| Wikipedia on the Web states that a heading indicator | | | | Directional gyros are normally calibrated to show an |
| (directional gyro or DG) is a flight instrument used in an | | | | apparent drift rate of 0 degrees for the location they |
| aircraft to inform the pilot of their heading. It is usually | | | | are calibrated at. |
| referred to as the directional gyro. | | | | A DG built perfect (no real drift) would show the |
| Most general aviation aircraft with Directional Gyros | | | | earth's turn rate on the heading card for example; (-15 |
| have the non-slaved type. This means they need to | | | | degrees/hour at the North Pole and +15 degrees/hour |
| reset to the magnetic compass at some time interval, | | | | at the South Pole). As noted above, directional gyros |
| depending on the apparent drift of the unit.) One good | | | | are normally calibrated for no apparent drift. |
| check for apparent drift is to fly aligned with a fixed | | | | This means a unit calibrated for the central latitudes of |
| land mark (section line road) while checking the gyro | | | | the Northern Hemisphere would need to be |
| heading and then return and fly the same land mark 1 | | | | recalibrated for the Southern Hemisphere for example; |
| 2-1 hour later. From this you can determine the drift | | | | South America, Africa, Australia, and etc. |
| rate degrees/10 minutes. (The FAA TSO spec. is a | | | | Changes in the apparent drift rate for your Directional |
| maximum of 4 degrees/10 minutes.) | | | | Gyro can indicate changes inside your unit. |