Cabin pressure is maintained during flight by which mechanism?

• A. Adjusting the bleed air temperature
• B. Opening and closing an outflow valve
• C. Bleed air source sequencing
• D. Manual cabin altitude control only

Answer: (B)

Maintaining cabin pressure is achieved by regulating how much air leaves the cabin. The pressurization system pushes conditioned air into the cabin, but the actual pressure is kept by venting air to the outside through the outflow valve. When the cabin needs to be pressurized less, the valve closes a bit to reduce venting; when it needs to be pressurized more (or to prevent overpressure), the valve opens to vent more air. An automatic pressurization controller uses cabin altitude and differential pressure sensors to adjust the valve so the cabin stays at the chosen level. Other options involve heating or conditioning the air or selecting air sources, which affect supply rather than how much air is bled off to maintain pressure.