To eliminate the difficulties imposed by magnetic compass unreliability in polar areas, you disregard the magnetic compass in favor of a free-running gyro. Gyro steering is used because it is stable and independent of magnetic influence. When used as a steering instrument, the gyro is restricted so its spin axis always remains horizontal to the surface of the earth and is free to turn only in this horizontal plane. Any movement of a gyro spin axis from its initial horizontal alignment is called precession. The two types of precession are real and apparent, with apparent broken into earth rate, transport, and grid transport precession. Total precession is the cumulative effect of real and apparent precession.
Real precession is the actual movement of a gyro spin axis from its initial alignment in space. [Figure 14-9] It is caused by such imperfections as power fluctuation, imbalance of the gyro, friction in gyro gimbal bearings, and acceleration forces. As a result of the improved quality of equipment now being used, real precession is considered to be negligible. Some compass systems have a real precession rate of less than 1° per hour. Electrical or mechanical forces are intentionally applied by erection or compensation devices to align the gyro spin axis in relation to the earth’s surface. In this manner, the effects of apparent precession are eliminated and the gyro can then be used as a reliable reference.
The spin axis of a gyro remains aligned with a fixed point in space, while your plane of reference changes, making it appear that the spin axis has moved. Apparent precession is this apparent movement of the gyro spin axis from its initial alignment.
Earth Rate Precession
Earth rate precession is caused by the rotation of the earth while the spin axis of the gyro remains aligned with a fixed point in space. Earth rate precession is divided into two components. The tendency of the spin axis to tilt up or down from the horizontal plane of the observer is called the vertical component. The tendency of the spin axis to drift around laterally; that is, to change in azimuth, is called the horizontal component. Generally, when earth rate is mentioned, it is the horizontal component that is referred to, since the vertical component is of little concern.
A gyro located at the North Pole, with its spin axis initially aligned with a meridian, appears to turn 15.04° per hour in the horizontal plane because the earth turns 15.04° per hour. [Figure 14-10] As shown in Figure 14-10A, the apparent relationship between the Greenwich meridian and the gyro spin axis changes by 90° in 6 hours, though the spin axis is still oriented to the same point in space. Thus, apparent precession at the pole equals the rate of earth rotation. At the equator, as shown in Figure 14-10B, no earth rate precession occurs in the horizontal plane if the gyro spin axis is still aligned with a meridian and is parallel to the earth’s spin axis.
When the gyro spin axis is turned perpendicular to the meridian, maximum earth rate precession occurs in the vertical component. [Figure 14-11] But the directional gyro does not precess vertically because of the internal restriction of the gyro movement in any but the horizontal plane. For practical purposes, earth rate precession is only that precession that occurs in the horizontal plane. Figure 14-11 illustrates earth rate precession at the equator for 6 hours of time.
Earth rate precession varies between 15.04°/hour at the poles and 0°/hour at the equator. It is computed for any latitude by multiplying 15.04° times the sine of the latitude. For example, at 30° N, the sine of latitude is 0.5. The horizontal component of earth rate is, therefore, 15°/hour right × 0.5 or 7.5°/hour right at 30° N. [Figure 14-12]
Steering by Gyro
Obviously, if the gyro is precessing relative to the steering datum of GN or TN, an aircraft steered by the gyro is led off heading at the same rate. To compensate for this precession, an artificial real precession is induced in the gyro to counteract the earth rate. At 30° N latitude, earth rate precession is equal to 15° × sin lat = 15 × .5 or 7.5° per hour to the right.
Offsetting Each Rate Effect
Hence, if at 30° N latitude, a real precession of 7.5° left per hour is induced in the gyro, it balances exactly and offsets earth rate effect. In ordinary gyros, a weight is used to produce this effect but, since the rate is fixed for a given latitude, the correction is good for only one latitude. The latitude chosen is normally the mean latitude of the area in which the aircraft operates. The N-1 and AHRS compass systems have a latitude setting knob that you can use to adjust for the earth rate corrections.
Earth Transport Precession (Horizontal Plane)
Earth transport precession is a form of apparent precession that results from transporting a gyro from one point on the earth’s surface to another. The gyro spin axis appears to move because the aircraft, flying over the curved surface of the earth, changes its attitude in relation to the gyro’s fixed point in space. [Figure 14-13] Earth transport precession causes the gyro spin axis to move approximately 1° in the horizontal plane for each true meridian crossed. This effect is avoided by using GN as the steering reference.
Grid Transport Precession
Grid transport precession exists because meridian convergence is not precisely portrayed on charts. The navigator wants to maintain a straight-line track, but the gyro follows a greatcircle track, which is a curved line on a chart. The rate at which the great-circle track curves away from a straight-line track is grid transport precession. This is proportional to the difference between convergence of the meridians as they appear on the earth and as they appear on the chart and the rate at which the aircraft crosses these meridians.
Summary of Precession
Real precession is caused by friction in the gyro gimbal bearings and dynamic unbalance. It is an unpredictable quantity and can be measured only by means of heading checks.
Earth rate precession is caused by the rotation of the earth. It can be computed in degrees per hour with the formula: 15.04 × sin lat. It is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. All gyros are corrected to some degree for this precession, many by means of a latitude setting knob.
Earth transport precession (horizontal plane) is an effect caused by using TN as a steering reference. It can be computed by using the formula (change longitude/hour × sine mid latitude). The direction of the precession is a function of the TC of the aircraft. If the course is 0° – 180°, precession is to the right; if the course is 180° – 360°, precession is to the left. This precession effect is avoided by using GN as a steering reference.
Grid transport precession is caused by the fact that the great circles are not portrayed as straight lines on plotting charts.
The navigator tries to fly the straight pencil-line course, the gyro a great circle course. The formula for grid transport precession is change longitude/hour (sin lat – CF), where CF is the chart convergence factor. The direction of this precession is a function of the chart used the latitude and the TC. Direct substitution into the formula produces an answer valid for easterly courses, such as 0° – 180°. For westerly courses, the sign of the answer must be reversed.
Gyro steering is much the same as magnetic steering, except that GH is used in place of true heading (TH). GH has the same relation to GC as TH has to true course (TC). The primary steering gyro in most aircraft provides directional data to the autopilot and maintains the aircraft on a preset heading. When the aircraft alters heading, it turns about the primary gyro while the gyro spin axis remains fixed in azimuth. If the primary gyro precesses, it causes the aircraft to change its heading by an amount equal to the precession.