Definition
In aircraft instrument systems, suction is the partial vacuum (pressure lower than the surrounding atmosphere) used to drive air through gyroscopic flight instruments such as the attitude indicator and heading indicator. A vacuum pump or venturi removes air from the instrument case, and outside air rushes in through a filter to spin the gyros. The level of suction is shown on a suction gauge, typically calibrated in inches of mercury (in. Hg).
Plain English
Suction is a pulling force created when the pressure inside something is lower than the pressure outside. In an aircraft, a pump lowers the pressure inside certain instruments so that outside air rushes in and spins their internal gyros, keeping them working.
Context Anchor
Seen in discussions of airflow over the wing, lift, and slow-speed flight at higher angles of attack.
Derivation
From the Latin sugere, meaning 'to suck.' In everyday speech, 'suction' suggests something pulling things in. In aircraft systems, nothing is really 'pulling' — air is simply moving from higher pressure outside to lower pressure inside the instrument. The pump creates the low pressure; the atmosphere does the pushing.
Why Pilots Care
Loss of suction at high angles of attack is what causes a stall, so recognizing it helps pilots manage slow flight safely.
Analogy
If you sip through a straw, you lower the pressure in the straw, and the higher outside air pressure pushes the drink upward. Wing suction is also a pressure difference, not an invisible hand pulling on the airplane.
Grounding Statement
Faster air over the curved top of the wing thins out and pulls the wing upward.
Intuition Check
Suction does not mean the wing is being pulled by empty space. Here it means lower air pressure on one side, with higher pressure elsewhere creating the force.
Example Sentence 1
During the runup, the pilot checked that the suction gauge showed a reading within the green arc before departing on an IFR flight.
Example Sentence 2
If the angle of attack increases too far, suction collapses and the wing stalls.