As one moves upward in altitude, the pressure of air decreases, making it harder for humans to breathe. Mount Everest’s peak lies at 29,000 feet, and oxygen levels have been described similar to breathing through a straw while on a treadmill. Thus, at 30,000 feet where many aircraft fly, it is a technological marvel that they are able to provide oxygen pressure that allows for safe and comfortable breathing for passengers. This is due to a complex aircraft cabin pressurization system which has been designed and improved upon by countless aircraft manufacturers over the history of aviation.
While many solutions for pressurized oxygen have existed over the years, the current method of providing pressurized air for the cabin of an aircraft is by using bleed air and compressors. Within the air compressor of the engine, air from the atmosphere is directed into spinning blades which heat up and compress air before it continues through the system. While much of this air is directed to the combustion chamber for fuel ignition and propulsion generation, some of the hot, compressed air is also used for other purposes. This clean and pressurized air is referred to as “bleed air” and it may be used for cabin pressurization, deicing operations, pneumatic pumps, and aiding engine starter motors.
For cabin pressurization and many other bleed air functions, the air must first be cooled down with the use of intercoolers which shed heat to ambient air. Air continues to cool with the use of air packs within the system that act as a refrigeration unit. Air is compressed to heat it, and another intercooler filters out the heat and expels it from the aircraft. The cooled air then expands within an expansion turbine before being mixed with recirculated air to be distributed throughout the airplane fuselage. Automatic outflow valves with pressure sensors regulate the amount of pressure to maintain safe levels. These valves are also used to cycle out old air that is vented from the aircraft.
Outside of preflight checks to ensure the functionality of the automatic pressure control system, pilots do not control pressurization during the flight operation. Nevertheless, in the case of a malfunction or emergency pilots can take control of automated functions and can manually adjust the outflow valve positioning. During ground support operations in between flights, testing equipment is used to detect pressure leaks, inoperable equipment, or any other issues that warrant immediate attention for safety measures.
At an altitude of 10,000 feet, pressure of the atmosphere is viable for safe breathing conditions, and this is the altitude that planes will fly at in an emergency situation caused by depressurization of the cabin. Nevertheless, aircraft cannot permanently fly at this ideal altitude for a number of reasons. As many mountain ranges and terrain surpass elevations of 10,000 feet, it would be unsafe for aircraft to fly at such a level, and harsher weather conditions often remain at such an altitude as well. Aircraft engines are also designed for optimal operation at much higher altitudes, thus it would be very inefficient to stay at 10,000 feet.
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