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