Plain English Meaning

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.

Full 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 air across the gyro rotors, spinning them up to operating . Because the system depends on through the venturi, it produces no suction on the and requires the to be flying before the gyros become reliable.