How does Cabin Pressurization work on an Airplane?

Cabin Pressurization System in modern aircraft

The airplane operates at altitudes where the oxygen density is not sufficient to sustain life. The pressurization control system keeps the airplane cabin interior at a safe altitude. This protects the passengers and crew from the effects of oxygen starvation called hypoxia.

Composition of Atmosphere

The mixture of gases that make up the earth’s atmosphere is commonly called air. It is composed principally of :

• 78 percent nitrogen,
• 21 percent oxygen, and
• 1 percent remaining various gases in smaller quantities.

As altitude increases, the total quantity of all the atmospheric gases reduces rapidly. However, the relative proportions of nitrogen and oxygen remain unchanged up to about 50 miles above the surface of the earth. Atmospheric pressure at sea level is 14.7 psi because if a one square inch column of air stretching from sea level into space weighs 14.7 pounds.

Atmospheric pressure decreases with increasing altitude. Most civilian aviation takes place in the troposphere in which temperature decreases as altitude increases. The troposphere is the lowest layer of Earth’s atmosphere and is also where nearly all weather conditions take place.

Source of air for cabin pressurization

The air conditioning packs force air into the airplane pressure vessel (cabin). Pressurization Control System maintains a safe cabin altitude. This is to ensure that passengers and crew have enough oxygen present at sufficient pressure to facilitate full blood saturation.

Control of Cabin Pressure

Controlling cabin pressurization is accomplished by regulating the amount of air that flows out of the cabin. The major components for the pressurization control are the Cabin pressure controller (CPC), Outflow valve, Safety valve, and Negative pressure relief valve.

Cabin Pressure Controller (CPC)

The cabin pressure controller is the device used to control the cabin air pressure. Each CPC has its own systems interface and valve motor system. Cabin altitude is normally rate-controlled by the cabin pressure controller up to a cabin altitude of 8,000 feet.

Outflow Valve

A cabin outflow valve opens, closes, or modulates to establish the amount of air pressure maintained in the cabin.

Positive Pressure Relief Valve

Pressurization safety valves are used to prevent the over-pressurization of the aircraft cabin. They open at a preset differential pressure and allow air to flow out of the cabin, also act as a dump valve, allowing the crew to dump cabin air manually. On most aircraft, safety valves are set to open between 8 and 10 psid. Some large transport category aircraft cabins may have more than one cabin pressurization safety valve. Pressurization safety valves also called positive pressure relief valves that prevent overpressure damage to the airplane structure. The positive pressure relief valves are fail-safe devices that bleed fuselage pressure overboard if the outflow valve fails closed.

Negative Pressure Relief Valve

The negative pressure relief valve prevents negative differential pressure (vacuum pressure) damage to the airplane structure. This can prevent structural damage during a rapid descent. The spring-loaded relief valve opens inward to allow ambient air to enter the cabin. Too much negative pressure can cause difficulty when opening the cabin door. If high enough, it could cause structural damage since the pressure vessel is designed for cabin pressure to be greater than ambient.

Ventilation System

Ventilation is achieved by controlling the rate at which the air enters the cabin and the rate at which it is allowed to leave. Supply air is the air pumped into the cabin, and if is allowed to leave at a slower rate than it is pumped in then the cabin becomes pressurized. The ventilation system uses differential pressure, cabin to ambient, to remove air by suction.