direction/heading indicator

The Directional Gyro is another vacuum driven gyroscope. It looks much like a compass. A major difference it has with the compass is that it doesn’t rely on the earth’s magnetic field to operate. When the gyroscope is spinning it has a principle of remaining rigid in space. That is the spinning wheel will resist any change in position. The DG takes advantage of that principle. When an airplane is turning the gyroscope will resist moving with the turn. The energy used to resist the turn instead moves the compass card which will indicate the heading of the airplane. DG’s are used because they are not effected by magnetic disturbances or have turning errors inherent to the compass. They are susceptible to gyroscopic precession which are errors due to the mechanical friction imposed on the spinning gyroscope.

Because the heading indicator has no direction-seeking qualities of its own, it must be set to agree with the magnetic compass. This should be done only on the ground or in straight-and-level, unaccelerated flight when magnetic compass indications are steady and reliable.

The pilot should set the heading indicator by turning the heading indicator reset knob at the bottom of the instrument to set the compass card to the correct magnetic heading.

The pilot of a light aircraft should check the heading indicator against the magnetic compass at least every 15 minutes to assure accuracy. Because the magnetic compass is subject to certain errors , the pilot should ensure that these errors are not transferred to the heading indicator.

The operation of the heading indicator depends upon the principle of rigidity in space of a universally mounted gyroscope. The rotor turns in a vertical plane. Fixed at right angles to the plane of the rotor (to the vertical gimbal) is a circular compass card. Since the rotor remains rigid in space, the points on the card hold the same position in space relative to the vertical plane. As the instrument case revolves about the vertical gimbal, the card provides clear and accurate heading references.

The source of power for the gyro is the engine-driven vacuum pump (or venturi) which sucks air from the rear of the instrument case. This causes air under atmospheric pressure to pass through the filtering system, thence through an air bearing into the hollow vertical gimbal ring. The air then passes through the air nozzle and jets, striking the rotor at a point just above the plane of the horizontal gimbal.

The speed of the rotor may vary from 10,000 to 18,000 rpm, depending on instrument design. For proper operation, the suction gauge reading can be as low as 3.5 and as high as 5.0 inches of mercury, with 4.0 the desired suction. Limits for adjustment of the vacuum are 3.75 and 4.25, or as specified in your aircraft operating hand-book.

erecting mechanism

During flight, precessing forces displace the rotor from the vertical plane. To compensate for precession and to provide better airflow distribution against the rotor buckets, the air is divided by two parallel jets at the tip of the nozzle.

Each jet strikes the buckets at points equidistant from the centre of the buckets when the rotor is perpendicular in its normal rotating plane. When the gyro precesses, both jets strike one side of the buckets and cause the plane of the rotor to again become parallel to the flow of air from the jets.


The design of the vacuum-driven directional gyro imposes limitations on rotation about the gimbals preventing operation of the instrument in abnormal flight attitudes. If the plane of rotation of the rotor were able to become parallel to the base of the case, it would lose its ability to hold the card in a stationary position, since its axis would be in line with the vertical gimbal and the card would tend to spin with the rotor. The stop, or limiting factor, in the instrument is the caging arm. In the uncaged position, the caging arm rests on the bottom of the vertical gimbal ring and in that position restricts the movement of the vertical gimbal ring about the rotor or the horizontal gimbal.

The caging arm is held against the bottom of the vertical gimbal ring by means of a small spring so that rough air cannot cause it to fly up and tumble the instrument. Beyond the normal operating limits - 55° of pitch and bank - when the horizontal gimbal touches the stop, the precessional force causes the card to spin rapidly. This may be corrected by caging, resetting, and uncaging the instrument.


The chief cause of precession, causing the card to creep or drift, is bearing friction. Normal movement of the gimbal rings produces friction, which is increased if the bearings are worn, dirty, or improperly lubricated. Other sources of precession error include unbalanced gyro components and the effect of the earth's rotation. The latter effect depends upon the position of the instrument in relation to the earth, and is not appreciable unless a flight involves considerable change in latitude.

An apparent error frequently results from misuse of the magnetic compass when the directional gyro is set. Unless magnetic deviations are applied, the indicator may appear to drift several degrees after a turn is completed. Another common error results from failure to maintain straight-and-level flight while reading the magnetic compass for the heading to set in the directional gyro. Errors in the magnetic compass induced by attitude changes are thus duplicated in the heading indicator.

The instrument should be checked at least every 15 minutes during flight and reset to the correct heading. An error of no more than 3° in 15 minutes is acceptable for normal operations.

caging mechanism

 The heading indicator can be adjusted by pushing in on the caging knob to mesh pinion and ring gears, thereby permitting rotation of the vertical gimbal and card. (Another type of caging mechanism utilizes friction between rubber and metal rings.) After setting, the gyro is uncaged by pulling out the caging knob to release the gimbals from the caging mechanism. Before setting the instrument during ground operations, allow 5 minutes after engine starting for the gyro to reach operating speed.