Definition
An instrument vacuum source that uses an externally mounted venturi tube to generate suction for driving gyroscopic flight instruments. As air flows through the narrow throat of the venturi during flight, its pressure drops, creating a partial vacuum that is plumbed to the gyro instrument cases. This suction draws filtered cabin air across the gyro rotors, spinning them up to operating speed. Because the system depends on airflow through the venturi, it produces no suction on the ground and requires the aircraft to be flying before the gyros become reliable.
Plain English
A small horn-shaped tube mounted on the outside of the airplane that uses moving air to create suction. That suction is what spins the gyro instruments inside the cockpit. Until the airplane is flying, the system isn't producing suction, so the gyros aren't fully working yet.
Context Anchor
Seen in instrument-system discussions, especially with older aircraft that use suction-powered instruments instead of an engine-driven vacuum pump.
Derivation
Named after Giovanni Venturi, an 18th-century Italian physicist who studied how fluid speeds up and loses pressure when forced through a narrowing in a tube. That pressure drop -- now called the Venturi effect -- is exactly what creates the suction used to drive the gyros.
Why Pilots Care
Supplies the suction needed to keep attitude and heading information available when the engine-driven vacuum pump fails or is not installed.
Analogy
It is like using moving air to create a gentle pull instead of using a pump. The faster the air moves through the narrowed tube, the stronger the pull becomes.
Intuition Check
Do not think of a venturi tube system as blowing air into the instruments. It creates a low-pressure area that pulls air through them.
Example Sentence 1
The old Cub uses a venturi tube system, so the attitude indicator doesn't settle down until a few minutes after takeoff.
Example Sentence 2
On the preflight, the instructor pointed out the venturi tube systems mounted on the side of the fuselage as the backup vacuum source.