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the acceleration error will become. Refer to Figure 13.13:
AFPAM11-216 1 MARCH 2001 291
Figure 13.12. Acceleration/Deceleration Errors.
Figure 13.13. Groundspeed Acceleration Error.
292 AFPAM11-216 1 MARCH 2001
13.13.1.1. Enter with Zn-Track.
13.13.1.2. Extract acceleration error and apply sign.
13.13.2. Example: Track = 080o, Zn = 060o, Beginning GS – 500 knots, ending GS – 515 knots.
060o – 080o = 340o = -1.40
515 – 500 = 15 knots
-1.40 x 15 = -21 correction to the Ho.
13.14. Wander Error. A change in track can be produced by changes in the wind, heading changes
caused by the autopilot or changing magnetic variation, or by heading changes caused by pilot manual
steering errors. As with the Coriolis force and rhumb line errors, correction tables have been developed
for wander error. Values extracted from the wander correction table, shown in Figure 13.14, are to be
applied to the Ho. Use the following information as entering arguments for the determination of the
correction taken from the table: (1) the heading at the beginning of the observation was 079o, (2) the
heading at the end of the observation was 081o, (3) the observation was taken over a 2-minute period, (4)
the GS was 450 knots, and (5) the Zn of the body was 130o.
Figure 13.14. Wander Correction Tables.
AFPAM11-216 1 MARCH 2001 293
13.14.1. Following the instructions shown at the bottom of the table, enter the numerical portion of the
table with the values of GS and the change of track per 2 minutes. In this case, the GS is 450 knots and
the change in track per 2 minutes is 2o. Since the heading at the end of the observation is greater than the
heading at the beginning, the change is 2o to the right. Notice that you must know whether the change is
to the right or to the left to determine the sign of the correction. The factor obtained from the table is 12
x 2 = 24.
13.14.2. Next, enter the graph portion of the table with the value of the factor (24) and the value of the
azimuth of the body minus the value of track. The graph is so constructed that it must be entered with Zn
- Tr. Zn - Tr = 130o – 080o; so use 050o. Following the rules in steps two and three in the table; the
correction is 19'. Since the change in track is to the right, the correction is subtracted from the Ho. This
is determined by referring to the signs shown at the ends of the arc in the table. Figure 13.15 shows the
effect of this correction.
13.14.3. If the track and groundspeed are the same at the beginning and the end of a shooting period,
there will be no wander error.
Figure 13.15. Wander Correction Applied to Ho.
13.15. Instrument Error. Index error is usually the largest mechanical error in the sextant. This error is
caused by improper alignment of the index prism with the altitude counter. No matter how carefully a
sextant is handled, it is likely to have some index error. If the error is small, the sextant need not be
readjusted; rather, each Hs can be corrected by the amount of the error. This means that the index error
of the sextant must be known to obtain an accurate celestial LOP. Another mechanical error found in
sextants is backlash. This is caused by excessive play in the gear train connecting the index prism to the
altitude counter.
294 AFPAM11-216 1 MARCH 2001
13.15.1. Usually, index and backlash errors are nearly constant through the altitude range of the sextant.
Therefore, if the error at one altitude setting is determined, the correction can be applied to any Hs or
Hc. The correction is of equal value to the error but the opposite sign.
13.15.2. The sextant should be checked on the ground before every celestial flight. Preflighting the
sextant can determine the sextant error of an individual instrument. The sextant error can also be
determined in-flight and a correction can be applied to the precomp to compensate for the error. To
determine the error and correction in-flight, one must have a celestial LOP, a Zn and the actual (or bestknown)
position of the aircraft at the same time. Refer to Figure 13.16.
Figure 13.16. Determining Sextant Error Correction.
13.15.3. The fix symbol represents the best-known position at the time of the celestial LOP. To
determine the actual value of the correction, measure the shortest distance between the position and the
LOP. This tells you how many minutes of arc (NM) the Ho must be adjusted on subsequent shots to get
an accurate LOP (in this case, the value is 10'). To determine whether this value must be added or
subtracted, note whether the LOP needs to be adjusted toward the Zn or away from the Zn. Remember
the rule HOMOTO? It applies here, too. If the LOP needs to be moved toward the Zn in order to be
　
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