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Gyroscopes operate on a principle called "rigidity in space". They tend to remain in position due to their resistance to deflection of the spinning wheel or disc. Gyroscopes form the stable platform required by certain flight instruments. In glass cockpit aircraft, conventional spinning gyroscopes are replace by solid-state devices contained in a control unit called the AHRS (Attitude Heading Reference System).
These conventional instruments are operated by gyroscopes. Any can be operated by the electrical system, but traditionally they are powered as indicated below:
Turn coordinator: electrical system
Turn and slip indicator: electrical system
Attitude indicator: vacuum system
Heading Indicator: vacuum system
Aircraft use two types of turn indicators: turn-and-slip indicators and turn coordinators. Because of the way the gyro is mounted, the turn-and-slip indicator shows only the rate of turn in degrees per second. The turn coordinator is mounted at an angle, or canted, so it can initially show roll rate. When the roll stabilizes, it indicates rate of turn. Both instruments indicate turn direction and quality (coordination), and also serve as a backup source of bank information in the event an attitude indicator fails. Coordination is achieved by referring to the inclinometer, which consists of a liquid-filled curved tube with a ball inside.
Prior to engine start, electrically-operated gyros may be checked (to some degree) by listening for unusual (or absent) sounds as they spin up.
Within five minutes of engine start, a vacuum-driven heading indicator should show accurate readings during taxi turns. In the attitude indicator, the horizon bar should be erect and stable. The horizon bar should tilt no more than 5° during taxi turns.
In flight and during coordinated turns, centrifugal force moves the pendulous vanes of a vacuum-driven attitude indicator resulting in precession of the gyro toward the inside of the turns.