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Pilot's Handbook of Aeronautical Knowledge
Aircraft Performance
Structure of the Atmosphere
Atmospheric Pressure

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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

It must be emphasized that the manufacturers' information
and data furnished in the AFM/POH is not standardized.
Some provide the data in tabular form, while others use
graphs. In addition, the performance data may be presented
on the basis of standard atmospheric conditions, pressure
altitude, or density altitude. The performance information in
the AFM/POH has little or no value unless the user recognizes
those variations and makes the necessary adjustments.

To be able to make practical use of the aircraft's capabilities
and limitations, it is essential to understand the significance
of the operational data. The pilot must be cognizant of the
basis for the performance data, as well as the meanings of
the various terms used in expressing performance capabilities
and limitations.

Since the characteristics of the atmosphere have a major
effect on performance, it is necessary to review two dominant
factors—pressure and temperature.

Structure of the Atmosphere

The atmosphere is an envelope of air that surrounds the Earth
and rests upon its surface. It is as much a part of the Earth as
its land and water. However, air differs from land and water
inasmuch as it is a mixture of gases. It has mass, weight, and
indefinite shape.

Air, like any other fluid, is able to .ow and change its shape
when subjected to even minute pressures because of the lack of
strong molecular cohesion. For example, gas will completely
fill any container into which it is placed, expanding or
contracting to adjust its shape to the limits of the container.
The atmosphere is composed of 78 percent nitrogen, 21 percent
oxygen, and 1 percent other gases, such as argon or helium.
Most of the oxygen is contained below 35,000 feet altitude.

Atmospheric Pressure

Though there are various kinds of pressure, pilots are mainly
concerned with atmospheric pressure. It is one of the basic
factors in weather changes, helps to lift the aircraft, and
actuates some of the most important flight instruments in
the aircraft. These instruments often include the altimeter,
the airspeed indicator (ASI), the vertical speed indicator, and
the manifold pressure gauge.

Though air is very light, it has mass and is affected by the
attraction of gravity. Therefore, like any other substance, it
has weight; because it has weight, it has force. Since it is a
fluid substance, this force is exerted equally in all directions,
and its effect on bodies within the air is called pressure.

Under standard conditions at sea level, the average pressure
exerted by the weight of the atmosphere is approximately 14.7
pounds per square inch (psi). The density of air has significant
effects on the aircraft's performance. As air becomes less
dense, it reduces:
• Power, because the engine takes in less air.
• Thrust, because the propeller is less efficient in thin
air.
• Lift, because the thin air exerts less force on the
airfoils.

The pressure of the atmosphere varies with time and altitude.
Due to the changing atmospheric pressure, a standard
reference was developed. The standard atmosphere at sea level
is a surface temperature of 59 degrees Fahrenheit (°F) or 15
degrees Celsius (°C) and a surface pressure of 29.92 inches of
mercury ("Hg) or 1013.2 millibars (mb). [Figure 10-1]

Standard sea level pressure.
Figure 10-1. Standard sea level pressure.

A standard temperature lapse rate is one in which the
temperature decreases at the rate of approximately 3.5 °F or
2 °C per thousand feet up to 36,000 feet. Above this point,
the temperature is considered constant up to 80,000 feet. A
standard pressure lapse rate is one in which pressure decreases
at a rate of approximately 1 "Hg per 1,000 feet of altitude gain
to 10,000 feet. [Figure 10-2] The International Civil Aviation
Organization (ICAO) has established this as a worldwide
standard, and it is often referred to as International Standard
Atmosphere (ISA) or ICAO Standard Atmosphere. Any
temperature or pressure that differs from the standard lapse
rates is considered nonstandard temperature and pressure.
Adjustments for nonstandard temperatures and pressures are
provided on the manufacturer's performance charts.

 

10-2