| Home | Privacy | Contact |

Pilot's Handbook of Aeronautical Knowledge
Aircraft Performance

| First | Previous | Next | Last |

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




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 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, or travel long distances is essential performance for
operators 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.

Drag versus speed.
Figure 10-5. Drag versus speed.