DG Tips

DG Tips

We get many calls from customers asking us to replace their DG because it won't hold a heading. They complain they have to continually re-set the DG in flight because it is drifting off of it's heading.

In our experience, about 1/2 of these complaints are bad DG's, and the other 1/2 are compass calibration problems. If the compass is out of calibration, it will make the DG appear bad. When your compass goes out of calibration, it is almost never a linear error. The compass may have 1 degree of error on a northerly heading, and 20 degrees of error on an easterly heading. The result is that the pilot will set the DG to the compass then some time later (usually after the aircraft heading has changed), the pilot will check the DG against the compass and find an error. The pilot will then re-set the DG to the compass and assume the DG has just pressesed. What can happen is that the DG was fine, but the compass has not displayed the change in aircraft heading accurately due to internal errors.

Whenever you suspect your DG may need overhauled, perform this simple test:

Perform a simple 15 minute test flight
Before taking off, with the aircraft nose pointed down the runway, set the DG to the runway heading.
Take off and fly around for about 10 minutes (do not re-set the DG in flight leave it alone, ignore the compass)
Land the plane on the same runway you departed from and before you taxi off the runway, note the DG heading.
How much has the DG heading changed from the take-off value?
The rule of thumb is that the DG should not presses more than 5 deg in 15 minutes.
If your DG is within 5 deg or less of the value on take off, your DG is probably OK, get the compass calibrated

If your compass has not been calibrated in 5 years or more, have it re-calibrated. Find a shop with a calibrated master compass to perform the compass swing and adjustments. Don't try to use the runway heading or a compass rose to calibrate your compass, you won't be able to position the nose of the aircraft in 1 degree increments. Most windshield mounted compasses can be accurate within a degree or two when accurately adjusted. Instrument panel mounted compass will have more error due to local disturbances. Penn Avionics offers compass calibration services for $95.00.

Compass being calibrated with external master compass. One technician outside the aircraft uses the calibrated master compass to sight the heading of the aircraft. Another technician inside the aircraft adjusts the aircraft compass accordingly. This is performed on all cardinal headings.