Heading indicator
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The heading indicator (or HI) is an instrument used in an aircraft to inform the pilot of his heading. It is sometimes referred to by its older name, the directional gyro, or (UK usage) direction indicator or DI. The primary means of establishing heading in most small aircraft is the magnetic compass, but that suffers from errors 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, making it difficult to use in any flight condition other than 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 HI to the heading shown on the magnetic compass.
The HI works using a gyroscope to establish an inertial platform, which will remain fixed in space. The HI is arranged so that only the horizontal 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 air from a vacuum pump (sometimes a pressure pump in high alititude aircraft) driven from the aircraft's engine. Because the earth rotates (15° per hour), and because of small accumulated errors caused by friction, the HI will drift over time, and must be reset from the compass periodically. Normal procedure is to reset the heading indicator once each fifteen minutes of flight. Failure to do this is a common source of navigation errors among beginner pilots.
Some more expensive heading indicators are 'slaved' to a sensor (called a 'flux gate'). The flux gate continuously senses the earth's magnetic field, and a servo mechanism constantly corrects the heading indicator. These 'slaved gyros' reduce pilot workload by eliminating the need to be manually reset every fifteen minutes.