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. 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.
Read more about this topic: Heading Indicator
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