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something about the constant pressure surface. The constant pressure surface is one on which the
pressure is the same everywhere, even though its height above sea level will vary from point to point as
shown in Figure 15.1. The pressure altimeter will show a constant reading. A constant pressure surface
is shown on a constant pressure chart (CPC) as lines that connect points of equal height above sea level.
These lines are referred to as contours (Figure 15.2) and are analogous to contour lines on land maps.
The intersection of altitude MSL and constant pressure surfaces form isobars. A comparison of isobars
and contours is shown in Figure 15.2. The geostrophic wind will blow along and parallel to the contours
of a CPC just as it blows along and parallel to the isobars of a constant level chart.
15.3. Geostrophic Wind. The shape and configuration of the constant pressure surface determine the
velocity and direction of the geostrophic wind. Flying with 29.92 set in the pressure altimeter will cause
the aircraft to follow a constant pressure surface and change its true height as the contours change
(Figure 15.3). The slope of the pressure surface, also known as the pressure gradient, is the difference in
pressure per unit of distance as shown in Figure 15.4. The pressure gradient force (PGF) or slope of the
pressure surface and Coriolis combine to produce the geostrophic wind. The speed of the geostrophic
wind is proportional to the spacing of the contours or isobars. Closely spaced contours form a steep
slope and produce a stronger wind, while widely spaced contours produce relatively weak winds.
According to Buys-Ballots Law, if you stand in the Northern Hemisphere with your back to the wind,
the lower pressure is to your left (Figure 15.5). The opposite is true in the Southern Hemisphere where
Coriolis deflection is to the left. Further study of Figure 15.5 shows that as you enter a low or a high
system, your drift will be right or left, respectively. The opposite is true as you exit the system. Since the
geostrophic wind is based on a constant pressure surface, you must fly a constant pressure altitude. A
minimum of 2,000 to 3,000 feet above the surface will usually eliminate distortion introduced through
surface friction. Near the equator (20o N to 20o S), Coriolis force approaches zero, and pressure
navigation is unreliable, pressure differential navigation is reliable in midlatitudes.
314 AFPAM11-216 1 MARCH 2001
Figure 15.1. Constant Pressure Surface.
Figure 15.2. Contours.
AFPAM11-216 1 MARCH 2001 315
Figure 15.3. Changing Contours of Constant Pressure Surface.
Figure 15.4. Pressure Gradient.
316 AFPAM11-216 1 MARCH 2001
Figure 15.5. Buys-Ballots Law.
15.4. Pressure Computations and Plotting. In determining a PLOP or Bellamy drift by pressure
differential techniques, use the crosswind component of the geostrophic wind over a given period of
time. To determine your pressure pattern displacement (ZN), use the following equation:
This formula gives the direction and crosswind displacement effect of the pressure system you've flown
through. To solve for ZN, you must understand how to obtain and apply such special factors as D
readings, effective true airspeed (ETAS), effective airpath (EAP), effective air distance (EAD), and K
values.
15.5. D Readings. The symbol D stands for the difference between the true altitude (TA) of the aircraft
and the pressure altitude (PA) of the aircraft. There are two methods for obtaining D values. The first
uses an absolute altimeter to measure TA on overwater flights and the pressure altimeter to measure PA.
The second method uses outside air temperature (OAT) readings to determine equivalent D values if the
absolute altimeter fails. For both methods, the D value is expressed in feet as a plus or minus value. To
determine the correct D reading using the altimeter method, assign a plus (+) to TA, a minus (-) to PA,
and algebraically add the two. Remember the city in Florida (TAMPA) to keep the signs right. Take the
first D reading in conjunction with the initial fix for the pressure navigation leg. This is D1. Take the
second reading (D2) at the next fix. Always take the readings at the same time relative to the fix (usually
about 4 minutes before fix time). The value, D2 – D1, is an expression of the slope or pressure gradient
experienced by the aircraft. Subtracting D1 from D2 determines the change in aircraft TA between
readings. When this altitude change is compared with the distance flown, the resulting value becomes an
expression of the slope. The value of D2 – D1 indicates whether the aircraft has been flying upslope (+)
or downslope (-).
AFPAM11-216 1 MARCH 2001 317
15.5.1. Take readings carefully, because an erroneous reading of either altimeter will produce an
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