Pressurization System Operation

Key Components of Pressurization Systems

• Pneumatic System: Supplies compressed air, typically from engine bleed air, which is the power source for pressurization.
• Outflow Valve: Regulates the amount of air leaving the cabin, directly controlling cabin pressure.
• Outflow Valve Actuator: Mechanism (usually electrically or pneumatically operated) that adjusts the outflow valve position.
• Pressure Controller: Automatically manages the outflow valve to maintain the selected cabin altitude and rate of change (cabin vertical speed).
• Pressure Relief Valves:
  • Excessive Differential Pressure-Relief Valve: Prevents the cabin-to-outside pressure difference from exceeding structural limits.
  • Negative Differential Pressure-Relief Valve: Protects against excessive negative pressure differentials, which could damage the fuselage.

Operating Principle

The system introduces compressed air into the cabin and controls its escape via the outflow valve. The pressure controller adjusts the outflow valve to maintain a set cabin altitude and vertical speed, ensuring a gradual pressure change during climb and descent. Cabin pressure is always kept higher than outside pressure at altitude, but within safe structural limits.

Key Terms

• Cabin Altitude: The equivalent altitude corresponding to the current cabin pressure.
• Cabin Vertical Speed: The rate at which the cabin altitude changes, controlled for passenger comfort.
• Differential Pressure: The difference between cabin and outside pressure, limited by design.
• Ground Pressurization: The system is typically unpressurized on the ground, with outflow valves open for boarding and maintenance.

System Indications and Warnings

Typical cockpit indications include cabin altitude, cabin vertical speed, and differential pressure. Warning systems alert the crew if cabin altitude becomes excessive (e.g., above 10,000 ft), or if differential pressure limits are approached.

Manual Operation

If the automatic controller fails, pilots can manually operate the outflow valve, usually via an alternate electrical circuit. This allows direct control of cabin pressure, but requires careful monitoring to avoid exceeding structural limits.

Integration with Air Conditioning

Pressurization and air conditioning are closely linked, as the same pneumatic air is used for both ventilation and temperature control, in addition to pressure regulation.