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incorrect D reading and a bad LOP. Gently tap the pressure altimeter before reading it to reduce
hysteresis error.
15.5.2. Maintain a constant PA to ensure consistent D readings. If you change altitudes, start with a new
D at the new altitude, or correct the previous reading by use of a pastagram. The pastagram will allow
you to continue accurately even though you've changed altitude. The pastagram uses average altitude
and average temperature change to determine a correction to the D reading taken before the altitude
change. Figure 15.6 shows a pastagram with instructions for its use and a sample problem.
15.6. Effective True Airspeed (ETAS). To determine a PLOP, you must compute the ETAS from the
last D reading. The ETAS is the TAS that the aircraft flew from the last fix to the next fix air position
(Figure 15.7). If the aircraft has maintained a constant true heading (TH) between D readings, the ETAS
equals the average TAS. But, if the aircraft has altered heading substantially between the D readings, the
effective TAS is derived by drawing a straight line from the fix at the first D reading to the final air
position. This line is called the effective airpath (EAP). ETAS is computed by measuring the effective
air distance (EAD) and dividing it by the elapsed time. In Figure 15.7, an aircraft flew at 400 knots TAS
from the 0820 fix to the 1020 air position via a dogleg route. The EAD is 516 NM; consequently, the
ETAS is 258 knots.
15.7. K Factor. The constant K takes into account Coriolis and the gravity constant for particular
latitudes.
15.7.1. Midlatitude is the average latitude between D1 and D2. It is in tabular form in Figure 15.8. In the
table, this constant is plotted against latitude since Coriolis force varies with latitude. In using the ZN
formula, enter the table with midlatitude and extract the corresponding K factor.
318 AFPAM11-216 1 MARCH 2001
Figure 15.6. Pastagram.
AFPAM11-216 1 MARCH 2001 319
Figure 15.7. Effective True Airspeed.
15.7.2. On MB-4 computers, a subscale of latitude appears opposite the values for K factors on the
minutes scale. K is computed so that with slope expressed in feet and distance in NM, the geostrophic
windspeed is in knots. For training purposes only, the K factors for 20o N or S to 14o N or S are listed in
Figure 15.9.
15.8. Crosswind Displacement. ZN is the displacement from the straight-line airpath between the
readings. Therefore, a PLOP must be drawn parallel to the effective airpath. With all the necessary
values available, the ZN formula can be rearranged for convenient solution on the DR computer as
follows:
Printed instructions on the face of MB-4 computers specify that to compute crosswind component, set
EAD on the minutes scale opposite D2 - D1 on the miles scale. The crosswind component (V) is not to be
confused with ZN. The crosswind component (V) is crosswind velocity in knots. This component (V)
must then be multiplied by the elapsed time between D2 and D1 in order to compute the ZN. Substitute
ETAS for EAD on the MB-4 computer, and read the ZN over the K factor (or latitude on the subscale).
320 AFPAM11-216 1 MARCH 2001
Figure 15.8. Pressure Pattern Worksheet/K Factors Table.
Figure 15.9. K Factors Table Below 20o.
AFPAM11-216 1 MARCH 2001 321
15.9. Pressure Line of Position (PLOP). After you determine ZN, you need to figure out whether to
plot it left or right of the EAP. Recall that wind circulation is clockwise around a high and
counterclockwise around a low in the Northern Hemisphere; the opposite is true in the Southern
Hemisphere. In the Northern Hemisphere, when the value of D increases (a positive D2 - D1), the aircraft
is flying into an area of higher pressure and the drift is left. (Refer to A of Figure 15.10.) When the value
of D decreases (a negative D2 - D1), the aircraft is flying into an area of lower pressure and the drift is
right (B of Figure 15.10). Use the memory device PLOP to remember Plot Left On Positive (in the
Northern hemisphere) Always plot the PLOP parallel to the EAP, as shown in Figure 15.11. Cross the
PLOP with another LOP to form a fix, or use it with a DR position to construct an MPP.
Figure 15.10. Pressure Pattern Displacement.
15.10. Bellamy Drift. Bellamy drift is a mean drift angle calculated for a past period of time. It is
named for Dr John Bellamy who first demonstrated that drift could be obtained from the use of pressure
differential information. Bellamy drift is used in the same way as any other drift reading.
15.10.1. An advantage of Bellamy drift is its independence from external sources. It can serve as a
backup if the primary drift source fails, but will not give groundspeed. Bellamy drift is less accurate than
Doppler or INS derived sources, but is better than using forecast drift or having none at all.
　
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