Plain English Meaning

These are spinning-wheel instruments that get their spin from moving air rather than from electricity. A pump in the engine, or a small funnel on the outside of the aircraft, sucks air through the instrument, and that moving air pushes against the wheel and keeps it spinning fast.

Full Definition

Gyroscopic flight instruments whose rotors are spun by a stream of air directed against buckets cut into the rotor's outer rim. The airflow is produced either by an engine-driven vacuum pump that draws filtered cabin air through the instrument, or by a venturi mounted on the airframe that uses the slipstream to create suction. Pneumatic gyros are commonly used to drive the attitude indicator and heading indicator in older or simpler aircraft.