Why Are Commercial Aircraft Cabins Pressurized?

Commercial aircraft cabins are pressurized to provide a comfortable environment for passengers and crew. The air pressure inside the cabin is kept at a level similar to that found at an altitude of 8,000 feet, which is much lower than the aircraft’s actual cruising altitude. This helps reduce the effects of altitude sickness and fatigue while also providing a more comfortable cabin environment for passengers.

Jet engines are notoriously loud, although increasingly quieter as high-bypass turbofans continue to push the limits of increased thrust and efficiency, and pressurizing the cabin helps reduce the amount of noise that penetrates into the cabin. The noise reduction benefit of pressurization makes it significantly easier for passengers to communicate and relax during their flight.

Why Is Cabin Pressurization So Important?

Aircraft cabins aren’t just pressurized for comfort. It’s also for the safety of passengers and crew members on board.

Commercial airlines fly at high altitudes to enhance fuel efficiency and avoid the bad weather and turbulence present at lower elevations. The cruising altitude for most airplanes is between 30,000 and 40,000 feet.

There isn’t enough oxygen for people to breathe at that altitude. In fact, prolonged exposure to those types of elevations without pressurization or a personal air source would cause a rapid onset of altitude sickness (hypoxia).

Hypoxia is a condition in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level. Symptoms of hypoxia vary depending on the severity and duration of oxygen deprivation, but typically include confusion, dizziness, shortness of breath, loss of consciousness and potentially even death.

Most modern commercial aircraft are specifically designed to maintain a “cabin altitude” of 8,000 ft. and can’t hold sea level pressure while flying at traditional commercial air travel altitudes. There are some unique scenarios (especially for things like aeromedical flights where a person with an internal injury is being transported by plane) where it is necessary to maintain near-sea level air pressure. In those circumstances the aircraft will have to maintain a significantly lower cruising altitude to avoid structural damage to the plane.

There have been specific types of jets that have had higher or lower cabin pressure. For example, the supersonic Concorde flew at significantly higher altitudes of roughly 60,000 ft. while maintaining a cabin altitude of 6,000 ft. Maintaining structural integrity required things like a heavier airframe than would be expected for a jet of its size and significantly smaller cabin windows.

There are some modern airliners, like the 787 Dreamliner and the Airbus A350 XWB, that are also being designed for approximately 6,000 ft. cabin altitude to enhance passenger comfort.

What Is Used to Control the Cabin Air Pressure?

An outflow valve regulates the air pressure inside the cabin. It is typically located near the rear of the aircraft and is responsible for controlling the amount of air that is allowed to enter the cabin and regulating its pressure with the use of pressure controllers.

The outflow valve helps maintain a safe and comfortable cabin environment by preventing rapid changes in air pressure, which can cause discomfort for passengers and crew. Because this system is not only vital to passenger comfort but also safety, redundancies are built in to ensure cabin pressurization equipment is robust.

What Causes a Loss in Cabin Air Pressure?

Although it’s a rare occurrence, cabin air pressure can be lost. The loss can happen for a variety of reasons:

  • Leak in the pressurization system: A leak in the pressurization system can cause a sudden drop in cabin pressure. This could be due to a faulty seal, a crack in the fuselage or a malfunctioning valve.
  • Rapid decompression: Rapid decompression occurs when the aircraft climbs or descends too quickly, causing air to escape from the cabin faster than it can be replaced. This can happen if the pilot fails to adjust the rate of climb or descent properly.
  • Faulty equipment: Faulty equipment, such as a regulator or door seal, can cause a sudden drop in cabin pressure if it malfunctions during flight.
  • Unusual weather conditions: Unusual weather conditions, such as turbulence or thunderstorms, can cause air pressure to fluctuate rapidly, resulting in a sudden drop in cabin pressure.

What Happens If Cabin Pressure Is Lost Mid-Flight?

In the unlikely event that cabin pressure is lost during flight, the pilots will immediately put on their oxygen masks, and initiate an emergency descent.

The pilots will then begin to descend the aircraft to a lower altitude where the air pressure is sufficient for passengers to breathe without supplemental oxygen. During this time, passengers will be instructed to put on their oxygen masks and remain seated with their seatbelts fastened until the aircraft reaches a safe altitude.

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