Direction indicator

The direction indicator (DI) or gyrocompass is an instrument used in an aircraft to inform the pilot of his heading. The primary means of establishing heading in most small aircraft is the compass, but that suffers from being constantly affected by the aircraft's motion, making it hard to use. To remedy this, the compass reading is periodically transferred to the DI, which gives a very stable, steady reading.

The DI works using a gyroscope to establish an inertial platform, which will remain fixed in space. The DI is arranged so that only the horizontal axis is used to drive the display, which consists of a circular compass card calibrated in degrees. The gyroscope is spun either electrically, or using air from a vacuum pump driven from the aircraft's engine. Because the earth rotates (15° per hour), and because of small accumulated errors caused by friction, the DI will drift over time, and must be reset from the compass periodically. Failure to do this is a common source of navigation errors among beginner pilots.