Aircraft Pressurization Systems

Main Components of a Pressurization System

• Pneumatic System (Power Source): Pressurization relies on compressed air, typically supplied by engine bleed air, which is directed into the cabin.
• Outflow Valve & Actuator: The outflow valve regulates how much air leaves the cabin. Its actuator, often electrically or pneumatically controlled, adjusts the valve position based on system commands.
• Pressure Controller: This device automatically manages the outflow valve to maintain the desired cabin pressure, using preset cabin altitude and rate-of-change profiles.
• Excessive Differential Pressure-Relief Valve: Prevents the pressure difference between inside and outside the cabin from exceeding structural limits.
• Negative Differential Pressure-Relief Valve: Protects the structure if outside pressure becomes greater than cabin pressure, such as during rapid descent.

Operating Principle

Pressurization works by introducing conditioned air into the sealed cabin and controlling its escape via the outflow valve. The pressure controller ensures the cabin altitude (the pressure inside the cabin expressed as an equivalent altitude) follows a safe profile during climb and descent. Cabin vertical speed is the rate at which cabin altitude changes, kept within comfortable limits to avoid discomfort. Differential pressure is the difference between cabin and outside pressure, and is closely monitored to prevent structural damage. On the ground, the system is set to maintain ambient pressure (ground pressurization) for comfort and safety during boarding and maintenance.

Air-Conditioning and Pressurization

The air-conditioning and pressurization systems work together to provide:

• Ventilation: Ensuring a continuous supply of fresh air.
• Temperature Control: Mixing bleed air with cooler air as needed.
• Pressure Regulation: Maintaining safe and comfortable cabin pressure.

Typical Indications

Flight deck displays show:

• Cabin altitude
• Cabin vertical speed
• Differential pressure
• Outflow valve position
• Warnings for overpressure or system faults

Electronic Cabin-Pressure Controllers

Modern controllers automate the entire process. Pilots input landing field elevation and desired profiles; the controller adjusts the outflow valve electrically to follow a programmed schedule, minimizing manual input and reducing the risk of error.