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Pilot's Handbook of Aeronautical Knowledge
Principles of Flight
Structure of the Atmosphere

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Pilot's Handbook of Aeronautical Knowledge

Preface

Acknowledgements

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

Appendix

Glossary

Index

The pressure altitude can be determined by either of two
methods:
1. Setting the barometric scale of the altimeter to 29.92
and reading the indicated altitude.
2. Applying a correction factor to the indicated altitude
according to the reported altimeter setting.

Density Altitude
SDP is a theoretical pressure altitude, but aircraft operate in a
nonstandard atmosphere and the term density altitude is used
for correlating aerodynamic performance in the nonstandard
atmosphere. Density altitude is the vertical distance above sea
level in the standard atmosphere at which a given density is
to be found. The density of air has significant effects on the
aircraft's performance because as air becomes less dense,
it reduces:
• Power because the engine takes in less air.
• Thrust because a propeller is less efficient in thin
air.
• Lift because the thin air exerts less force on the
airfoils.

Density altitude is pressure altitude corrected for nonstandard
temperature. As the density of the air increases (lower density
altitude), aircraft performance increases and 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, because under standard atmospheric
conditions, air at each level in the atmosphere not only has
a specific density, its pressure altitude and density altitude
identify the same level.

The computation of density altitude involves 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 with
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
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.

Air density is affected by changes in altitude, temperature,
and humidity. High density altitude refers to thin air while
low density altitude refers to dense air. The conditions that
result in a high density altitude are high elevations, low
atmospheric pressures, high temperatures, high humidity, or
some combination of these factors. Lower elevations, high
atmospheric pressure, low temperatures, and low humidity
are more indicative of low density altitude.


Effect of Pressure on Density
Since air is a gas, it can be compressed or expanded. When
air is compressed, a greater amount of air can occupy a given
volume. Conversely, when pressure on a given volume of air
is decreased, the air expands and occupies a greater space.
At a lower pressure, the original column of air contains
a smaller mass of air. The density is decreased because
density is directly proportional to pressure. If the pressure is
doubled, the density is doubled; if the pressure is lowered, the
density is lowered. This statement is true only at a constant
temperature.

Effect of Temperature on Density
Increasing the temperature of a substance decreases its density.
Conversely, decreasing the temperature increases the density.
Thus, the density of air varies inversely with temperature.
This statement is true only at a constant pressure.
In the atmosphere, both temperature and pressure decrease
with altitude, and have conflicting effects upon density.
However, the fairly rapid drop in pressure as altitude is
increased usually has the dominating effect. Hence, pilots
can expect the density to decrease with altitude.

Effect of Humidity (Moisture) on Density
The preceding paragraphs refer to air that is perfectly dry. In
reality, it is never completely dry. The small amount of water
vapor suspended in the atmosphere may be almost negligible
under certain conditions, but in other conditions humidity
may become an important factor in the performance of an
aircraft. Water vapor is lighter than air; consequently, moist
air is lighter than dry air. Therefore, as the water content
of the air increases, the air becomes less dense, increasing
density altitude and decreasing performance. It is lightest or
least dense when, in a given set of conditions, it contains the
maximum amount of water vapor.

 

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