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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



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




Atmospheric Pressure
Although 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 an aircraft,
and actuates some of the important flight instruments. These
instruments are the altimeter, airspeed indicator, vertical
speed indicator, and manifold pressure gauge.

Air is very light, but it has mass and is affected by the
attraction of gravity. Therefore, like any other substance, it
has weight, and because of its 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 14.70
pounds per square inch (psi) of surface, or 1,013.2 millibars
(mb). Its thickness is limited; therefore, the higher the altitude,
the less air there is above. For this reason, the weight of the
atmosphere at 18,000 feet is one-half what it is at sea level.

The pressure of the atmosphere varies with time and location.
Due to the changing atmospheric pressure, a standard
reference was developed. The standard atmosphere at sea
level is a surface temperature of 59 °F or 15 °C and a surface
pressure of 29.92 inches of mercury ("Hg), or 1,013.2 mb.
[Figure 3-1]

Standard sea level pressure
Figure 3-1. 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 which is
approximately -65 °F or -55 °C. 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 3-2] 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.
Since aircraft performance is compared and evaluated with
respect to the standard atmosphere, all aircraft instruments
are calibrated for the standard atmosphere. In order to account
properly for the nonstandard atmosphere, certain related
terms must be defined

Properties of standard atmosphere.
Properties of standard atmosphere.

Pressure Altitude
Pressure altitude is the height above a standard datum plane
(SDP), which is a theoretical level where the weight of the
atmosphere is 29.92 "Hg (1,013.2 mb) as measured by a
barometer. An 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. As atmospheric pressure changes, the
SDP may be below, at, or above sea level. Pressure altitude
is important as a basis for determining airplane performance,
as well as for assigning flight levels to airplanes operating at
or above 18,000 feet.