What is the function of the outflow valve and how does it affect cabin pressure during climb and descent?

• A. The outflow valve injects air into the cabin to raise pressure.
• B. The outflow valve changes bleed air temperature to regulate temperature.
• C. The outflow valve regulates the amount of air leaving the cabin to control cabin pressure; adjusting it maintains the desired cabin altitude and differential pressure.
• D. The outflow valve closes automatically at takeoff and has no effect during climb or descent.

Answer: (C)

The key idea is that the outflow valve is the last control of the pressurization system, and it sets cabin pressure by letting air escape from the cabin to the outside. The cabin pressure controller uses this valve to maintain the desired cabin altitude and the differential pressure between the cabin and the outside air.

During climb, as outside pressure falls, the system tends to retain air to keep the cabin at a comfortable, lower altitude. The outflow valve is modulated toward restricting venting, so less air leaves the cabin and the cabin pressure rises toward the target (keeping cabin altitude from increasing too quickly). As you reach cruising height, the valve continuously adjusts to hold the target cabin pressure while the packs and other components supply conditioned air.

During descent, outside pressure rises. The outflow valve opens more to vent air from the cabin, allowing cabin pressure to fall toward the increasing outside pressure and keeping the differential pressure within limits. In short, more venting lowers cabin pressure; less venting raises it, and the system balances to maintain the intended cabin altitude and differential pressure.

The valve does not inject air or regulate temperature; its job is to control how much air leaves the cabin to maintain the proper pressure.