Definition
Gyroscopic flight instruments that display aircraft heading by sensing rotation about the vertical axis. A directional gyro uses a spinning rotor whose rigidity in space provides a stable reference; the aircraft turns around the gyro, and that relative motion is shown on a heading card. Directional gyros are not magnetic and must be manually set to agree with the magnetic compass, since they drift over time due to mechanical friction and Earth's rotation.
Plain English
Heading instruments that use a spinning wheel to keep a steady reference, so the pilot can read the aircraft's heading on a smooth, stable dial instead of the jumpy magnetic compass.
Context Anchor
Seen in instrument flying when using the heading indicator to maintain or turn to a specific heading.
Derivation
Directional' comes from Latin directio, meaning 'a setting straight' -- here, a heading. 'Gyro' is from Greek gyros, meaning 'a circle' or 'ring,' and refers to the spinning rotor inside. So a directional gyro is literally a 'spinning-wheel device that shows direction.'
Why Pilots Care
Supplies a stable heading reference when the magnetic compass swings or lags during turns, climbs, or acceleration.
Analogy
A directional gyro is a little like a clock that keeps good time only after you set it correctly. If it slowly gets off, you have to compare it with the correct reference and reset it.
Intuition Check
A directional gyro is not the same as a magnetic compass. It displays the heading it was set to and can slowly drift away from the correct heading.
Example Sentence 1
Before taxiing, the pilot aligned the directional gyro with the magnetic compass so the heading indicator would read correctly.
Example Sentence 2
During the instrument scan the directional gyro showed a slow drift that matched the expected precession error.