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.
Plain English
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.
Context Anchor
Seen in instrument flying when discussing vacuum or pressure systems that power the attitude indicator and heading indicator.
Derivation
Pneumatic comes from the Greek pneuma, meaning breath or air. So a pneumatic gyro is simply an air-driven gyro — the wheel inside is spun by airflow rather than by an electric motor.
Why Pilots Care
They keep working during an electrical failure and supply the pilot with essential attitude and heading information.
Intuition Check
Do not read pneumatic gyros as gyros that measure air. Here, pneumatic means air-powered: airflow is used to spin the gyro inside the instrument.
Example Sentence 1
After the vacuum pump failed, the pilot noted that the pneumatic gyros were beginning to wind down and switched to the electric backup attitude indicator.
Example Sentence 2
When the alternator failed, the pneumatic gyros continued to display accurate pitch and bank information.