The directional gyro is a fairly simple instrument. It indicates the direction that the aircraft is heading. However, it does not sense the direction that the aircraft is heading.
How does the DG know which way the aircraft is heading?
The heading must be set in advance and the DG will keep track of whatever was set.
The DG does not have the acceleration and turning issues that a compass has, so it is much easier to turn to a given heading without having to think about the lead and lag of the compass.
The presentation is just plain nicer and easier to read, but it also allows for more advanced add-ons like a heading bug all the way up to a full HSI.
For these reasons, you will be very hard-pressed to find a plane without a DG, even though a magnetic compass is sufficient to determine your heading.
When do I need to set my DG?
The DG needs to be set before takeoff and sometimes in flight.
Look at the knob in the image above. If you push that knob in and turn it, the whole card will rotate left or right as you turn. The tiny airplane image stays fixed in place as the headings turn under it. This knob is how the DG is set. Usually, this is done before taxi and then checked again after the runup.
In order to set it, we need to know the plane’s current heading. Luckily, on the ground a magnetic compass works pretty well so we simply read the magnetic compass and then turn the knob until that heading is set at the top of the instrument.
The instrument can be wrong for a number of reasons. Simply put, once the gyro spools down (when the plane is off), there is nothing to hold the compass card rigid in space. It tends to naturally be off by a few degrees every time you start up.
Imagine if you shut down with the DG pointing to 050 (like in the image above). Then you hook up the tow bar and move the plane into a tie-down spot facing the opposite direction. You have just turned the plane all the way around. What will happen to the DG? You will get in the plane and start-up facing 230, but the DG will still show 050.
Checking that the DG matches the compass is critical to removing these errors and being able to rely on the DG.
But there’s more. You will also need to set the DG in flight. Over time, due to internal friction, the gyro will precess out of place a little and the heading will be off by a few degrees. Personally, I set this using a mixture of procedures and situational awareness.
If I am on a cross-country, travelling somewhere, it is good to do this periodically as part of a cruise checklist (which you should be doing every so often during cruise).
As for situational awareness, there are times when the heading doesn’t seem to make perfect sense, or some reference point on the ground is 10+ degrees from where you thought it would be. This is a great time to check the DG and see that it matches the compass.
To set the DG in flight make sure that the wings are level and the plane is not accelerating (or slowing down). Then simply set it like you would on the ground. That’s all there is to it.