30
25
20
15
10
5
0
Inches of
Mercury Millibars
1016
847
677
508
339
170
0
29.92
Standard
Sea Level
Pressure
Hg
1013
Standard
Sea Level
Pressure
mb
Atmospheric Pressure
Figure 4-2. Standard sea level pressure.
Altitude (ft) Pressure (Hg)
Temperature
(°C) (°F)
Standard Atmosphere
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
11,000
12,000
13,000
14,000
15,000
16,000
17,000
18,000
19,000
20,000
29.92
28.86
27.82
26.82
25.84
24.89
23.98
23.09
22.22
21.38
20.57
19.79
19.02
18.29
17.57
16.88
16.21
15.56
14.94
14.33
13.74
15.0
13.0
11.0
9.1
7.1
5.1
3.1
1.1
-0.9
-2.8
-4.8
-6.8
-8.8
-10.8
-12.7
-14.7
-16.7
-18.7
-20.7
-22.6
-24.6
59.0
55.4
51.9
48.3
44.7
41.2
37.6
34.0
30.5
26.9
23.3
19.8
16.2
12.6
9.1
5.5
1.9
-1.6
-5.2
-8.8
-12.3
Figure 4-3. Properties of standard atmosphere.
Once the boundary layer of the air adheres to the wing by
friction, further resistance to the airflow is caused by the
viscosity, the tendency of the air to stick to itself. When
these two forces act together to resist airflow over a wing,
it is called drag.
Pressure
Pressure is the force applied in a perpendicular direction to the
surface of an object. Often, pressure is measured in pounds of
force exerted per square inch of an object, or PSI. An object
completely immersed in a fluid will feel pressure uniformly
around the entire surface of the object. If the pressure on one
surface of the object becomes less than the pressure exerted
on the other surfaces, the object will move in the direction
of the lower pressure.
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
air is a fluid substance, this force is exerted equally in all
directions. 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). The thickness of the atmosphere 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 4-2]
A standard temperature lapse rate is when 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
when pressure decreases at a rate of approximately 1 "Hg
per 1,000 feet of altitude gain to 10,000 feet. [Figure 4-3]
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 properly
account for the nonstandard atmosphere, certain related terms
must be defined.
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.
The pressure altitude can be determined by one of the
following 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; 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 temperatures 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 of the
aircraft, 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, a fairly rapid drop in pressure as altitude increases
usually has a dominating effect. Hence, pilots can expect the
density to decrease with altitude.