Page 100 - Planning and Design of Airports

70 Airp o r t Pl anning

Air Pressure and Temperature
Since aircraft are designed to operate in the altitudes of the earth’s
atmosphere from sea level to nearly 50,000 ft above sea level, it is
important to understand the characteristics of the atmosphere at
these altitudes and how altitudes, as well as other atmospheric char-
acteristics, affect aircraft performance.
The performance of all aircraft is affected significantly by the atmos-
pheric conditions in which they operate. These conditions are constantly
varying, based simply on the daily heating and cooling of the earth by
the sun, and the associated winds and precipitation that occur.
In general, the performance of aircraft depends primarily on the
density of the air through which it is operating. The greater the den-
sity of the air, the more air molecules flow over the wings, creating
more lift, allowing the aircraft to fly. As air density decreases, aircraft
require larger airspeed to maintain lift. For airport design, for exam-
ple, this translates to longer runway length requirements when air is
less dense. The density of the air is primarily a function of the air
pressure, measured in English units as inches of mercury (inHg) and
in metric units as millibars (mb) or hectopascals.
Air density is affected by air pressure and air temperature. As air
pressure decreases, there are less air molecules per unit volume and thus
air density decreases. As air temperature increases, the velocity and thus
spacing between air molecules increases, thus reducing air density.
While these characteristics of the atmosphere vary from day to day
and from place to place, for practical convenience for comparing the
performance of aircraft, as well as for planning and design of airports,
a standard atmosphere has been defined. A standard atmosphere represents
the average conditions found in the actual atmosphere in a particular
geographic region. Several different standard atmospheres are in use,
but the one most commonly used is the one proposed by ICAO.
In the standard atmosphere it is assumed that from sea level to an
altitude of about 36,000 ft, known as the troposphere, the temperature
decreases linearly. Above 36,000 to about 65,000 ft, known as the strato-
sphere, the temperature remains constant; and above 65,000 ft, the tem-
perature rises. Many conventional jet aircraft fly as high as 41,000 ft. The
supersonic transports flew at altitudes on the order of 60,000 ft or more.
In the troposphere the standard atmosphere is defined as follows:

1. The temperature at sea level is 59°F or 15°C. This is known as
the standard temperature at sea level.
2. The pressure at sea level is 29.92126 inHg or 1015 mb. This is
known as the standard pressure at sea level.
3. The temperature gradient from sea level to the altitude at
which the temperature becomes −69.7°F is 3.566°F per thou-
sand feet. That is, for every increase in altitude of 1000 ft, the
temperature decreases by approximately 3.5°F or 2°C.