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associated with precession. Since the full effect of precession does not occur at one time, we have to
account for the gradual increase of precession.
14.22.1. Step 1 — Determine the Hourly Rate. In Figure 14.16, grid entry occurred at 1700. At 1720,
the navigator obtained a heading shot or MPP. The heading shot determined precession correction to be
-2 and the compass was reset to the GH. On the MB-4 computer place the -2 correction on the outside
scale and the time since grid entry (20 minutes) on the inside scale. The hourly rate now appears above
the index (6.0R). To minimize error the hourly rate has to be computed to the nearest tenth of a degree.
AFPAM11-216 1 MARCH 2001 311
Figure 14.16. Typical Mission In-Flight Log.
14.22.2. Step 2 — Compute "All Behind/Half Ahead." Since precession begins at the last time the
gyro was reset, for this example we need to start at grid entry 1700. At 1700 "all behind" would be
determined to be zero minutes and half-ahead to the next DR (1706) would be 3 minutes. To determine
the amount of precession correction to be used, leave the hourly rate (6.0) over the index and look above
3 minutes. The computed precession correction for the 1706 DR is -0.3o or 0 for use on the log. Next we
need to determine the precession correction for the 1720 DR. At 1706, "all behind" is 6 minutes and
"half ahead" is 7 minutes. The total time used to compute the precession correction for this DR is 13
minutes. Again, using the hourly rate precession correction for the 1720 DR is -1.3o or -1o for the log.
14.23. KC-135 Method. Since the "all behind/half ahead" method tends to keep you behind, the KC-
135 method is used by some navigators to predict precession. This method basically uses half of the
computed precession correction for future DRs/MPPs when the precession correction is determined
between two positions. Though not as accurate as the "all behind/half ahead" method, the KC-135
method can be effective if used with short DRs. Using the KC-135 method, compensate for precession
around the turn by getting a heading shot immediately before and after the turn, resetting the gyro after
the heading shot restarts precession.
14.24. False Latitude. A second method of compensating for precession while in-flight involves the use
of false latitude inputs into the gyro compass. Most gyro compasses have a latitude control, which
allows the navigator to compensate for earth rate precession (ERP). Normally, the latitude control is set
to the actual latitude of the aircraft. However, other values may be set. For example, if the aircraft is at
30o N and the latitude control knob is set to 70o N, the gyro will overcorrect for ERP. Since ERP is right
in the Northern Hemisphere, the correction will be to the left. Thus, setting a higher than actual latitude
will correct for right precession over and above that for ERP. Since ERP 15o/hr X sine latitude, a table
such as Figure 14.17 can be developed to use this procedure.
312 AFPAM11-216 1 MARCH 2001
Figure 14.17. False Latitude Correction Table.
14.25. Summary. The USAF Grid Overlay and the free-running gyro are used to overcome the
difficulties of converging meridians and the unreliability of the magnetic compass when navigating in
high latitudes. When using gyro steering, maintain a record of the precession of both the primary and
secondary gyros. The gyro log provides you with the information necessary to predict values when it is
impossible to obtain heading checks because of overcast conditions or twilight. By maintaining a log on
the secondary gyro, the navigator can change gyros in case of malfunction of the primary gyro. Use the
information recorded in the gyro log in conjunction with the navigator's log to plot position and compute
winds, headings, alter headings, and ETAs. And never forget grid is green.
AFPAM11-216 1 MARCH 2001 313
Chapter 15
PRESSURE PATTERN NAVIGATION
Section 15A— Pressure Differential Techniques
15.1. Basics. Pressure differential flying is based on a mathematically derived formula. The formula
predicts windflow based on the fact that air moves from a high pressure system to a low pressure
system. This predicted windflow, the geostrophic wind, is the basis for pressure navigation. The formula
for the geostrophic wind (modified for a constant pressure surface) combined with in-flight information
makes available two aids to navigation: Bellamy drift and the pressure line of position (PLOP). Bellamy
drift gives information about aircraft track by supplying net drift over a set period of time. Using the
same basic information, the PLOP provides an LOP as valid as any other type.
15.2. Constant Pressure Surface. To understand pressure differential navigation, you should know
　
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