Editing Heading indicator (section)

== 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.<ref name="Bowditch" />{{efn|As a heading indicator ages and its ball bearings become worn and noisy, thus increasing friction, the tendency to drift will increase.}}  The apparent drift is predicted by '''ω sin Latitude''' and will thus be greatest over the poles. 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. Otherwise it would be necessary to manually 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. Another sort of apparent drift exists in the form of transport wander, caused by the aircraft movement and the convergence of the meridian lines towards the poles. It equals the course change along a [[great circle]] (orthodrome) flight path. 
<ref>{{cite magazine |last=Peck |first=James L. H. |date=March 1944 |title=How Aircraft Instruments Work |url=https://books.google.com/books?id=0SkDAAAAMBAJ&pg=PA119 |magazine=Popular Science |location=New York, New York |publisher=Popular Science Publishing Company |volume=144 |issue=3 |page=119 |access-date=31 January 2023}}</ref>