A directional gyro, with the word gyro standing for gyroscope, is an instrument used to indicate the direction in which an aircraft is traveling. Using magnetic compasses alone can sometimes give rise to errors during certain aircraft flight modes. Directional gyros are typically used in combination with a magnetic compass and are located in a pilot’s cockpit. A variety of directional gyros with varying levels of complexity can be found.
When an aircraft banks, or rolls on its side, a magnetic compass needle generally will begin to point in an incorrect direction. This is called dip error, and it increases the more a pilot banks an aircraft. Similarly, compasses will read incorrectly during periods of acceleration and deceleration. Except in moments of steady, level flight, it is necessary to factor in an aircraft’s orientation for accurate heading measurements.
To determine an aircraft’s orientation, a gyroscope is used. Gyroscopes rely on the principle of conservation of angular momentum. A disk is mounted in a pivoting frame and is spun to a high rotation rate. The result is a spinning disk that will maintain the same orientation regardless of changes of the frame’s orientation. In an aircraft, for example, the spinning disk will point in the same direction even as the aircraft pitches, yaws and rolls.
The fact that Earth rotates means a directional gyro needs to be reset from a compass reference occasionally. This need for periodic correction is experienced the most near the poles and the least near Earth’s equator. In a simple aircraft, the pilot may manually recalibrate the directional gyro several times an hour. More sophisticated aircraft use what are called slaved gyros, which are continuously updated electronically.
The output display of a directional gyro is similar to that of a compass. There is often an outline of an airplane with its nose pointed in the direction of travel. Numbers typically begin at the north position and increase clockwise in a full circle. The numbers may stand for tens of degrees ranging from 0 to 36 instead of 0 to 360 degrees.
In a spacecraft, magnetic compasses are typically not helpful. A form of directional gyro may be used to determine orientation, however. Gyroscopes operate perfectly well in space because the law of conservation of angular momentum applies universally. So long as the spinning gyroscope disk does not slow down excessively because of friction, the gyro’s spin axis will maintain the same orientation.