AS9120B, ISO 9001:2015, AND FAA AC 0056B ACCREDITED

How Does an Aircraft Cabin Pressurization System Work?

As aircraft operate at high altitudes, they must be pressurized or have breathable oxygen provided for the safety of passengers and aircrew alike. If such heights pose safety risks for humans, then why do we fly so high? First, airplanes at high altitudes consume less fuel for a given airspeed than they do for the same speed at lower altitudes, making it more efficient to soar high. Second, bad weather and unprecedented turbulence can be avoided by flying in the smooth air above dangerous weather patterns closer to the surface.

With fuel efficiency and weather in mind, pressurization systems are incorporated to make aircraft safe for passenger travel. A typical pressurization system has the cabin, flight compartment, and baggage compartments sealed into a unit containing air that has a higher pressure than the outside atmospheric pressure. On airplanes equipped with turbine engines, bleed air from the engine compressor section is utilized to pressurize the cabin. For older turbine-powered aircraft, superchargers can be used to pump air into the sealed fuselage. Piston-powered aircraft, on the other hand, take advantage of the air supplied from each engine turbocharger through a sonic venturi.

A cabin pressurization system is tasked with maintaining a cabin pressure altitude of about 8,000 feet at the maximum designated cruising altitude of an airplane. This prevents rapid shifts in cabin altitude which can be uncomfortable or cause injury to all on board as a result of hypoxia. More than that, this system enables the fast exchange of air from the inside to the outside of the cabin, ensuring that odor and stale air is eliminated. For these reasons, the flight crew in aircraft with pressurization systems should be aware of the danger of accidental loss in cabin pressure.

Terms Associated with Pressurization Systems

To better understand the operating principles of pressurization and air conditioning systems, we will outline some critical terms.

Aircraft Altitude is defined as the actual height above sea level at which the airplane is operating.

Ambient Temperature is the temperature of the area surrounding the aircraft.

Ambient Pressure is the pressure of the area around the aircraft.

Cabin Altitude is the equivalent cabin pressure in terms of altitude above sea level.

Differential Pressure is the difference in pressure between cabin pressure and atmospheric pressure.

The cabin pressure control system provides cabin pressure regulation, pressure relief, vacuum relief, and the ability to keep the desired cabin altitude in the isobaric and differential range. To apply these functions, a cabin pressure regulator, outflow valve, and safety valve are used. The cabin pressure regulator controls cabin pressure to a predetermined value in the isobaric range and limits cabin pressure to a preset value in the differential range. First, the aircraft must reach an altitude at which the difference between the pressure inside and outside the cabin is equal to the highest different pressure that the fuselage structure can withstand. This increase in aircraft altitude will be followed by an increase in cabin altitude as well.

The safety valve is a combination pressure relief, vacuum relief, and dump valve all in one. The pressure relief valve prevents cabin pressure from exceeding a predetermined differential pressure above ambient pressure. The vacuum relief prevents ambient pressure from surpassing cabin pressure by enabling external air to enter the cabin when ambient pressure exceeds cabin pressure. The flight deck control switch actuates the dump valve. When the switch is positioned to ram, a solenoid valve opens, dumping cabin air into the atmosphere. 

There are several critical design factors that limit the degree of pressurization and operating altitude of an aircraft. For instance, the fuselage is designed to handle a specified maximum cabin differential pressure. There are a few key instruments that work in tandem with the pressurization controller. The cabin differential pressure gauge indicates the difference between the pressure inside and outside of the aircraft. The gauge must be monitored so the cabin does not exceed the maximum permissible differential pressure. A cabin altimeter is also utilized to check on the overall performance of the system, and a third instrument is used to acquire information about the cabin rate of climb of descent.

Decompression is defined as the inability of the pressurization system to maintain its designed pressure differential. They generally fall into two categories, those of which are explosive and rapid. Explosive decompression happens when cabin pressure changes faster than the lung can decompress, resulting in lung damage. In contrast, rapid decompression is the change in cabin pressure where the lungs decompress faster than the cabin.

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