Why do aircraft use cabin pressurization

What cabin pressurization means and why it's important

Cliff Garrett was an American entrepreneur and founder of the Garrett AiResearch company. In the late 1930s, Cliff Garrett’s company solved one of the biggest challenges for long-range military flights. They invented the world's first volume production of a cabin pressurization system for the B-29 Superfortress. The invention by Garrett AiResearch, now Honeywell, was to become the foundation for cabin pressurization systems on all modern aircraft flying nowadays.

Then, in 1946 the first commercial cabin pressurization system came into use.

Boeing's 307 Stratoliner – nicknamed the Flying Whale – began flying passengers in pressurized comfort at 20,000 feet. It was the first in-service pressurized airplane and airliner in history. Then came the first digital electronic cabin pressure control system in 1977. This was followed in 1979 by fully-automatic digital cabin pressure control systems using converging nozzle thrust recovery valves. 

Today, most commercial airplanes have pressurized cabins to help passengers and flying crew breathe comfortably, which is the short answer to an essential question. 

Why do aircraft use cabin pressurization?

Commercial aircraft fly best at high altitudes – that is a fact. This enables them to enhance fuel consumption efficiency and avoid potential bad weather and turbulence factors. However, for humans the situation is precisely the opposite. The higher we go, the less oxygen there is available to breathe. This happens because air density decreases with altitude.

Thus, air molecules spread out more, decreasing their density and – with that – there is less oxygen available for each breath of air. All this makes it increasingly harder to breathe for us. At 18,000 feet, the amount of oxygen halves compared to what we normally have at sea level. As a matter of fact, going much higher than 8,000 feet without the help of modern technology can cause altitude sickness, also known as hypoxia. Hypoxia can lead to dizziness, headache, difficulty thinking, unconsciousness and eventually death.

Thankfully, modern jet airliners are engineering miracles. Apart from getting us across the world safely in a matter of hours, they also act as a flying pressure chamber, controlling the air entering and exiting the pressurized cabin. The aircraft's cabin pressurization system helps create – alongside other technologies such as the air management systems – the necessary pressure that you and I would need to breathe comfortably during a flight that typically takes place at a cruising altitude of around 36,000 feet.

What is a cabin pressure system?

To keep the cabin pressure at a comfortable level for people onboard even at altitudes higher than 36,000 feet, airplanes pump pressurized air into it. The air that goes into an aircraft's cabin via this process is called conditioned air. This air enters the plane's pneumatic system through its engine compressors and gets directed into the primary heat exchanger. Then, it goes through a turbine and compressor and other heat exchangers and control valves that cool the air and regulate its pressure and temperature, and then is finally transferred into the cabin to pressurize it and control its temperature. Once the cabin achieves an ideal pressure level, the aircraft will limit the cabin air exhaust to keep the cabin pressure under control and maintain it at a constant level throughout the entire flight. 

Put simply, cabin pressurization is a process in which conditioned air is pumped into and exhausted out of the cabin of an aircraft to keep the pressure in the cabin between sea level and 8000 feet. Comparable to the air pressure we'd experience on a mountain of around 8,000 feet, this is called cabin altitude and allows still for normal breathing. 

What is the device used to control the cabin air pressure?

Airplanes control their cabin pressure via an outflow valve. This valve helps keep the incoming air inside the cabin and then releases it at a rate that is regulated by pressure controllers.

Since modern-day commercial airplanes don't have oxygen tanks onboard due to weight constraints, they have to supplement the air in the cabin from some other source. They do it with the help of their jet turbines that suck the air inside. The engines compress this air, and this compression pushes oxygen closer together, making it breathable for the passengers inside.

The air inside the cabin continuously needs to be refreshed with clean air. Thus, fresh air from the engine must always enter the cabin to pressurize it (and control the temperature) and air must always leave the cabin to exhaust away harmful contaminants. This is achieved by opening the outflow valve, a process that decreases the pressure inside the cabin. The cabin pressure is maintained constant with the help of a pressure regulator.

What controls the operation of the cabin pressure regulator?

The cabin pressure regulator controls the opening and closing of an aircraft's outflow valve, and – in turn – its proper operation is controlled by computers installed onboard the aircraft. An excellent example is that of the Honeywell cabin pressure control systems.

These provide clear advantages such as thrust recovery outflow valve systems. They optimize cabin air exhaust speed for improved fuel efficiency, single or multiple outflow systems to aid in cabin comfort and ventilation of heat and odors, and pneumatic safety valves for simple control and backup positive and negative pressure relief functionality.

With innovations like the latest-generation digital cabin pressure control system, Honeywell continues its long legacy of pressure control system leadership that dates back more than 75 years – a heritage from names like Garrett, AiResearch and NormalAir.

Honeywell’s new fourth-generation Combined Hydrocarbon Ozone Catalyst is another important innovation. This part of the cabin air system filters out contaminants, such as engine exhaust or deicing fluid, that may get into the bleed air. It improves air quality and reduces “smell in cabin” events, which cost airlines millions of dollars a year in delays or cancellations.