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Pilot's Handbook of Aeronautical Knowledge
Aircraft Performance
Pressure Altitude
Density Altitude

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Properties of standard atmosphere.
Figure 10-2. Properties of standard atmosphere.

Since all aircraft performance is compared and evaluated with
respect to the standard atmosphere, all aircraft instruments
are calibrated for the standard atmosphere. Thus, certain
corrections must apply to the instrumentation, as well as the
aircraft performance, if the actual operating conditions do
not fit the standard atmosphere. In order to account properly
for the nonstandard atmosphere, certain related terms must
be defined.

Pressure Altitude

Pressure altitude is the height above the standard datum
plane (SDP). The aircraft altimeter is essentially a sensitive
barometer calibrated to indicate altitude in the standard
atmosphere. If the altimeter is set for 29.92 "Hg SDP, the
altitude indicated is the pressure altitude—the altitude in the

standard atmosphere corresponding to the sensed pressure.
The SDP is a theoretical level where the pressure of the
atmosphere is 29.92 "Hg and the weight of air is 14.7 psi. As
atmospheric pressure changes, the SDP may be below, at, or
above sea level. Pressure altitude is important as a basis for
determining aircraft performance, as well as for assigning
flight levels to aircraft operating at above 18,000 feet.

The pressure altitude can be determined by any of three
1. By setting the barometric scale of the altimeter to
29.92 "Hg and reading the indicated altitude, or
2. By applying a correction factor to the indicated altitude
according to the reported "altimeter setting" (See figure
3. By using a flight computer.

Density Altitude

The more appropriate term for correlating aerodynamic
performance in the nonstandard atmosphere is density
altitude—the altitude in the standard atmosphere
corresponding to a particular value of air density.

Density altitude is pressure altitude corrected for nonstandard
temperature. As the density of the air increases (lower
density altitude), aircraft performance increases. Conversely,
as air density decreases (higher density altitude), aircraft
performance decreases. A decrease in air density means a
high density altitude; an increase in air density means a lower
density altitude. Density altitude is used in calculating aircraft
performance. Under standard atmospheric condition, air at
each level in the atmosphere has a specific density; under
standard conditions, pressure altitude and density altitude
identify the same level. Density altitude, then, is the vertical
distance above sea level in the standard atmosphere at which
a given density is to be found.

The computation of density altitude must involve consideration
of pressure (pressure altitude) and temperature. Since aircraft
performance data at any level is based upon air density under
standard day conditions, such performance data apply to air
density levels that may not be identical to altimeter indications.
Under conditions higher or lower than standard, these levels
cannot be determined directly from the altimeter.

Density altitude is determined by first finding pressure
altitude, and then correcting this altitude for nonstandard
temperature variations. Since density varies directly with
pressure, and inversely with temperature, a given pressure
altitude may exist for a wide range of temperature by allowing
the density to vary. However, a known density occurs for any
one temperature and pressure altitude. The density of the air,
of course, has a pronounced effect on aircraft and engine
performance. Regardless of the actual altitude at which the
aircraft is operating, it will perform as though it were operating
at an altitude equal to the existing density altitude.

For example, when set at 29.92 "Hg, the altimeter may
indicate a pressure altitude of 5,000 feet. According to the
AFM/POH, the ground run on takeoff may require a distance
of 790 feet under standard temperature conditions.

However, if the temperature is 20 °C above standard, the
expansion of air raises the density level. Using temperature
correction data from tables or graphs, or by deriving the
density altitude with a computer, it may be found that the
density level is above 7,000 feet, and the ground run may be
closer to 1,000 feet.