When does a negative pressure relief valve open?

• A. Cabin pressure is greater than ambient pressure
• B. Cabin pressure equals ambient pressure
• C. Cabin pressure is less than ambient pressure
• D. Cabin pressure is unknown

Answer: (C)

The air system uses relief valves to respond to the pressure difference between the cabin and the outside. A negative pressure relief valve is there to prevent the cabin from falling below the outside pressure. It opens when the cabin pressure becomes lower than ambient pressure, allowing outside air to flow into the cabin until the pressures are closer to equal. It stays closed when the cabin pressure is equal to or higher than ambient. This is the safety mechanism that prevents a dangerous negative differential (vacuum) inside the cabin.