The function of a pressure relief valve in an aircraft's pneumatic system is to

• A. Prevent seals from bleeding
• B. Maintain cabin pressure
• C. Vent excess pressure to atmosphere
• D. Regulate system pressure

Answer: (A)

In an aircraft's pneumatic system, the pressure relief valve acts as a safety device that opens to vent air when the system pressure exceeds a preset limit. This prevents over-pressurization, which could damage components such as ducts, valves, and fittings or cause seal failures. Under normal operation it stays closed, and only vents when the pressure climbs high enough, releasing the excess to the atmosphere. This is why venting excess pressure to atmosphere is the correct way to describe its function.

Cabin pressure is controlled by the environmental control system and outflow valves, not by the relief valve. Regulation of normal operating pressure is handled by a regulator or control system, while the relief valve’s job is specifically to prevent dangerous over-pressure. It isn’t about preventing seals from bleeding.