Definition
A spring-loaded valve installed in the vacuum (suction) system of an aircraft that limits the amount of suction applied to the gyroscopic instruments. When suction in the system exceeds a preset value, the valve opens to admit filtered outside air, holding the suction at the correct operating level.
Plain English
A valve that keeps the vacuum pulling on the gyro instruments from getting too strong. If the pull becomes too great, the valve cracks open and lets in a little outside air to bring it back to the right level.
Context Anchor
Seen in aircraft vacuum-system descriptions, maintenance checks, and troubleshooting related to the suction gauge and vacuum-powered instruments.
Derivation
‘Suction’ comes from the Latin sugere, meaning ‘to suck’ — here it refers to the partial vacuum the system creates. ‘Relief’ comes from the Old French relever, ‘to lift up or ease’ — the valve eases excess suction. Together: a valve that relieves too much suction.
Why Pilots Care
Excessive suction can destroy expensive gyro instruments and produce unreliable attitude information in instrument conditions.
Intuition Check
A suction relief valve does not create suction. It limits excessive suction by opening when the pull becomes too strong.
Example Sentence 1
During the annual inspection, the mechanic adjusted the suction relief valve to bring the vacuum gauge reading back into the normal operating range.
Example Sentence 2
During the instrument check, proper operation of the suction relief valve kept vacuum within the green arc.