Definition
The component of an aircraft's pneumatic system that drives air through the gyroscopic flight instruments, either by drawing air out (creating suction/vacuum) or by pumping air in (creating positive pressure). In most light aircraft this is an engine-driven vacuum pump or pressure pump; in some installations a venturi tube mounted on the airframe provides suction from airflow in flight.
Plain English
The pump (or sometimes a venturi in the airflow) that moves air through the gyro instruments to make them spin. It either sucks air through them or blows air through them, depending on the system.
Context Anchor
Seen in instrument flying when discussing pneumatic system failure and loss of air-driven flight instruments.
Derivation
‘Suction’ comes from the Latin sugere, ‘to suck’ — meaning air is drawn through the system by reducing pressure inside it. ‘Pressure’ here means the opposite: air is pushed through the system above outside pressure. Knowing this clarifies that both methods achieve the same goal — moving air across the gyro vanes — just from opposite ends.
Why Pilots Care
Loss of the suction or pressure source disables key attitude and directional gyros, forcing reliance on partial-panel instruments in instrument meteorological conditions.
Grounding Statement
The key idea is simple: no moving air from the source means the air-driven instruments may no longer be trustworthy.
Intuition Check
Do not read “suction or pressure source” as just any low-pressure or high-pressure area. Here it means the specific pump or supply that powers the aircraft’s air-driven instrument system.
Example Sentence 1
During the runup, the pilot checked the suction or pressure source by verifying the gauge reading was within the green arc.
Example Sentence 2
When the suction or pressure source failed in clouds, the pilot transitioned to the turn coordinator and magnetic compass for attitude reference.