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in the standard atmosphere at (A') is called the DENSITY ALTITUDE.
Similarly, if you are at atmosphere (B) (colder or high pressure shown as blue on the left) the air will be
more dense than standard. Therefore you will have to go down to a lower actual altitude in the standard
atmosphere at (B') to find the equivalent air density. This equivalent altitude in the Standard Atmosphere
is the DENSITY ALTITUDE.
The reason that you need to convert your actual non-standard altitude (and thus your non-standard air
density) to the standard density altitude is that all performance charts and data is based on a standard
atmosphere. For example, if you are at a high altitude runway already, and the atmosphere pressure is
low and temperature is high, it will require a significantly longer take off run than you may be
accustomed to at your lower home base. If you are not aware of the effects of density altitude on your
aircraft performance, it could lead to serious consequences.
Density Altitude Calculations
Density Altitude can be found in two ways
· Using conversion charts
· Using the E6B Flight Computer
Density Altitude calculation is a 2 step process.
Step 1. Find Pressure Altitude
Pressure Altitude adjusts for pressure difference between your air and standard atmosphere. The question
is “What would your altimeter read if you were in a standard atmosphere at your current actual
altitude?” This altitude is called PRESSURE ALTITUDE.
Pressure Altitude can be determined two ways.
· In the aircraft, adjust your altimeter setting to 29.92 in. Hg (standard pressure), and read the altitude
value shown by the altimeter needles. Or...
· Apply a correction factor from a Pressure Altitude Correction Table as shown below.
PRESSURE ALTITUDE CONVERSION TABLE
In. Hg Conv. Factor In. Hg Conf. Factor In. Hg Conv. Factor In. Hg Conv. Factor
28.0 1824 28.8 1053 29.6 298 30.3 -348
28.1 1727 28.9 975 29.7 205 30.4 -440
Aircraft Performance
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28.2 1630 29.0 863 29.8 112 30.5 -531
28.3 1533 29.1 768 29.9 20 30.6 -622
28.4 1336 29.2 673 29.92 0 30.7 -712
28.5 1340 29.3 579 30.0 -73 30.8 -803
28.6 1148 29.4 485 30.1 -175 30.9 -893
28.7 1148 29.5 392 30.2 -257 31.0 -983
EXAMPLE:
Airport Altitude = 2367 ft
Altimeter Setting = 30.40 In. Hg
Conversion Factor= -440 feet (from table )
Pressure Altitude = Airport Altitude + Conversion Factor =2367+(- 440) = 1927
NOTE: If your barometric pressure is not shown in the table (say a value such as 30.35) you will have to
interpolate to get the correct pressure altitude adjustment.
Step 2. Find Density Altitude
Density Altitude uses Pressure Altitude as a basis, and adds in a correction factor for non-standard
temperature.
Calculate Density Altitude using:
1. PRESSURE ALTITUDE and
2. Outside Air Temperature (OAT)
· Use E6B Flight Computer (see E6B instruction book)
· Use Density Altitude Chart like the one shown below.
Density Alt For Example: If you found the Pressure Altitude, doing either of the steps cited above, to be
4000 feet, and the outside Air Temperature (OAT) is 16° , do the following on the chart to
find Density Altitude.
Locate 16° C on bottom scale. Go vertically up to intersect the 4000 foot Pressure Altitude slanted line
(blue line). Go left horizontally (blue line) to read Density Altitude = 5000 feet from the left side scale.
You have now adjusted for the difference from standard temperature by using the chart.
The red line on the chart is a Standard Atmosphere Temperature line.
Performance charts provided by the manufacturer are based on Standard Atmosphere. Therefore you
must adjust your current situation (barometric pressure and temperature) to Standard Atmosphere. This is
done by calculating your Density Altitude, then using this Density Altitude as the altitude in the
Aircraft Performance
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manufacturers performance table when interpreting the performance table data.
Aircraft Performance Charts
Aircraft Performance Charts state performance figures in standard atmosphere conditions.
Takeoff Performance
You should consult the manufacturers Pilot Operating Handbook for the aircraft to be flown for take-off
performance tables or graphs.
Takeoff performance is influenced by several factors.
· Adverse conditions
1. High density altitude (high altitude runway, low pressure, high temperature)
2. Runway conditions - mud, soft field, slush, snow, tall grass, rough surface, uphill
3. Tailwind (downwind takeoff)
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