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Computed: Applied:
From (subpolar) GC 316o Old (subpolar) GH 320o
GC difference –8o GC difference –8 o
To (polar) GC 308o New (polar) GH 312o
CAUTION: Do not alter the aircraft heading; instead, simply reposition the DG pointer to the new GH.
14.11. Crossing 180th Meridian on Subpolar Chart. When a flight crosses the 180th meridian on a
subpolar grid chart, the GH changes because of the convergence of grid meridians along this true
meridian. This is very similar to the chart transition procedure described above. When using a subpolar
chart that crosses the 180th meridian on an easterly heading (A to B in Figure 14.7), the apical angle
must be subtracted from the GH. Conversely, the apical angle must be added to the GH when on a
westerly heading (B to A in Figure 14.7). The apical angle can be measured on the chart at the 180th
meridian between the converging GN references. The angle can also usually be found on the chart
border, or computed by use of the following formula:
Apical angle = 360o – (360o x convergence factor)
EXAMPLE:
Given: Chart convergence factor 0.785
Find: Apical angle
Apical angle = 360o – (360o x 0.785)
Apical angle = 360o – 283o
Apical angle = 77o
CAUTION: Do not alter the aircraft heading when crossing the l80th meridian; instead, simply reset the
DG pointer to the new GH.
302 AFPAM11-216 1 MARCH 2001
Figure 14.7. Crossing 180th Meridian on Subpolar Chart.
14.12. Grivation. The difference between the directions of the magnetic lines of force and GN is called
grivation (GV). GV is similar to variation and used to convert MH to GH and vice versa. Figure 14.8
shows the relationship between GN, TN, and MN. Lines of equal GV (isogrivs) are plotted on grid
charts.
Figure 14.8. Grivation.
AFPAM11-216 1 MARCH 2001 303
14.12.1. The formulas for computing GV in the Northern Hemisphere are:
GV = (–W conv angle) + W variation
GV = (–W conv angle) – E variation
GV = (+E conv angle) + W variation
GV = (+E conv angle) – E variation
14.12.2. If GV is positive, it is W grivation; if grivation is negative, it is E grivation. For example, if
variation is 17o E and convergence angle is 76o W, using the formula:
GV = (–West conv angle) + (–East variation)
GV = (–76) + (–l7) = –93
GV = 93o East
14.12.3. To compute MH from GH, use the formula:
MH = GH +W grivation
MH = GH – E grivation
14.12.4. In the Southern Hemisphere, reverse the signs of west and east convergence angles in the
formula above.
Section 14D— Gyro Precession
14.13. Basics. 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.
14.14. Real Precession. Real precession, illustrated in Figure 14.9, is the actual movement of a gyro
spin axis from its initial alignment in space. 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 1o 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.
304 AFPAM11-216 1 MARCH 2001
Figure 14.9. Real Precession.
14.15. Apparent Precession. 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.
14.15.1. 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
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