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