Definition
A cockpit instrument that displays the amount of negative pressure (vacuum) being produced by the aircraft's vacuum system, typically measured in inches of mercury (in. Hg). The gauge confirms that the vacuum pump is generating enough suction to drive the gyroscopic instruments, such as the attitude indicator and heading indicator.
Plain English
A small dial in the cockpit that shows whether the vacuum pump is pulling hard enough to spin the gyro instruments. If the reading is in the normal range, those instruments can be trusted. If it's low, they may be unreliable.
Context Anchor
Seen during cockpit checks before engine start, after engine start, and during instrument scan checks in aircraft that use a vacuum system.
Derivation
Suction comes from the Latin sugere, meaning 'to suck.' In this context it refers to the partial vacuum (lower-than-atmospheric pressure) that the pump creates to draw air through the gyro instruments and spin their rotors.
Why Pilots Care
Low or zero suction means the attitude and heading indicators will not function reliably, creating a serious risk of spatial disorientation in instrument conditions.
Intuition Check
Do not read “suction” here as general engine airflow or the engine “sucking” in air for power. In this context, it means the vacuum pressure used to operate certain flight instruments.
Example Sentence 1
During the run-up, the pilot checked the suction gauge and confirmed it was reading within the green arc before departing into the clouds.
Example Sentence 2
In cruise the suction gauge showed 4.8 inches of mercury, confirming the vacuum system was supplying adequate pressure to the gyros.