. An altitude of 20,000 feet above sea level might be perfectly fine while cruising over the Pacific or the Atlantic ocean, but will be very dangerous while flying over a high rising land-form, such as the Himalayas.

For this reason, aircraft are equipped with altimeters and radars, which measure and display the vertical separation of the aircraft from the surface below. Pilots are also provided with aeronautical charts and maps with different terrain heights marked on them.

The military also uses very high altitude planes for surveillance. An example of such a plane is the U2 spy plane, the current version of which can cruise at 90,000 feet.