For example, using the formula for the same case
we just calculated with the table, we find the following:
PA= H_{A} + PAV
PA = 1,500 + 1,000(29.92  29.41)
PA = 1,500 + 510
PA = 2,010 feet
By comparison, you can see that this value is 34 feet
higher than we found by using the table, but it is a close
enough approximation when nothing else is available.
And, it may be done quickly in your head. With the
pressure reduction computer, the same case yields a
pressure altitude of 1,979 feet.
Pilots of aircraft, especially rotary wing aircraft,
frequently ask for maximum pressure altitude for
takeoff and for all destinations. This is calculated using
the lowest expected altimeter setting (QNH) for the
destination. The forecaster may have to interpret the
other station’s forecast to determine if the forecast QNH
will be valid during the time the aircraft will be in the
vicinity. Many rotary wing aircraft have a table in their
aircraft technical data that is entered using maximum
pressure altitude and maximum temperature to find the
maximum permissible load that can be carried.
Maximum pressure altitude may be used by the pilot in
lieu of density altitude.
DENSITY ALTITUDE
Density altitude is defined as the altitude at which a
given air density is found in the standard atmosphere.
For a given altitude, density altitude changes with
changes in pressure, air temperature, and humidity. An
increase in pressure increases air density, so it decreases
density altitude. An increase in temperature decreases
air density, so it increases density altitude. An increase
in humidity decreases air density, so itincreases density
altitude. Changes in pressure and temperature have the
greatest effect on density altitude, and changes in
humidity have the least effect.
If, for example, the pressure at Cheyenne,
Wyoming, (elevation 6,140 feet) is equal to the pressure
of the standard atmosphere at that elevation, and the
temperature is 101°F, the density would be the same as
that found at 10,000 feet. Therefore, the air is less dense
than normal, and an aircraft on takeoff (at
approximately constant weight and power setting) will
take longer to get airborne. Air density also affects
airspeed. True airspeed and indicated airspeed are equal
only when density altitude is zero. True airspeed
exceeds indicated airspeed when density altitude
increases.
No instrument is available to measure density
altitude directly. It must be computed from the pressure
(for takeoff, station pressure) and the virtual
temperature at the particular altitude under
consideration. This may be accomplished by using the
Density Altitude Computer (CP718/UM) or from
Table 69, Density Altitude Diagram, of Smithsonian
Meteorological Tables, NA50lB521. Remember,
virtual temperature is used in the computation of
density altitude.
The quickest method of calculating density altitude
is to use the Density Altitude Computer (CP718/UM),
discussed in chapter 2.
Density altitude must be
computed from the pressure (for takeoff, station
pressure) and the virtual temperature at the particular
altitude under consideration. Specific instructions are
printed on the device. Density altitude results from the
computer may be estimated to the nearest 10 feet
between the marked increments of 100 feet. If you are
in a situation where you do not have a density altitude
computer or the Smithsonian Meteorological Tables
available, you may ignore the humidity value and
calculate density altitude by the formula
DA = PA + (120 V_{t}),
where
DA = density altitude,
PA = pressure altitude at the level you desire
density altitude,
120 = a temperature constant (120 feet per 1°C),
and
V_{t} = actual temperature minus standard
temperature at the level of the pressure
altitude.
For example, let's say the surface temperature is
30°C and your pressure altitude is 2,010 feet. Look at
table 16 and find the standard temperature
corresponding to 2,000 feet. You should find 11°C.
Plug these values into the formula to find the following:
DA = PA + (120 V_{t})
DA = 2,010 feet + [120(3°C  11°C)]
DA = 2,010 + 120(19)
DA = 2,010 + 2,280
DA = 4,290 feet
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