Introduction
This chapter discusses the factors that affect aircraft
performance, which include the aircraft weight, atmospheric
conditions, runway environment, and the fundamental
physical laws governing the forces acting on an aircraft.
Importance of Performance Data
The performance or operational information section of the
Aircraft Flight Manual/Pilot’s Operating Handbook (AFM/
POH) contains the operating data for the aircraft; that is, the
data pertaining to takeoff, climb, range, endurance, descent,
and landing. The use of this data in flying operations is
mandatory for safe and efficient operation. Considerable
knowledge and familiarity of the aircraft can be gained by
studying this material.
Aircraft
Performance
Chapter 11
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 11-1. Standard sea level pressure.
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 is land and water. However, air differs from land
and water in that it is a mixture of gases. It has mass, weight,
and indefinite shape.
Air, like any other fluid, is able to flow 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 (VSI),
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 may vary with time but more
importantly, it varies with altitude and temperature. Due to
the changing atmospheric pressure, a standard reference
was developed. The standard atmosphere at sea level has
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 11-1]
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 11-2] 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.
Altitude (ft) Pressure
("Hg)
Temperature
(°C) (°F)
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 11-2. Properties of standard atmosphere.
Altimeter
setting
Altitude
correction
28.0
28.1
28.2
28.3
28.4
28.5
28.6
28.7
28.8
28.9
29.0
29.1
29.2
29.3
29.4
29.5
29.6
29.7
29.8
29.9
29.92
30.0
30.1
30.2
30.3
30.4
30.5
30.6
30.7
30.8
30.9
31.0
1,824
1,727
1,630
1,533
1,436
1,340
1,244
1,148
1,053
957
863
768
673
579
485
392
298
205
112
20
0
−73
−165
−257
−348
−440
−531
−622
−712
−803
−893
−983
Subtract Add
To field
elevation
From field
elevation
To get
pressure altitude
Alternate Method for Determining
Pressure Altitude
Method for Determining
Pressure Altitude
Field elevation is sea level
Figure 11-3. Field elevation versus pressure. The aircraft is located
on a field that happens to be at sea level. Set the altimeter to the
current altimeter setting (29.7). The difference of 205 feet is added
to the elevation or a PA of 205 feet.
Since all aircraft performance is compared and evaluated
using the standard atmosphere, all aircraft instruments
are calibrated for the standard atmosphere. Thus, certain
corrections must apply to the instrumentation, as well as the
aircraft performance, if the actual operating conditions do
not fit the standard atmosphere. In order to account properly
for the nonstandard atmosphere, certain related terms must
be defined.
Pressure Altitude
Pressure altitude is the height above the standard datum
plane (SDP). The aircraft 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—the altitude in the
standard atmosphere corresponding to the sensed pressure.
The SDP is a theoretical level at which the pressure of the
atmosphere is 29.92 "Hg and the weight of air is 14.7 psi. As
atmospheric pressure changes, the SDP may be below, at, or
above sea level. Pressure altitude is important as a basis for
determining aircraft performance, as well as for assigning
flight levels to aircraft operating at above 18,000 feet.
The pressure altitude can be determined by any of the three
following methods:
1. By setting the barometric scale of the altimeter to
29.92 "Hg and reading the indicated altitude,
2. By applying a correction factor to the indicated
altitude according to the reported “altimeter setting,”
[Figure 11-3]
3. By using a flight computer
Density Altitude
The more appropriate term for correlating aerodynamic
performance in the nonstandard atmosphere is density
altitude—the altitude in the standard atmosphere
corresponding to a particular value of air density.
Density altitude is pressure altitude corrected for nonstandard
temperature. As the density of the air increases (lower
density altitude), aircraft performance increases. Conversely,
Outside air temperature (OAT)
Density altitude (feet)
15,000
14,000
13,000
12,000
11,000
10,000
9,000
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
Sea level
14,000
13,000
12,000
11,000
10,000
9,000 Pressure altitude (feet)
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
-1,000
Sea level
-2,000
-20° -10° 0° 10° 20° 30° 40°
0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100°
Standard temperature
C
F
Figure 11-4. Density altitude chart.
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. Under standard atmospheric condition, air at
each level in the atmosphere has a specific density; under
standard conditions, pressure altitude and density altitude
identify the same level. Density altitude, then, is the vertical
distance above sea level in the standard atmosphere at which
a given density is to be found.
Density altitude is computed using 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 to 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, of course, 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.
For example, when set at 29.92 "Hg, the altimeter may
indicate a pressure altitude of 5,000 feet. According to the
AFM/POH, the ground run on takeoff may require a distance
of 790 feet under standard temperature conditions. However,
if the temperature is 20 °C above standard, the expansion of
air raises the density level. Using temperature correction data
from tables or graphs, or by deriving the density altitude with
a computer, it may be found that the density level is above
7,000 feet, and the ground run may be closer to 1,000 feet.
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.
Using a flight computer, density altitude can be computed by
inputting the pressure altitude and outside air temperature at
flight level. Density altitude can also be determined by referring
to the table and chart in Figures 11-3 and 11-4 respectively.
Effects 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. That is, the original column of air at a lower pressure
contains a smaller mass of air. In other words, the density is
decreased. In fact, density is directly proportional to pressure.
If the pressure is doubled, the density is doubled, and if the
pressure is lowered, so is the density. This statement is true
only at a constant temperature.
Effects 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 dominant effect. Hence, pilots can
expect the density to decrease with altitude.
Effects of Humidity (Moisture) on Density
The preceding paragraphs are based on the presupposition of
perfectly dry air. In reality, it is never completely dry. The
small amount of water vapor suspended in the atmosphere
may be 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.
Humidity, also called relative humidity, refers to the amount
of water vapor contained in the atmosphere and is expressed
as a percentage of the maximum amount of water vapor
the air can hold. This amount varies with the temperature;
warm air can hold more water vapor, while colder air can
hold less. Perfectly dry air that contains no water vapor has
a relative humidity of zero percent, while saturated air that
cannot hold any more water vapor has a relative humidity
of 100 percent. Humidity alone is usually not considered an
essential factor in calculating density altitude and aircraft
performance; however, it does contribute.
The higher the temperature, the greater amount of water
vapor that the air can hold. When comparing two separate air
masses, the first warm and moist (both qualities making air
lighter) and the second cold and dry (both qualities making
it heavier), the first must be less dense than the second.
Pressure, temperature, and humidity have a great influence
on aircraft performance because of their effect upon density.
There is no rule-of-thumb or chart used to compute the effects
of humidity on density altitude, but it must be taken into
consideration. Expect a decrease in overall performance in
high humidity conditions.
Performance
Performance is a term used to describe the ability of an
aircraft to accomplish certain things that make it useful for
certain purposes. For example, the ability of an aircraft to land
and take off in a very short distance is an important factor
to the pilot who operates in and out of short, unimproved
airfields. The ability to carry heavy loads, fly at high altitudes
at fast speeds, and/or travel long distances is essential for the
performance of airline and executive type aircraft.
The primary factors most affected by performance are the
takeoff and landing distance, rate of climb, ceiling, payload,
range, speed, maneuverability, stability, and fuel economy.
Some of these factors are often directly opposed: for example,
high speed versus short landing distance, long range versus
great payload, and high rate of climb versus fuel economy.
It is the preeminence of one or more of these factors that
dictates differences between aircraft and explains the high
degree of specialization found in modern aircraft.
The various items of aircraft performance result from the
combination of aircraft and powerplant characteristics. The
aerodynamic characteristics of the aircraft generally define
the power and thrust requirements at various conditions of
flight, while powerplant characteristics generally define the
power and thrust available at various conditions of flight.
The matching of the aerodynamic configuration with the
powerplant is accomplished by the manufacturer to provide
maximum performance at the specific design condition (e.g.,
range, endurance, and climb).
Straight-and-Level Flight
All of the principal components of flight performance involve
steady-state flight conditions and equilibrium of the aircraft.
For the aircraft to remain in steady, level flight, equilibrium
must be obtained by a lift equal to the aircraft weight and a
powerplant thrust equal to the aircraft drag. Thus, the aircraft
drag defines the thrust required to maintain steady, level
flight. As presented in Chapter 4, Aerodynamics of Flight,
all parts of an aircraft contribute to the drag, either induced
(from lifting surfaces) or parasite drag.
While parasite drag predominates at high speed, induced drag
predominates at low speed. [Figure 11-5] For example, if
an aircraft in a steady flight condition at 100 knots is then
accelerated to 200 knots, the parasite drag becomes four
times as great, but the power required to overcome that
drag is eight times the original value. Conversely, when the
Power required
Speed
Low cruise speed
Min. speed
High cruise speed
Maximum available power
Maximum level flight speed
Figure 11-6. Power versus speed.
Drag
Speed
Total drag
Parasite drag
Stall
Induced drag
L/DMAX
Figure 11-5. Drag versus speed.
aircraft is operated in steady, level flight at twice as great a
speed, the induced drag is one-fourth the original value, and
the power required to overcome that drag is only one-half
the original value.
When an aircraft is in steady, level flight, the condition of
equilibrium must prevail. The unaccelerated condition of
flight is achieved with the aircraft trimmed for lift equal
to weight and the powerplant set for a thrust to equal the
aircraft drag.
The maximum level flight speed for the aircraft is obtained
when the power or thrust required equals the maximum power
or thrust available from the powerplant. [Figure 11-6] The
minimum level flight airspeed is not usually defined by thrust
or power requirement since conditions of stall or stability and
control problems generally predominate.
Climb Performance
If an aircraft is to move, fly, and perform, work must act
upon it. Work involves force moving the aircraft. The aircraft
acquires mechanical energy when it moves. Mechanical
energy comes in two forms: (1) Kinetic Energy (KE), the
energy of speed; (2) Potential Energy (PE), the stored energy
of position.
Aircraft motion (KE) is described by its velocity (airspeed).
Aircraft position (PE) is described by its height (altitude).
Both KE and PE are directly proportional to the object’s
mass. KE is directly proportional to the square of the object’s
velocity (airspeed). PE is directly proportional to the object’s
height (altitude). The formulas below summarize these
energy relationships:
KE = ½ × m × v2 m = object mass
v = object velocity
m = object mass
PE = m × g × h g = gravity field strength
h = object height
We sometimes use the terms “power” and “thrust”
interchangeably when discussing climb performance. This
erroneously implies the terms are synonymous. It is important
to distinguish between these terms. Thrust is a force or
pressure exerted on an object. Thrust is measured in pounds
(lb) or newtons (N). Power, however, is a measurement of
the rate of performing work or transferring energy (KE and
PE). Power is typically measured in horsepower (hp) or
kilowatts (kw). We can think of power as the motion (KE
and PE) a force (thrust) creates when exerted on an object
over a period of time.
Positive climb performance occurs when an aircraft gains PE
by increasing altitude. Two basic factors, or a combination
of the two factors, contribute to positive climb performance
in most aircraft:
1. The aircraft climbs (gains PE) using excess power
above that required to maintain level flight, or
2. The aircraft climbs by converting airspeed (KE) to
altitude (PE).
As an example of factor 1 above, an aircraft with an engine
capable of producing 200 horsepower (at a given altitude)
is using only 130 horsepower to maintain level flight at that
altitude. This leaves 70 horsepower available to climb. The
pilot holds airspeed constant and increases power to perform
the climb.
As an example of factor 2, an aircraft is flying level at 120
knots. The pilot leaves the engine power setting constant but
applies other control inputs to perform a climb. The climb,
sometimes called a zoom climb, converts the airspeed (KE)