Introduction
An aircraft is a device that is used, or intended to be used, for
flight according to the current Title 14 of the Code of Federal
Regulations (14 CFR) part 1, Definitions and Abbreviations.
Categories of aircraft for certification of airmen include
airplane, rotorcraft, glider, lighter-than-air, powered-lift,
powered parachute, and weight-shift control aircraft. Title
14 CFR part 1 also defines airplane as an engine-driven,
fixed-wing aircraft that is supported in flight by the dynamic
reaction of air against its wings. Another term, not yet
codified in 14 CFR part 1, is advanced avionics aircraft,
which refers to an aircraft that contains a global positioning
system (GPS) navigation system with a moving map display,
in conjunction with another system, such as an autopilot.
This chapter provides a brief introduction to the structure
of aircraft and uses an airplane for most illustrations. Light
Sport Aircraft (LSA), such as weight-shift control aircraft,
balloon, glider, powered parachute, and gyroplane, have their
own handbooks to include detailed information regarding
aerodynamics and control.
Aircraft
Construction
Chapter 3
Aircraft Design, Certification, and
Airworthiness
The FAA certifies three types of aviation products: aircraft,
aircraft engines, and propellers. Each of these products
has been designed to a set of airworthiness standards.
These standards are parts of Title 14 of the Code of
Federal Regulations (14 CFR), published by the FAA. The
airworthiness standards were developed to help ensure that
aviation products are designed with no unsafe features.
Different airworthiness standards apply to the different
categories of aviation products as follows:
• Normal, Utility, Acrobatic, and Commuter Category
Airplanes- 14 CFR part 23
• Transport Category Airplanes—14 CFR part 25
• Normal Category—14 CFR part 27
• Transport Category Rotorcraft—14 CFR part 29
• Manned Free Balloons—14 CFR part 31
• Aircraft Engines—14 CFR part 33
• Propellers—14 CFR part 35
Some aircraft are considered “special classes” of aircraft and
do not have their own airworthiness standards, such as gliders
and powered lift. The airworthiness standards used for these
aircraft are a combination of requirements in 14 CFR parts
23, 25, 27, and 29 that the FAA and the designer have agreed
are appropriate for the proposed aircraft.
The FAA issues a Type Certificate (TC) for the product
when they are satisfied it complies with the applicable
airworthiness standards. When the TC is issued, a Type
Certificate Data Sheet (TCDS) is generated that specifies
the important design and operational characteristics of the
aircraft, aircraft engine, or propeller. The TCDS defines the
product and are available to the public from the FAA website
at www.faa.gov.
A Note About Light Sport Aircraft
Light sport aircraft are not designed according to FAA
airworthiness standards. Instead, they are designed to a
consensus of standards agreed upon in the aviation industry.
The FAA has agreed the consensus of standards is acceptable
as the design criteria for these aircraft. Light sport aircraft do
not necessarily have individually type certificated engines
and propellers. Instead, a TC is issued to the aircraft as a
whole. It includes the airframe, engine, and propeller.
Aircraft, aircraft engines, and propellers can be manufactured
one at a time from the design drawings, or through an FAA
approved manufacturing process, depending on the size and
capabilities of the manufacturer. During the manufacturing
process, each part is inspected to ensure that it has been built
exactly according to the approved design. This inspection is
called a conformity inspection.
When the aircraft is complete, with the airframe, engine, and
propeller, it is inspected and the FAA issues an airworthiness
certificate for the aircraft. Having an airworthiness
certificate means the complete aircraft meets the design and
manufacturing standards, and is in a condition for safe flight.
This airworthiness certificate must be carried in the aircraft
during all flight operations. The airworthiness certificate
remains valid as long as the required maintenance and
inspections are kept up to date for the aircraft.
Airworthiness certificates are classified as either “Standard”
or “Special.” Standard airworthiness certificates are white,
and are issued for normal, utility, acrobatic, commuter, or
transport category aircraft. They are also issued for manned
free balloons and aircraft designated as “Special Class.”
Special airworthiness certificates are pink, and are issued
for primary, restricted, and limited category aircraft, and
light sport aircraft. They are also issued as provisional
airworthiness certificates, special flight permits (ferry
permits), and for experimental aircraft.
More information on airworthiness certificates can be found
in Chapter 9, in 14 CFR parts 175-225, and also on the FAA
website at www.faa.gov.
Lift and Basic Aerodynamics
In order to understand the operation of the major components
and subcomponents of an aircraft, it is important to
understand basic aerodynamic concepts. This chapter briefly
introduces aerodynamics; a more detailed explanation can be
found in Chapter 5, Aerodynamics of Flight.
Four forces act upon an aircraft in relation to straight-and-
level, unaccelerated flight. These forces are thrust, lift,
weight, and drag. [Figure 3-1]
Thrust is the forward force produced by the powerplant/
propeller. It opposes or overcomes the force of drag. As a
general rule, it is said to act parallel to the longitudinal axis.
This is not always the case as explained later.
Drag is a rearward, retarding force and is caused by disruption
of airflow by the wing, fuselage, and other protruding objects.
Drag opposes thrust and acts rearward parallel to the relative
wind.
Yawing Rolling Pitching
Lateral axis Longitudinal axis Vertical axis
Figure 3-2. Illustrates the pitch, roll, and yaw motion of the aircraft along the lateral, longitudinal, and vertical axes, respectively.
Lift
Weight
Drag
Thrust
Figure 3-1. The four forces.
Weight is the combined load of the aircraft itself, the crew,
the fuel, and the cargo or baggage. Weight pulls the aircraft
downward because of the force of gravity. It opposes lift
and acts vertically downward through the aircraft’s center
of gravity (CG).
Lift opposes the downward force of weight, is produced by
the dynamic effect of the air acting on the wing, and acts
perpendicular to the flight path through the wing’s center
of lift (CL).
An aircraft moves in three dimensions and is controlled by
moving it about one or more of its axes. The longitudinal,
or roll, axis extends through the aircraft from nose to tail,
with the line passing through the CG. The lateral or pitch
axis extends across the aircraft on a line through the wing
tips, again passing through the CG. The vertical, or yaw, axis
passes through the aircraft vertically, intersecting the CG. All
control movements cause the aircraft to move around one or
more of these axes and allows for the control of the aircraft
in flight. [Figure 3-2]
One of the most significant components of aircraft design is
CG. It is the specific point where the mass or weight of an
aircraft may be said to center; that is, a point around which,
if the aircraft could be suspended or balanced, the aircraft
would remain relatively level. The position of the CG of
an aircraft determines the stability of the aircraft in flight.
As the CG moves rearward (towards the tail), the aircraft
becomes more and more dynamically unstable. In aircraft
with fuel tanks situated in front of the CG, it is important
that the CG is set with the fuel tank empty. Otherwise, as the
fuel is used, the aircraft becomes unstable. [Figure 3-3] The
CG is computed during initial design and construction and
is further affected by the installation of onboard equipment,
aircraft loading, and other factors.
Major Components
Although airplanes are designed for a variety of purposes, most
of them have the same major components. [Figure 3-4] The
overall characteristics are largely determined by the original
design objectives. Most airplane structures include a fuselage,
wings, an empennage, landing gear, and a powerplant.
Fuselage
The fuselage is the central body of an airplane and is designed
to accommodate the crew, passengers, and cargo. It also
provides the structural connection for the wings and tail
assembly. Older types of aircraft design utilized an open truss
structure constructed of wood, steel, or aluminum tubing.
[Figure 3-5] The most popular types of fuselage structures
used in today’s aircraft are the monocoque (French for
“single shell”) and semimonocoque. These structure types
are discussed in more detail under aircraft construction later
in the chapter.
Wings
The wings are airfoils attached to each side of the fuselage
and are the main lifting surfaces that support the airplane in
Empennage
Powerplant
Fuselage
Wing
Landing gear
Figure 3-4. Airplane components.
Bulkhead Stringers
Longerons
Struts
Longerons
Figure 3-5. Truss-type fuselage structure.
independent of airspeed
Nose-down force
dependent upon airspeed
Nose-up force
Fixed
Variable
Vertical forces acting on an airplane in flight.
CG
CL
Lift
CG too far aft
nose-down force
Insufficient elevator
If the CG is too far aft, there might not be enough elevator nose-down
force at the low stall airspeed to get the nose down for recovery.
Lift
CG
CL
CG too far forward
nose-up force
Insufficient elevator
If the CG is too far forward, there will not be enough elevator
nose-up force to flare the airplane for landing.
CG
Lift
CL
Figure 3-3. Center of gravity (CG).
flight. There are numerous wing designs, sizes, and shapes
used by the various manufacturers. Each fulfills a certain need
with respect to the expected performance for the particular
airplane. How the wing produces lift is explained in Chapter
5, Aerodynamics of Flight.
Wings may be attached at the top, middle, or lower portion
of the fuselage. These designs are referred to as high-, mid-,
and low-wing, respectively. The number of wings can also
vary. Airplanes with a single set of wings are referred to as
monoplanes, while those with two sets are called biplanes.
[Figure 3-6]
Many high-wing airplanes have external braces, or wing
struts that transmit the flight and landing loads through the
struts to the main fuselage structure. Since the wing struts
are usually attached approximately halfway out on the wing,
this type of wing structure is called semi-cantilever. A few
high-wing and most low-wing airplanes have a full cantilever
wing designed to carry the loads without external struts.
The principal structural parts of the wing are spars, ribs,
and stringers. [Figure 3-7] These are reinforced by trusses,
I-beams, tubing, or other devices, including the skin. The
wing ribs determine the shape and thickness of the wing
(airfoil). In most modern airplanes, the fuel tanks are either
an integral part of the wing’s structure or consist of flexible
containers mounted inside of the wing.