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altimeter for accuracy before every flight. To check and set the altimeter:
3.11.6.1. Set the current altimeter setting on the barometric scale.
3.11.6.2. Check altimeter at a known elevation and note the error in feet.
3.11.6.3. Set the reported altimeter setting on the barometric scale and compare the indicated altitude to
the elevation of a known checkpoint.
AFPAM11-216 1 MARCH 2001 101
3.12. Nonstandard Atmospheric Effects. The altimeter setting is a correction for nonstandard surface
pressure only. Atmospheric pressure is measured at each station and the value obtained is corrected to
sea level according to the surveyed field elevation. Thus, the altimeter setting is the computed sea level
pressure and should be considered valid only in close proximity to the station and the surface. It does not
reflect nonstandard temperature nor distortion of atmospheric pressure at higher altitudes.
3.13. Types of Altitude:
3.13.1. Indicated Altitude. This is the value of altitude that is displayed on the pressure altimeter.
3.13.2. Calibrated Altitude. Calibrated altitude is indicated altitude corrected for installation or position
error.
3.13.3. Pressure Altitude (PA). The height above the standard datum plane (29.92" Hg and 15 oC) is
PA (Figure 3.13.)
3.13.4. Density Altitude (DA). Density is mass per unit volume. The density of the air varies with
temperature and with height. Warm air expands and is less dense than cold air. Normally, the higher the
PA, the less dense the air becomes. The density of the air can be expressed in terms of the standard
atmosphere. DA is the PA corrected for temperature in the density altitude window of the dead
reckoning (DR) computer. This calculation converts the density of the air to the standard atmospheric
altitude having the same density. DA is used in performance data and true airspeed (TAS) calculations.
3.13.5. True Altitude (TA). TA is the actual vertical distance above mean sea level (MSL), measured in
feet. It can be determined by two methods:
3.13.5.1. Set the local altimeter setting on the barometric scale of the pressure altimeter to obtain the
indicated true altitude (ITA). The ITA can then be resolved to TA by use of the DR computer (Figure
3.17).
3.13.5.2. Measure altitude over water with an absolute altimeter.
3.13.6. Absolute Altitude. The height above the terrain is called absolute altitude. It is computed by
subtracting terrain elevation from TA, or it can be read directly from a radar altimeter.
3.14. Computer Altitude Solutions. The two altitudes most commonly accomplished on the computer
are TA and DA. Nearly all DR computers have a window by which DA can be determined; however, be
certain that the window is labeled density altitude.
3.14.1. True Altitude (TA) Determination. In the space marked FOR ALTITUDE COMPUTATIONS
are two scales: (1) a centigrade scale in the window and (2) a pressure altitude (PA) scale on the upper
disk. When a PA is placed opposite the temperature at that height, all values on the outer (miles) scale
are equal to the corresponding values on the inner (minutes) scale increased or decreased by 2 percent
for each 5.5 oC that the actual temperature differs from the standard temperature at that PA, as set in the
window. Although the PA is set in the window, the ITA is used on the inner (minutes) scale for finding
the TA, corrected for difference in temperature lapse rate.
102 AFPAM11-216 1 MARCH 2001
EXAMPLE:
Given: PA 8,500 feet
Given: ITA 8,000 feet
Given: Air Temperature –16 oC
To Find: TA
Procedure: Place PA (8,500 feet) opposite the temperature (-16) on the FOR ALTITUDE
COMPUTATIONS scale. Opposite the ITA (8,000 feet) on the inner scale, read the TA (7,600 feet) on
the outer scale. The solution is illustrated in Figure 3.l7.
Figure 3.17. Finding True Altitude.
3.14.2. Density Altitude (DA) Determination. DA determination on the computer is accomplished by
using the window just above FOR AIRSPEED AND DENSITY ALTITUDE COMPUTATIONS and the
small window just above that marked DENSITY ALTITUDE.
EXAMPLE:
Given: PA 9,000 feet
Given: Air temperature 10o C
To Find: DA
Procedure: Place pressure altitude (9,000 feet) opposite air temperature (10) in window marked FOR
AIRSPEED AND DENSITY ALTITUDE, read DA (10,400 feet). The solution is illustrated in Figure
3.18.
Figure 3.18. Finding Density Altitude.
AFPAM11-216 1 MARCH 2001 103
Section 3E— Absolute Altimeter
3.15. Basics. Accurate absolute altitude is an important requisite for navigation, airdrop, and bombing as
well as for safe aviating. It is particularly important in pressure pattern navigation. Absolute altitude may
be computed from the PA readings if the position of the aircraft is known, but the results are often
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