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How High Do Planes Fly? It’s Simple Science

In August 2001, the official world record for the maximum-altitude flight by a non-rocket powered aircraft was set by the light-weight, solar-powered aircraft Helios designed by NASA, when it flew at an altitude of 96,863 feet above sea level.
When our ancestors used to trudge along trackless lands or wade through precarious waters, they used to look up at the birds, covetous of their ability to soar and fly above all obstacles on the infinite open highway of the sky. This strong desire for flight among them was passed down through generations, until centuries later, in 1903, the ‘flying machine’, invented by the Wright brothers finally allowed us to conquer the skies.

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.

Obstacles & Air Resistance
For flying an aircraft, high altitudes are chosen to avoid the different obstacles that may be present at lower levels, such as tall trees, buildings, bridges, skyscrapers, mountains, etc. These flying heights are chosen so that the plane flies at a safe margin above all these. Also, many different birds and insects tend to get in the path of an airplane at lower levels, making greater flying heights preferable for avoiding them.
Another factor for choosing higher flying altitudes is the reduced air resistance. Air cover around the Earth exists in the form of a gradient. Air is thickest at the surface, and continually gets thinner as one moves vertically upwards away from it, until one reaches the void of space. Thicker air offers greater resistance to the objects moving through it. To overcome this resistance, a lager amount of forward thrust needs to be created by burning more fuel, to keep the aircraft moving at the desired speed.

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.

Air Thinning
From the above discussion on air resistance, you might think that the higher an airplane flies, the more efficient its flight would be. So why not go all the way up to space where there is zero air resistance, and fly there? The answer to this question is two-fold.

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.

Typically, an airplane takes off the Earth’s surface and steadily climbs upwards till it reaches the cruising altitude, and then continues onward at that height till the landing. Cruising altitudes vary depending upon the type of aircraft and the distance that needs to be covered. Internationally, for most conventional airplanes, these altitudes range from 25,000 to 40,000 feet.
Airplanes on short flights tend to maintain a fixed cruising altitude throughout, while those on longer flights change the cruising altitudes several times, gradually working their way to higher levels as they burn their on-board fuel and become lighter in weight.
Altitude Separation
The most preferred cruising altitude for jet-powered aircraft is 30,000 feet. However, the large amount of air traffic in the skies at present doesn’t permit every airplane to fly at this level. In order to avoid collision, air traffic is constantly monitored, and different planes are assigned different altitudes at different times.
In aviation lingo, different cruising altitudes are referred to as ‘flight levels’. These flight levels are obtained by dividing a given altitude by 100. Thus, an altitude of 30,000 feet is represented as flight level 300, and an altitude of 25,000 as flight level 250.
To minimize the possibility of a collision, flight levels are assigned based upon the direction on the compass that the airplane is flying towards. For example, an airplane flying east would be assigned an odd flight level such as 359, while the one flying opposite would be assigned an even flight level such as 320. Therefore, even if two planes are on the same course, flying directly towards each other, they would still have at least a thousand feet of vertical altitude separation between them.
Geographical Considerations
Since the ground level constantly dips and rises at different places on the surface of the Earth, it becomes impractical to measure flight altitudes in relation to it. Therefore, altitude level measurements for flights are normally carried out by taking sea level as the reference. Each different altitude represents the number of feet above the level of sea, which is taken as a constant value. Since the geographical characteristics vary from location to location, it is very important that the flying altitudes be chosen and adjusted accordingly.

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.

Effects of Weather
Weather is an unpredictable factor that can affect the cruising altitude of a plane. While flying, pilots receive timely weather reports from air traffic control stations on the ground as well as from other pilots flying in the area. In case a thunderstorm is building on the way, or there are reports of clear air turbulence up ahead, pilots may choose to change the cruising altitude in order to avoid going through them.
Duration of the Flight
The length of the flight also decides the cruising altitude of an airplane. For short duration flights, cruising altitudes are typically kept low, even though the aircraft has to face increased air resistance at that level. This is because, rising to higher levels requires the aircraft to burn more fuel, which becomes uneconomical considering the short duration of the flight.
For example, for a flight that is going to last for about an hour, the chosen altitude may be 25,000 feet. This is because, even though by rising to 35,000 feet it will encounter lower air resistance, it wouldn’t make any practical scene in expending fuel for climbing up to that level, as the plane would likely have to begin descending to its destination the moment it reaches that height.
Type of Aircraft
Different types of aircraft are designed for attaining different flying and cruising heights. Typically, commercial planes fly at 28,000 to 35000 feet. Many of these are actually capable of flying much higher, but fly lower to maintain a safety margin. For instance, the new generation Boeing 737 is capable of flying at 41,000 feet, but actually flies much lower than that.
An exception among commercial airliners was the Concorde, which was designed to fly at around 45,000 feet. This was to allow it to fly faster, by taking advantage of the lower air resistance at that height.
Most military planes are designed to fly at much higher altitudes than that of civilian aircraft, so that they can attain higher speeds. The famous Stealth Bomber, for example, cruises at around 50,000 feet. At that height, it travels at supersonic speeds, and escapes detection on the enemy’s radar.

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.

For Skydiving
Skydiving is an extreme sport, wherein a plane carriers skydivers to certain heights above the ground, from where they jump off and glide down to the ground with the help of a parachute.
The most important consideration while choosing the correct altitude for skydiving is the availability of air and oxygen at that height. A common altitude chosen by many skydivers is around 13,000 feet. At this height there is enough oxygen for breathing, and the skydiver gets about 60 seconds of free fall, after which the parachute has to be opened. It is even possible to skydive from 16,000 feet without using an oxygen mask, which will give the diver up to 75 seconds of free fall.
As we can see, the altitude at which an aircraft flies depends upon several factors, as mentioned above. For commercial airliners, while choosing the right altitude, flight-economy is the major point of consideration, while in military aircraft, speed and stealth are the main points of consideration. Hence, according to the different requirements, different flight altitudes are chosen, while always keeping in mind the actual capability of the aircraft.

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