How is cabin pressurization managed and what are the basic limits?

• A. Manual cabin pressurization by crew only.
• B. Semi-automatic pressurization with fixed 8,000 ft cabinet altitude.
• C. Automatic pressurization via the outflow valve with manual descent if needed.
• D. Automatic pressurization via the outflow valve with automatic descent to maintain cabin pressure; limits include a maximum cabin altitude and a differential pressure limit.

Answer: (D)

Cabin pressurization is governed automatically by the system. The pressure controller sets the desired cabin pressure and the outflow valve adjusts to maintain that pressure by venting air from the cabin to the outside as needed. If the cabin pressure would rise too high (air inside is too pressurized), the valve opens more; if it would fall too low (pressure too low), the valve closes to keep air in. Automatic descent is built in to bring the aircraft to an altitude that lets the cabin pressure stay within limits, without requiring crew intervention.

Two safety limits define the envelope: the maximum cabin altitude, which is the highest altitude at which the cabin can be held (typically about 8,000 feet), and the differential pressure limit, which is the maximum allowed difference between cabin pressure and outside ambient pressure (around 8.6 psi, or roughly 0.6 bar). These prevent structural stress and ensure passenger comfort.