# Pressure Differential Techniques (Part One)

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 inflight 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 a line of position (LOP) as valid as any other type.

## Constant Pressure Surface

To understand pressure differential navigation, you should know 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 and are analogous to contour lines on land maps. [Figure 15-2] The intersection of altitude mean sea level (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.

Figure 15-1. Constant pressure surface.

Figure 15-2. Contours. [click image to enlarge]

## 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 (20° N to 20° S), Coriolis force approaches zero, and pressure navigation is unreliable, pressure differential navigation is reliable in midlatitudes.

Figure 15-3. Changing contours of constant pressure surface. [click image to enlarge]

## 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.