Why Are Airplane Cabins Pressurized?

In the 1930s, American entrepreneur and founder of the Garrett AiResearch company Cliff Garrett devised a solution to one of the biggest challenges for long-range military aircraft. Garrett’s company invented the world’s first cabin pressurization system for the B-29 Superfortress. The invention by AiResearch, which now goes by Honeywell, became the foundation for cabin pressurization systems on modern aircraft today.

History of Cabin Pressurization

By 1946, the first commercial cabin pressurization system came into use on Boeing’s 307 Stratoliner, providing passenger comfort at 20,000 feet. This achievement was followed by the development of the first digital electronic cabin pressure control system in 1977, and the fully-automatic digital cabin pressure control system using converging nozzle thrust recovery valves in 1979. Presently, most commercial aircraft have pressurized cabins to help passengers and aircrew breathe with ease.

Commercial aircraft fly best at high altitudes, enabling them to enhance fuel consumption efficiency and avoid unprecedented weather and turbulence. However, for humans, high altitudes can be risky territory. The higher we go, the less oxygen there is available to breathe. This is due to the fact that air density decreases with altitude. For instance, at 18,000 feet, the amount of oxygen is half of what it would be at sea level. In fact, going any higher than 8,000 feet without the help of modern cabin pressurization systems can lead to altitude sickness, also known as hypoxia. Hypoxia can cause dizziness, headaches, difficulty thinking, unconsciousness, and eventually death.

How Cabin Pressurization Is Achieved

To ensure passenger safety, airplane cabins must be pressurized. This is done by pumping pressurized air, or conditioned air, into the aircraft cabin. First, the air makes its way into the plane’s pneumatic system through its engine compressors and is guided into the primary heat exchanger. From here, it goes through a turbine and compressors, as well as other heat exchangers and control valves to cool it and regulate its pressure and temperature. Then, it moves into the cabin to pressurize it and control its temperature. Once the ideal pressure level is achieved, the aircraft will limit the cabin air exhaust to ensure the cabin pressure is maintained at a constant level throughout the entire flight. In simplistic terms, cabin pressurization consists of pumping conditioned air into and exhausting it out of an aircraft cabin to keep the pressure in the cabin between sea level and 8,000 feet.

Aircraft control this cabin pressure via an outflow valve that keeps the incoming air inside the cabin and releases it at a rate that is regulated by pressure controllers. With the help of jet turbines, air can be sucked inside, and the engines compress it to make it breathable for passengers. Meanwhile, the cabin pressure regulator controls the opening and closing of the aircraft outflow valve, both of which are controlled by computers installed onboard the aircraft. These cabin pressure control systems provide clear advantages. For instance, thrust recovery outflow valves systems optimize cabin air exhaust speed for improved fuel efficiency, and single or multiple outflow systems aid in cabin comfort and facilitate the ventilation of heat and odors.

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