While sitting comfortably in the cabin of a commercial aircraft, it is easy to forget the immense altitude at which the plane is flying. Reaching a cruising altitude of 30,000ft with every flight, airplanes are explicitly designed to protect occupants from the low oxygen concentrations found in this extreme environment. In order to accomplish this, all aircraft flying above 10,000ft have pressurized cabins, ensuring a safe and comfortable flight for both passengers and crew members. In this blog, we will discuss everything you need to know about cabin pressurization, including its purpose, design, and implementation.
Earth's atmosphere contains several gas-based constituents, including nitrogen, argon, carbon dioxide, methane, and oxygen. As humans, the most relevant of these is oxygen, being that it makes up nearly every macromolecule in our body while also supporting aerobic respiration. At standard elevation, there is enough partial pressure of oxygen to facilitate gas exchange in our lungs, but as altitude increases above 10,000ft, gas molecules begin to spread out, and there becomes less oxygen available to breathe. In addition to the lack of oxygen, there are other physiological disruptions that occur when pressure is disrupted. For example, our inner ears are very sensitive to changes in air pressure, which is why individuals "pop" their ears at elevation. If there is a dramatic fluctuation in air pressure, the inner ear would not be able to equalize, which would cause immediate disorientation and trauma.
Aircraft featuring pressurized cabins rely upon the environmental control system (ECS), which regulates an air compressor to achieve proper pressurization. The air source comes from the engine's compressor in most instances, removing a particular amount of bleed air for this purpose and other operations. This process is entirely automatic and requires no input from crew members. Bleed air is very hot when it comes out of the compressor since pressure is directly related to temperature. Therefore, it must be re-expanded to meet the desired pressure and temperature. Often, there is a tendency for the resulting air to be too cold, requiring it to pass through a heat exchanger before entering the cabin.
Since cabin pressurization is paramount to the safety of all occupants, the ECS can pull bleed air from a minimum of two engines, as well as the APU, in case of total engine failure. This pneumatic program is fully redundant with a minimum of two controllers, as well as a manual override in case of emergency. While not true of all aircraft, most pressurize the entire vessel, including the baggage area and other non-trafficked regions. This is done because a large pressure differential could damage any item containing sealed air.
Since air must be continuously circulated throughout the aircraft, some must be forced to exit the plane as exhaust. This action is facilitated through the outflow valve, which is also controlled by the ECS. Like other aspects of this system, the valve may be manually controlled at any time by the pilot in case of an emergency. While extremely rare, unplanned decompression may occur and result in equipment damage and endanger lives. In such an instance, pressurized masks providing 100% oxygen would be made available to all occupants, while the pilots would initiate an immediate descent to below 8,000ft.
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