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The heading indicator (also called an HI) is a flight instrument used in an aircraft to inform the pilot of the aircraft's heading. It is sometimes referred to by its older names, the directional gyro or DG, and also (UK usage) direction indicator or DI.
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.
The heading indicator works using a gyroscope, tied by an erection mechanism to the aircraft horizontal, i. e. the plane defined by the longitudinal and the lateral axis of the aircraft. As such, any configuration of the aircraft horizontal that does not match the local Earth horizontal results in a gimbal error, essentially leading to a variation in the predictable "apparent" wander, known in this instance as drift. The heading indicator 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 filtered air flow from a vacuum pump (sometimes a pressure pump in high altitude aircraft) driven from the aircraft's engine. Because the Earth rotates (ω, 15° per hour), and because of small accumulated errors caused by friction and imperfect balancing of the gyro, the heading indicator will drift over time, and must be reset from the compass periodically.  The apparent drift is predicted by ω sin Latitude and will thus be greatest over the poles. Another sort of apparent drift exist in the form of transport wander, where aircraft movement will essentially add or subtract to the effect of the Earth's rotation upon a gyroscope. To counter for the effect of Earth rate drift a latitude nut can be set (on the ground only) which induces a (hopefully equal and opposite) real wander in the gyroscope. Normal procedure is to realign the direction indicator once each ten to fifteen minutes during routine in-flight checks. Failure to do this is a common source of navigation errors among new pilots.
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. These "slaved gyros" reduce pilot workload by eliminating the need for manual realignment every ten to fifteen minutes.
The prediction of drift in degrees per hour, is as follows:
|SOURCE||Drift Rate (Degrees per Hour)||Sign (Northern Hemisphere)||Sign (Southern Hemisphere)|
|Earth Rate||15 * sin Mean Operating Latitude||- (causing an under-read)||+ (causing an over-read)|
|Latitude Nut||15 * sin Latitude of Setting||+||-|
|Transport Wander EAST||(East Ground Speed Component (or sine track angle x ground speed or change in longitude/flight time in hours) * tan Mean Operating Latitude)/60||-||+|
|Transport Wander WEST||(West Ground Speed Component (or sine track angle x ground speed or change in longitude/flight time in hours) * tan Mean Operating Latitude)/60||+||-|
|Real/Random Wander||As given in the Aircraft Operating Manual||As given||As given|
Although it is possible to predict the drift, there will be minor variations from this basic model, accounted for by gimbal error (operating the aircraft away from the local horizontal), among others. A common source of error here is the improper setting of the latitude nut (to the opposite hemisphere for example). The table however allows one to gauge whether an indicator is behaving as expected, and as such, is compared with the realignment corrections made with reference to the magnetic compass. Transport wander is an undesirable consequence of apparent drift.
- Acronyms and abbreviations in avionics
- Earth Inductor Compass
- Gyrocompass, a compass depending on gyroscopic precession effect instead of a basic gyroscopic effect
- Horizontal situation indicator
- Inertial navigation system, a far more complex system of gyroscopes that also employ accelerometers
- Bowditch, Nathaniel. American Practical Navigator, Paradise Cay Publications, 2002, pp.93-94, ISBN 978-0-939837-54-0.
- NASA NASA Callback: Heading for Trouble, NASA Callback Safety Bulletin website, December 2005, No. 305. Retrieved August 29, 2010.
- As a heading indicator ages and its ball bearings become worn and noisy, thus increasing friction, the tendency to drift will increase.
- "How Aircraft Instruments Work." Popular Science, March 1944, pp. 119, bottom half of page.