Pressure

PRESSURE

When an aircraft flies from one place to another at a constant indicated altitude, it is flying

along a surface of constant pressure. As the surface pressure varies, so do the heights of

the pressure levels aloft. Figure 4.4-1 shows the path of an aircraft as it follows a constant

pressure surface. As the surface pressure is reduced (all other conditions remaining the

same), the whole column of air aloft is lowered, causing an aircraft flying at a particular

pressure level to descend to a lower altitude.

Figure 4.4-1 Path of Aircraft Following

a Constant Pressure Surface

The current altimeter setting must be obtained by radio throughout the flight and it is

imperative to receive an altimeter setting from the tower at your destination prior to landing.

Without a current altimeter setting, a pilot flying toward an area where the pressure is

decreasing would be at an MSL altitude lower than indicated. A change in pressure of 0.10

in-Hg will change the altimeter reading 100 feet. A basic rule for altimeter errors is, when

flying from point to point and your flight takes you from

High pressure to Low pressure, your altimeter indicates

High but the aircraft is actually Lower

ALT

A/C

4.6-91