The direction indicator (also called an DI) or heading indicator (also called an HI) is a flight instrument used in an aircraft to inform the pilot of the aircraft’s heading.
The primary means of establishing the heading in most small aircraft is the magnetic compass, which suffers from several types of errors. The dip error causes 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 direction indicator, as the DI is unaffected by dip and acceleration errors. The pilot will periodically reset the heading indicator to the heading shown on the magnetic compass.