Most of us have risen as high as we ever did thanks to the airplane, which is considered one of the greatest inventions in the history of modern transportation. At the time, it lets man reach unprecedented highs.
Still, the development of aviation as we know it today – along with the beginning of the space era – set certain limits. Modern airplanes are designated to fly to meet their purpose most safely and efficiently.
So, here we will look at one of them, namely the height limit. And of course, to answer the questions such as ‘why do planes fly so high’ or ‘how high above do airplanes fly, we will gaze a little beyond such limits.
Today, almost every commercial airplane flies at an altitude that varies from 9000 to 13000 meters – or something between 30000 and 42000 feet.
The lower point is much higher than any of the highest obstacles that could obstruct their way across the skies.
The answer here is pretty simple as it all comes to the efficiency of the aircraft, and namely, fuel efficiency becomes perhaps the most important here.
And not just because more jet fuel costs more money – as just a few, primarily military aircraft, are capable of being refueled at higher altitude mid-flight, usually, the less fuel the aircraft uses, the further it can fly.
That’s where the plane can meet less thrust but, on the other hand, can maintain a high compression ratio and thermal efficiency.
And that is why an airplane engine is the most efficient when it runs very close to its maximum RPM limit, which could not be achieved when a plane’s engines run close to the ground.
As the air gets less dense at a much higher altitude, less strain is put on the engine here, making it run as efficiently as it was designed to. So, thin air creates excellent conditions for fuel efficiency as less fuel can be needed for higher-flying aircraft.
That’s why virtually any modern commercial jet aircraft, most of the time, will fly higher than 9 000 meters – but at an altitude that is lower than 13 000 meters.
While many military planes equipped with jet engines are designed to fly much lower – or much higher than that, they tend to demonstrate many not-so-economical design solutions such as stronger frames that use highly-valued titanium components.
In aeronautics, the term ‘service ceiling’ refers to the maximum usable altitude of an airplane. For example, the service ceiling of The Wright Flyer, which made the first sustained flight by a manned heavier-than-air powered and controlled aircraft in 1903, was as low as 9.1 m (30 ft). That was, of course, long ago, and almost nobody has flown higher before.
However, there were times before the piston-powered engine came into the game – and even longer before jet aircraft had conquered the skies and convinced us that everyone could fly so high. At the beginning of this century, the Concorde, the fastest commercial aircraft in history, flew at 18.3 km (60,000 ft).
Cruising altitude is when a plane flies during the cruise phase of the flight – which is usually the longest and at which most of the fuel is consumed.
Depending on the payload, center of gravity, air temperature, humidity, and the conditions, creating more or less resistance and lower or higher true airspeed, cruising altitude may vary for different commercial aircraft on different routes and in other situations.
It could be a bit higher or, in some circumstances, a little bit lower altitude is better. However, for most commercial airplanes, it is usually kept within the aircraft’s service ceiling limits.
Most planes on which we fly so high today maintain an altitude that does not exceed the range mentioned above.
For example, Airbus A320 has a service ceiling between 11 917 m and 12 497 m (39,100 to 41,000 feet), while Airbus A350 has a service ceiling of 12 700 m or 42,000 feet. Boeing 747, on its part, has a service ceiling of 13 137 m (43,100 ft).
As the less dense air is crucial for the plane you are flying to operate its jet engines efficiently, it also means that in such conditions, it contains not have enough oxygen for its passenger – or any crew member – to breathe normally. It is one of the reasons why aircraft depressurization at higher altitudes is extremely dangerous for everyone onboard.
Today, when commercial planes fly on their usual routes, they must warn the cabin crew and the pilots in the cockpit if the cabin pressure matches the normal pressure in the atmosphere at approximately 2 500 m.
When such a warning is received, it is time to find out if the only way to avoid more significant problems is to fly at a lower altitude, or if some other kind of action at such a high altitude – such as an emergency landing – is needed. The flight crew will receive the next warning as the aircraft fly when the so-called cabin altitude reaches 3 050 m.
The aircraft systems are set in such a way that oxygen masks are automatically deployed before the cabin altitude reaches 4 500 m.
Some modern planes are also equipped with a system that provides an automated announcement for the passengers to put on the oxygen masks just after they drop.
Fixed-wing aircraft, of course, is known to reach higher altitudes than most of the whirlybirds. As the design of the latter type of aircraft suggests that helicopters fly more efficiently at much lower altitudes than jet planes, the same design also makes it impossible for helicopters to fly as high as the Boeing 767, which is meeting less resistance at high altitudes above sea level.
As a matter of fact, the maximum height a helicopter can reach is usually related to the ability of the engine to breathe thinner air than to the rotor’s capability to give a constant lift.
Turbine-engined choppers can climb at altitudes exceeding even 7 600 meters – or about 25,000 feet. But the maximum altitude at which the pilot can let such aircraft fly, or more precisely, safely hover, is, of course, lower – at best, half of that height.
So no helicopters at 42,000 feet – at least, not until the technology which defines how high planes fly will reach such a level that today only some not-so-hard sci-fi pieces of art have to show.
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