Heading Indicator - Operation

Operation

The heading indicator works using a gyroscope, tied to the aircraft horizontal, to establish an inertial platform. As such, any configuration of the aircraft horizontal that does not match the local Earth horizontal results in 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 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.