What is the function of the outflow valve in cabin pressurization, and how does it behave during climb and descent?

• A. The outflow valve controls cabin air exhaust to regulate pressure; it modulates to maintain the set differential during climb, cruise, and descent.
• B. It controls cabin air intake from outside.
• C. It measures cabin pressure.
• D. It is used only on the ground.

Answer: (A)

The outflow valve is the mechanism that vent-air leaves the cabin to the outside, and it is the primary control for cabin pressurization. The system maintains a set cabin pressure by balancing the air being supplied (bleed air and recirculated air) with the air being exhausted through this valve. The valve opens or closes in response to the cabin pressure sensors and the push from the automatic pressurization controller to keep the differential between cabin and ambient pressure at the desired value.

During climb, ambient pressure drops as you rise. To keep the cabin from over-pressurizing relative to the air outside, the valve tends to close more, reducing exhaust so the cabin pressure can rise toward the target differential. During descent, ambient pressure increases, so the valve opens more to vent air out, allowing cabin pressure to fall back toward the lower target. In level flight, the valve continuously modulates to maintain the set differential as conditions change.