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precession. Apparent precession will cause an apparent change of heading equal to 15o per hour times
the sine of the latitude. Real precession, caused by defects in the gyro, may occur at any rate, but is
typically very small in current gyros. Apparent precession is a known value depending upon location and
can be compensated for. In some of the more complex gyro systems, apparent precession is compensated
for by setting in a constant correction equal to and in the opposite direction to the precession caused by
the earth's rotation.
Section 3D— Altitude and Altimeters
3.7. Basics. Altitude may be defined as a vertical distance above some point or plane used as a reference.
Knowledge of the aircraft altitude is imperative for terrain clearance, aircraft separation, and a multitude
of operational reasons. There are as many kinds of altitude as there are reference planes from which to
measure them. Only six concern the navigator: indicated altitude, calibrated altitude, pressure altitude
(PA), density altitude (DA), true altitude (TA), and absolute altitude. There are two main types of
altimeters; the pressure altimeter which is installed in every aircraft, and the absolute or radar altimeter.
To understand the pressure altimeter's principle of operation, a knowledge of the standard datum plane is
essential.
3.8. Standard Datum Plane. The standard datum plane is a theoretical plane where the atmospheric
pressure is 29.92 inches of mercury (Hg) and the temperature is 15 oC. The standard datum plane is the
zero elevation level of an imaginary atmosphere known as the standard atmosphere. In the standard
atmosphere, pressure is 29.92" Hg at 0 feet and decreases upward at the standard pressure lapse rate. The
temperature is 15 oC at 0 feet and decreases at the standard temperature lapse rate. Both the pressure and
temperature lapse rates are given in the table in Figure 3.12. The standard atmosphere is theoretical. It
was derived by averaging the readings taken over a period of many years. The list of altitudes and their
corresponding values of temperature and pressure given in the table were determined by these averages.
The height of the aircraft above the standard datum plane (29.92" Hg and 15 oC) is called pressure
altitude (PA) as illustrated in Figure 3.13.
96 AFPAM11-216 1 MARCH 2001
Figure 3.12. Standard Lapse Rate Table.
AFPAM11-216 1 MARCH 2001 97
Figure 3.13. Depiction of Altimetry Terms.
3.9. Pressure Altimeter Principles of Operation. The pressure altimeter is an aneroid barometer
calibrated to indicate feet of altitude instead of pressure. As shown in Figure 3.14, the pointers are
connected by a mechanical linkage to a set of aneroid cells. These aneroid cells expand or contract with
changes in barometric pressure. In this manner, the cells assume a particular thickness at a given
pressure level and, thereby, position the altitude pointers accordingly. On the face of the altimeter is a
barometric scale which indicates the barometric pressure (expressed in inches of mercury) of the point or
plane from which the instrument is measuring altitude. Turning the barometric pressure set knob on the
altimeter manually changes this altimeter setting on the barometric scale and results in simultaneous
movement of the altitude pointers to the corresponding altitude reading. Like all measurements, an
altitude reading is meaningless if the point from which it starts is unknown. The face of the pressure
altimeter supplies both values. The position of the pointers indicates the altitude in feet, and the
barometric pressure appearing on the barometric scale is that of the reference plane above which the
measurement is made.
98 AFPAM11-216 1 MARCH 2001
Figure 3.14. Altimeter Mechanical Linkage.
3.10. Altimeter Displays:
3.10.1. Counter-Pointer Altimeter. The counter-pointer altimeter has a two-counter digital display unit
located in the 9 o'clock position of the dial. The counter indicates altitude in 1,000-foot increments from
zero to 80,000 feet (Figure 3.15). A single conventional pointer indicates l00s of feet on the fixed
circular scale. It makes one complete revolution per 1,000 feet of altitude change and, as it passes
through the 900 to 1,000-foot area of the dial, the 1,000-foot counter is actuated. The shaft of the 1,000-
foot counter in turn actuates the 10,000-foot counter at each 10,000 feet of altitude change. To determine
the indicated altitude, first read the 1,000-foot counter and then add the 100-foot pointer indication.
Figure 3.15. Counter-Pointer.
AFPAM11-216 1 MARCH 2001 99
CAUTION: It is possible to misinterpret the counter-pointer altimeter by 1,000 feet immediately before
or after the 1,000-foot counter moves. This error is possible because the 1,000-foot counter changes
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