What is the function of a suction relief valve?

• A. A relief valve in an instrument vacuum system required to maintain the correct low pressure inside the instrument case for the proper operation of the gyros.
• B. A valve that drains fuel from the wing tank.
• C. A valve controlling cabin pressurization.
• D. A valve used to regulate hydraulic pressure.

Answer: (A)

The suction relief valve exists to regulate the vacuum in the instrument system, keeping the pressure inside the instrument case from becoming too low. In a vacuum-driven gyro system, the gyros need a precise level of suction to spin correctly and read accurately. If the pump pulls too much, the suction relief valve opens and lets ambient air enter to limit how negative the pressure can get. This protects the gyros from excessive load or erratic readings and keeps the instruments operating within their design range. When suction is within limits, the valve stays closed and the proper vacuum is maintained. This valve is specifically for the instrument vacuum system, not for fuel drainage, cabin pressurization, or hydraulic pressure.