Definition
Flight instruments that use a spinning rotor (gyroscope) driven by a flow of air to sense an aircraft's attitude, heading, or rate of turn. The air is supplied by an engine-driven vacuum or pressure pump that pulls or pushes air across small buckets cut into the rim of the rotor, spinning it at high speed. Common examples include the attitude indicator, heading indicator, and some turn indicators when configured as air-driven rather than electric.
Plain English
These are cockpit instruments that work by spinning a small wheel inside, and the wheel is kept spinning by a stream of air rather than by electricity. The spinning wheel stays steady in space, and the instrument uses that steadiness to show things like whether the wings are level or which way the nose is pointing.
Context Anchor
Seen during preflight and cockpit instrument checks, especially when checking instruments powered by the vacuum or pressure system.
Derivation
Gyro comes from the Greek gyros, meaning 'circle' or 'turn' — a reference to the spinning rotor at the heart of each instrument. 'Mechanical' and 'air-driven' simply distinguish these from newer electric or electronic (solid-state) instruments that do the same job without moving parts.
Why Pilots Care
These instruments supply attitude and heading information essential for instrument flight; loss of vacuum renders them unusable and forces partial-panel procedures.
Intuition Check
Do not assume “air-driven” means the outside wind is turning the instrument. In most airplanes, it means an internal vacuum or pressure system is moving air through the instrument to spin the gyro.
Example Sentence 1
During preflight, the pilot checked that the vacuum gauge was reading in the green, since the attitude indicator and heading indicator are mechanical air-driven gyro instruments and depend on it.
Example Sentence 2
A failed vacuum pump caused the mechanical air-driven gyro instruments to wind down and become unreliable within a few minutes.