thumb|right|A heading indicator in a [[light aircraft|small aircraft.]]
thumb|HI interior
The heading indicator (HI), also known as a directional gyro (DG) or direction indicator (DI), is a flight instrument used in an aircraft to inform the pilot of the aircraft's heading.
Use
The primary means of establishing the heading in most small aircraft is the magnetic compass, which, however, suffers from several types of errors, including that created by the "dip" or downward slope of the Earth's magnetic field. Dip error causes the magnetic compass to read incorrectly whenever the aircraft is in a bank, or during acceleration or deceleration, making it difficult to use in any flight condition other than unaccelerated, perfectly straight and level. To remedy this, the pilot will typically maneuver the airplane with reference to the heading indicator, as the gyroscopic heading indicator is unaffected by dip and acceleration errors. The pilot will periodically reset the heading indicator to the heading shown on the magnetic compass.
Operation
The heading indicator works using a gyroscope, tied by an erection mechanism to the aircraft yawing plane, i. e. the plane defined by the longitudinal and the horizontal axis of the aircraft. As such, any configuration of the aircraft yawing plane that does not match the local Earth horizontal results in an indication error. The heading indicator is arranged such that the gyro axis is used to drive the display, which consists of a circular compass card calibrated in degrees. The gyroscope is spun either electrically, or using filtered air flow from a suction pump (sometimes a pressure pump in high altitude aircraft) driven from the aircraft's engine. Because the Earth rotates (ω, 15° per hour, apparent drift), and because of small accumulated errors caused by imperfect balancing of the gyro, the heading indicator will drift over time (real drift), and must be reset using a magnetic compass periodically.
Variations
Some more expensive heading indicators are "slaved" to a magnetic sensor, called a flux gate. The flux gate continuously senses the Earth's magnetic field, and a servo mechanism constantly corrects the heading indicator.