Pressure navigation is limited by a few meteorological considerations. The basic accuracy of the LOP in average conditions is about 5 to 10 miles. It will rapidly become worse under the following conditions: tightly circulating pressure systems of highs and lows, flying through a front, or carelessness in reading or computing the information. Bellamy drift has another limitation. To determine drift you must stay on one heading long enough to take two readings about 20 minutes apart.
ZN is a displacement in NM perpendicular to the EAP. Compute ZN on the MB-4 using the equation:
Determine ETAS by using the EAD and time. Measure EAD along a straight line between the two points in question. In the Northern and Southern Hemispheres, the sign of the ZN is the sign of the drift correction. Use airplot in conjunction with a fix position to plot the PLOP, and plot it parallel to the EAP. If the absolute altimeter fails, use pressure by temperature as a backup. With this method, use temperature and pressure altitude to find equivalent D readings. If you change altitudes, restart pressure at the new altitude, or correct the last D reading prior to the altitude change with a pastagram. Another expression of the PLOP is Bellamy drift, used as a backup source of drift angle. Figure 15-15 shows a fix determined by a PLOP and a celestial LOP.
Figure 15-15. Fix using PLOP and celestial line of position. [click image to enlarge]