Heading Indicator (DG)
The Gyro of the Heading Indicator spins, as seen on the picture, vertically on a horizontal Gimbal. The horizontal Gimbal is attached to vertical Gimbal. As the airplane turns to different headings (rotation around the longitudinal axis of the a/c). The gyro will remain in its position at all times. It is also able to show rotation about the vertical (yaw).
What Errors is the HI subject to?
Due to bearing friction, the HI should be checked and adjusted as required about every
15 min with the readings of the Magnetic Compass. This check is especially advisable during maneuvers or if you fly a couple of turn and/or steep turns.
How to adjust the Instrument?
The HI has an 'Adjustment Knob'. Pushing it in, will momentarily block the gimbals, and will enable you to turn the compass card mechanically through gears as visible on the picture above.