What is the function of the negative pressure relief valve?

• A. To limit cabin altitude rise
• B. To vent cabin air to the outside
• C. To prevent fuselage external pressure from exceeding the fuselage internal pressure
• D. To provide emergency oxygen

Answer: (C)

The key idea is protecting the structure from a negative pressure differential. A negative pressure relief valve opens when the cabin pressure becomes lower than the outside air pressure by a set amount. In that situation, it allows outside air to flow into the cabin to raise internal pressure and prevent the fuselage from being stressed or crushed by the higher external pressure. It’s not about venting cabin air to the outside, and it doesn’t supply oxygen—the emergency oxygen system does that separately.