The heading indicator, or direction indicator, uses a rotating gyro to display the current compass direction (otherwise known as the heading) that the aircraft is flying. Using a 360° compass card with north as zero or ‘N’, the heading indicator shows headings in 5° increments with every 30° enumerated. To reduce crowding, the last ‘zero’ of the heading is omitted; ‘3’ is 30°, ’12’ is 120°, etc.
An adjustment knob turns the internal compass card to align with the aircraft’s magnetic compass.
In practice
The primary means of establishing the heading in most small aircraft is the magnetic compass, which unfortunately suffers from errors. These errors cause the magnetic compass to read incorrectly whenever the aircraft is in a bank, or during acceleration or deceleration, making it difficult to use in any flight condition other than unaccelerated, perfectly straight and level. To remedy this, the pilot will typically manoeuvre the airplane with reference to the heading indicator, as the gyroscopic heading indicator is unaffected by the same errors.
However, you shouldn’t always trust your heading indicator to tell you which way you’re heading. The heading indicator isn’t a very smart instrument, as it only repeats the heading that has been set into it. For that reason, it has an adjustment knob and must be set to correspond to the compass (or the runway heading) before it can be used for navigation.
If you fail to set the heading indicator properly before takeoff, and don’t notice that it disagrees with the compass, you can be many miles off course in a very short time.
While flying, the heading indicator will slowly drift away from the heading you have set. You should check the heading indicator against the compass at least every 15 minutes. Reset the heading indicator to the compass only when the airplane is in straight-and-level, unaccelerated flight.