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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. 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, balloon, glider, powered parachute, 

and gyroplane have their own handbooks to include detailed 

information regarding aerodynamics and control. 

Aircraft Structure

Chapter 2
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Pitching Rolling Yawing

Lateral Axis Longitudinal Axis Vertical Axis

Figure 2-2. Illustrates the pitch, roll, and yaw motion of the aircraft along the lateral, longitudinal, and vertical axes, respectively.

Figure 2-1. The four forces.

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 4, Aerodynamics of Flight.

Four forces act upon an aircraft in relation to straight-andlevel, unaccelerated flight. These forces are thrust, lift, 

weight, and drag. [Figure 2-1] 

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.

Weight is the combined load of the airplane itself, the crew, 

the fuel, and the cargo or baggage. Weight pulls the airplane 

downward because of the force of gravity. It opposes lift, 

and acts vertically downward through the airplane’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 flightpath through the wing’s center 

of lift.

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 

airplane in flight. [Figure 2-2] 

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 2-3] The CG is computed 

during initial design and construction, and is further affected 

by the installation of onboard equipment, aircraft loading, 

and other factors.
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Figure 2-4. Airplane components.

Figure 2-5. 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 Lift

CG too far aft

nose-down force Insufficient elevator

If the CG is too far aft, there might not be enough elevator nosedown 

force at the low stall airspeed to get the nose down for recovery. Lift

CG

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

Figure 2-3. Center of gravity (CG). Figure 2-3. Center of gravity.

Major Components

Although airplanes are designed for a variety of purposes, most 

of them have the same major components. [Figure 2-4] 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 2-5] 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 

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 

4, 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 
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Figure 2-6. Monoplane (left) and biplane (right).

Figure 2-7. Wing components.

monoplanes, while those with two sets are called biplanes. 

[Figure 2-6] 

Many high-wing airplanes have external braces, or wing 

struts, which 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 2-7] 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 either are 

an integral part of the wing’s structure, or consist of flexible 

containers mounted inside of the wing. 

Attached to the rear or trailing edges of the wings are two 

types of control surfaces referred to as ailerons and flaps. 

Ailerons extend from about the midpoint of each wing 

outward toward the tip, and move in opposite directions to 

create aerodynamic forces that cause the airplane to roll. 

Flaps extend outward from the fuselage to near the midpoint 

of each wing. The flaps are normally flush with the wing’s 

surface during cruising flight. When extended, the flaps move 

simultaneously downward to increase the lifting force of the 

wing for takeoffs and landings. [Figure 2-8]
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Plain flap

Basic section

Split flap

Slotted flap

Fowler flap

Slotted Fowler flap

Figure 2-8. Types of flaps.

Figure 2-9. Weight-shift control aircraft use the shifting of weight 

for control.

Figure 2-10. Empennage components.

Alternate Types of Wings

With the Federal Aviation Administration’s (FAA) recent 

addition of the LSA category, various methods are employed 

to control flight and to produce lift. These methods are 

discussed in Chapter 4, Aerodynamics of Flight, which 

provides information on the effect controls have on lifting 

surfaces from traditional wings to wings that use both flexing 

(due to billowing) and shifting (through the change of the 

aircraft’s CG). Handbooks specific to each category of LSA 

are available for the interested pilot. LSA illustrate various 

lifting surfaces and control methods. For example, the wing 

of the weight-shift control aircraft is highly swept, and the 

shifting of weight to provide controlled flight. [Figure 2-9]

Empennage

The empennage includes the entire tail group and consists of 

fixed surfaces such as the vertical stabilizer and the horizontal 

stabilizer. The movable surfaces include the rudder, the 

elevator, and one or more trim tabs. [Figure 2-10]

The rudder is attached to the back of the vertical stabilizer. 

During flight, it is used to move the airplane’s nose left 

and right. The elevator, which is attached to the back of the 

horizontal stabilizer, is used to move the nose of the airplane 

up and down during flight. Trim tabs are small, movable 

portions of the trailing edge of the control surface. These 

movable trim tabs, which are controlled from the flight deck, 

reduce control pressures. Trim tabs may be installed on the 

ailerons, the rudder, and/or the elevator.

A second type of empennage design does not require an 

elevator. Instead, it incorporates a one-piece horizontal 

stabilizer that pivots from a central hinge point. This type of 
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Figure 2-12. Types of landing gear: floats (top), skis (middle), and 

wheels (bottom).

Figure 2-11. Stabilator components.

design is called a stabilator, and is moved using the control 

wheel, just as the elevator is moved. For example, when a 

pilot pulls back on the control wheel, the stabilator pivots so 

the trailing edge moves up. This increases the aerodynamic 

tail load and causes the nose of the airplane to move up. 

Stabilators have an antiservo tab extending across their 

trailing edge. [Figure 2-11]

The antiservo tab moves in the same direction as the trailing 

edge of the stabilator and helps make the stabilator less 

sensitive. The antiservo tab also functions as a trim tab to 

relieve control pressures and helps maintain the stabilator in 

the desired position. 

Landing Gear

The landing gear is the principal support of the airplane when 

parked, taxiing, taking off, or landing. The most common type 

of landing gear consists of wheels, but airplanes can also be 

equipped with floats for water operations, or skis for landing 

on snow. [Figure 2-12] 

The landing gear consists of three wheels—two main wheels 

and a third wheel positioned either at the front or rear of the 

airplane. Landing gear with a rear mounted wheel is called 

conventional landing gear.

Airplanes with conventional landing gear are sometimes 

referred to as tailwheel airplanes. When the third wheel is 

located on the nose, it is called a nosewheel, and the design 

is referred to as a tricycle gear. A steerable nosewheel or 

tailwheel permits the airplane to be controlled throughout all 

operations while on the ground. Most aircraft are steered by 

moving the rudder pedals, whether nosewheel or tailwheel. 

Additionally, some aircraft are steered by differential 

braking.

The Powerplant

The powerplant usually includes both the engine and the 

propeller. The primary function of the engine is to provide 

the power to turn the propeller. It also generates electrical 

power, provides a vacuum source for some flight instruments, 

and in most single-engine airplanes, provides a source of 

heat for the pilot and passengers. [Figure 2-13] The engine 

is covered by a cowling, or a nacelle, which are both types 

of covered housings. The purpose of the cowling or nacelle 
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Figure 2-13. Engine compartment.

is to streamline the flow of air around the engine and to help 

cool the engine by ducting air around the cylinders. 

The propeller, mounted on the front of the engine, translates 

the rotating force of the engine into thrust, a forward acting 

force that helps move the airplane through the air. The 

propeller may also be mounted on the rear of the engine as 

in a pusher-type aircraft. A propeller is a rotating airfoil that 

produces thrust through aerodynamic action. A low pressure 

area is formed at the back of the propeller’s airfoil, and high 

pressure is produced at the face of the propeller, similar to the 

way lift is generated by an airfoil used as a lifting surface or 

wing. This pressure differential pulls air through the propeller, 

which in turn pulls the airplane forward. 

There are two significant factors involved in the design of 

a propeller which impact its effectiveness. The angle of a 

propeller blade, as measured against the hub of the propeller, 

keeps the angle of attack relatively constant along the span 

of the propeller blade, reducing or eliminating the possibility 

of a stall. The pitch is defined as the distance a propeller 

would travel in one revolution if it were turning in a solid. 

These two factors combine to allow a measurement of the 

propeller’s efficiency. Propellers are usually matched to a 

specific aircraft/powerplant combination to achieve the best 

efficiency at a particular power setting, and they pull or push 

depending on how the engine is mounted. 

Subcomponents

The subcomponents of an airplane include the airframe, 

electrical system, flight controls, and brakes. 

The airframe is the basic structure of an aircraft and is designed 

to withstand all aerodynamic forces, as well as the stresses 

imposed by the weight of the fuel, crew, and payload. 

The primary function of an aircraft electrical system is to 

generate, regulate, and distribute electrical power throughout 

the aircraft. There are several different power sources on 

aircraft to power the aircraft electrical systems. These power 

sources include: engine-driven alternating current (AC) 

generators, auxiliary power units (APUs), and external power. 

The aircraft’s electrical power system is used to operate the 

flight instruments, essential systems such as anti-icing, etc., 

and passenger services, such as cabin lighting. 

The flight controls are the devices and systems which govern 

the attitude of an aircraft and, as a result, the flightpath 

followed by the aircraft. In the case of many conventional 

airplanes, the primary flight controls utilize hinged, trailingedge surfaces called elevators for pitch, ailerons for roll, and 

the rudder for yaw. These surfaces are operated by the pilot 

in the flight deck or by an automatic pilot.

Airplane brakes consist of multiple pads (called caliper pads) 

that are hydraulically squeezed toward each other with a 

rotating disk (called a rotor) between them. The pads place 

pressure on the rotor which is turning with the wheels. As 

a result of the increased friction on the rotor, the wheels 

inherently slow down and stop turning. The disks and brake 

pads are made either from steel, like those in a car, or from a 

carbon material that weighs less and can absorb more energy. 

Because airplane brakes are used principally during landings 

and must absorb enormous amounts of energy, their life is 

measured in landings rather than miles.

Types of Aircraft Construction

The construction of aircraft fuselages evolved from the early 

wood truss structural arrangements to monocoque shell 

structures to the current semimonocoque shell structures.

Truss Structure

The main drawback of truss structure is its lack of a 

streamlined shape. In this construction method, lengths of 

tubing, called longerons, are welded in place to form a wellbraced framework. Vertical and horizontal struts are welded 

to the longerons and give the structure a square or rectangular 

shape when viewed from the end. Additional struts are needed 

to resist stress that can come from any direction. Stringers 

and bulkheads, or formers, are added to shape the fuselage 

and support the covering. 
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Figure 2-14. Semimonocoque and monocoque fuselage design.

As technology progressed, aircraft designers began to enclose 

the truss members to streamline the airplane and improve 

performance. This was originally accomplished with cloth 

fabric, which eventually gave way to lightweight metals such 

as aluminum. In some cases, the outside skin can support all 

or a major portion of the flight loads. Most modern aircraft 

use a form of this stressed skin structure known as monocoque 

or semimonocoque construction. [Figure 2-14] 

Monocoque

Monocoque construction uses stressed skin to support almost 

all loads much like an aluminum beverage can. Although 

very strong, monocoque construction is not highly tolerant 

to deformation of the surface. For example, an aluminum 

beverage can supports considerable forces at the ends of 

the can, but if the side of the can is deformed slightly while 

supporting a load, it collapses easily. 

Because most twisting and bending stresses are carried by 

the external skin rather than by an open framework, the need 

for internal bracing was eliminated or reduced, saving weight 

and maximizing space. One of the notable and innovative 

methods for using monocoque construction was employed by 

Jack Northrop. In 1918, he devised a new way to construct 

a monocoque fuselage used for the Lockheed S-1 Racer. 

The technique utilized two molded plywood half-shells that 

were glued together around wooden hoops or stringers. To 

construct the half shells, rather than gluing many strips of 

plywood over a form, three large sets of spruce strips were 

soaked with glue and laid in a semi-circular concrete mold 

that looked like a bathtub. Then, under a tightly clamped lid, a 

rubber balloon was inflated in the cavity to press the plywood 

against the mold. Twenty-four hours later, the smooth halfshell was ready to be joined to another to create the fuselage. 

The two halves were each less than a quarter inch thick. 

Although employed in the early aviation period, monocoque 

construction would not reemerge for several decades due to 

the complexities involved. Every day examples of monocoque 

construction can be found in automobile manufacturing where 

the unibody is considered standard in manufacturing.

Semimonocoque

Semimonocoque construction, partial or one-half, uses a 

substructure to which the airplane’s skin is attached. The 

substructure, which consists of bulkheads and/or formers 

of various sizes and stringers, reinforces the stressed skin 

by taking some of the bending stress from the fuselage. The 

main section of the fuselage also includes wing attachment 

points and a firewall. On single-engine airplanes, the engine is 

usually attached to the front of the fuselage. There is a fireproof 

partition between the rear of the engine and the flight deck or 

cabin to protect the pilot and passengers from accidental engine 

fires. This partition is called a firewall and is usually made 

of heat-resistant material such as stainless steel. However, 

a new emerging process of construction is the integration of 

composites or aircraft made entirely of composites.

Composite Construction

History

The use of composites in aircraft construction can be dated 

to World War II aircraft when soft fiberglass insulation was 

used in B-29 fuselages. By the late 1950s, European high 

performance sailplane manufacturers were using fiberglass 

as primary structures. In 1965, the FAA type certified the 

first all-fiberglass aircraft in the normal category, a Swiss 

sailplane called a Diamant HBV. Four years later, the FAA 

certified a four-seat single-engine Windecker Eagle in the 

normal category. By 2005, over 35 percent of new aircraft 

were constructed of composite materials. 

Composite is a broad term and can mean materials such as 

fiberglass, carbon fiber cloth, Kevlar© cloth, and mixtures 

of all of the above. Composite construction offers two 

advantages: extremely smooth skins and the ability to 

easily form complex curved or streamlined structures. 

[Figure 2-15]
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Figure 2-15. Composite aircraft.

Advantages of Composites

Composite construction offers several advantages over 

metal, wood, or fabric, with its lighter weight being the most 

frequently cited. Lighter weight is not always automatic. It 

must be remembered that building an aircraft structure out of 

composites does not guarantee it will be lighter, it depends on 

the structure, as well as the type of composite being used. 

A more important advantage is that a very smooth, compound 

curved, aerodynamic structure made from composites 

reduces drag. This is the main reason sailplane designers 

switched from metal and wood to composites in the 1960s. 

In aircraft, the use of composites reduces drag for the Cirrus 

and Columbia line of production aircraft, leading to their high 

performance despite their fixed landing gear. Composites also 

help mask the radar signature of “stealth” aircraft designs, 

such as the B-2 and the F-22. Today, composites can be found 

in aircraft as varied as gliders to most new helicopters.

Lack of corrosion is a third advantage of composites. Boeing 

is designing the 787, with its all-composite fuselage, to have 

both a higher pressure differential and higher humidity in 

the cabin than previous airliners. Engineers are no longer as 

concerned about corrosion from moisture condensation on the 

hidden areas of the fuselage skins, such as behind insulation 

blankets. This should lead to lower long-term maintenance 

costs for the airlines.

Another advantage of composites is their good performance 

in a flexing environment, such as in helicopter rotor blades. 

Composites do not suffer from metal fatigue and crack growth 

as do metals. While it takes careful engineering, composite 

rotor blades can have considerably higher design lives than 

metal blades, and most new large helicopter designs have all 

composite blades, and in many cases, composite rotor hubs.

Disadvantages of Composites

Composite construction comes with its own set of 

disadvantages, the most important of which is the lack of 

visual proof of damage. Composites respond differently from 

other structural materials to impact, and there is often no 

obvious sign of damage. For example, if a car backs into an 

aluminum fuselage, it might dent the fuselage. If the fuselage 

is not dented, there is no damage. If the fuselage is dented, 

the damage is visible and repairs are made.

In a composite structure, a low energy impact, such as a 

bump or a tool drop, may not leave any visible sign of the 

impact on the surface. Underneath the impact site there may 

be extensive delaminations, spreading in a cone-shaped area 

from the impact location. The damage on the backside of 

the structure can be significant and extensive, but it may be 

hidden from view. Anytime one has reason to think there 

Composite Materials in Aircraft

Composite materials are fiber-reinforced matrix systems. The 

matrix is the “glue” used to hold the fibers together and, when 

cured, gives the part its shape, but the fibers carry most of 

the load. There are many different types of fibers and matrix 

systems. 

In aircraft, the most common matrix is epoxy resin, which is 

a type of thermosetting plastic. Compared to other choices 

such as polyester resin, epoxy is stronger and has good hightemperature properties. There are many different types of 

epoxies available, with a wide range of structural properties, 

cure times and temperatures, and costs. 

The most common reinforcing fibers used in aircraft 

construction are fiberglass and carbon fiber. Fiberglass has 

good tensile and compressive strength, good impact resistance, 

is easy to work with, and is relatively inexpensive and readily 

available. Its main disadvantage is that it is relatively heavy, 

and it is difficult to make a fiberglass load-carrying structure 

lighter than a well designed equivalent aluminum structure. 

Carbon fiber is generally stronger in tensile and compressive 

strength than fiberglass, and has much higher bending 

stiffness. It is also considerably lighter than fiberglass. 

However, it is relatively poor in impact resistance; the fibers 

are brittle and tend to shatter under sharp impact. This can 

be greatly improved with a “toughened” epoxy resin system, 

as used in the Boeing 787 horizontal and vertical stabilizers. 

Carbon fiber is more expensive than fiberglass, but the price 

has dropped due to innovations driven by the B-2 program 

in the 1980s, and Boeing 777 work in the 1990s. Very welldesigned carbon fiber structures can be significantly lighter 

than an equivalent aluminum structure, sometimes by 30 

percent or so.
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Figure 2-16. Impact energy affects the visibility, as well as the 

severity, of damage in composite structures. High and medium 

energy impacts, while severe, are easy to detect. Low energy impacts 

can easily cause hidden damage.

If an impact results in delaminations, crushing of the surface, 

or a puncture, then a repair is mandatory. While waiting 

for the repair, the damaged area should be covered and 

protected from rain. Many composite parts are composed 

of thin skins over a honeycomb core, creating a “sandwich” 

structure. While excellent for structural stiffness reasons, 

such a structure is an easy target for water ingress (entering), 

leading to further problems later. A piece of “speed tape” 

over the puncture is a good way to protect it from water, but 

is not a structural repair. The use of a paste filler to cover up 

the damage, while acceptable for cosmetic purposes, is not 

a structural repair, either.

The potential for heat damage to the resin is another 

disadvantage of using composites. While “too hot” depends 

on the particular resin system chosen, many epoxies begin 

to weaken over 150° F. White paint on composites is often 

used to minimize this issue. For example, the bottom of 

a wing that is painted black facing a black asphalt ramp 

on a hot, sunny day, can get as hot as 220 °F. The same 

structure, painted white, rarely exceeds 140 °F. As a result, 

composite airplanes often have specific recommendations 

on allowable paint colors. If the airplane is repainted, these 

recommendations must be followed. Heat damage can also 

occur due to a fire. Even a quickly extinguished small brake 

fire can damage bottom wing skins, composite landing gear 

legs, or wheel pants.

Also, chemical paint strippers are very harmful to composites, 

and must not be used on them. If paint needs to be removed 

from composites, only mechanical methods are allowed, such 

as gentle grit blasting or sanding. Many expensive composite 

parts have been ruined by the use of paint stripper, and such 

damage is generally not repairable.

Fluid Spills on Composites

Some owners are concerned about fuel, oil, or hydraulic fluid 

spills on composite surfaces. These are generally not a problem 

with modern composites using epoxy resin. Usually, if the spill 

doesn’t attack the paint, it won’t hurt the underlying composite. 

Some aircraft use fiberglass fuel tanks, for example, in which 

the fuel rides directly against the composite surface with no 

sealant being used. If the fiberglass structure is made with 

some of the more inexpensive types of polyester resin, there 

can be a problem when using auto gas with ethanol blended 

into the mixture. The more expensive types of polyester resin, 

as well as epoxy resin, can be used with auto gas, as well as 

100 octane aviation gas (avgas) and jet fuel.

Lightning Strike Protection

Lightning strike protection is an important consideration in 

aircraft design. When an aircraft is hit by lightning, a very 

may have been an impact, even a minor one, it is best to 

get an inspector familiar with composites to examine the 

structure to determine underlying damage. The appearance 

of “whitish” areas in a fiberglass structure is a good tip-off 

that delaminations of fiber fracture has occurred. 

A medium energy impact (perhaps the car backing into the 

structure) results in local crushing of the surface, which 

should be visible to the eye. The damaged area is larger 

than the visible crushed area, and will need to be repaired. 

A high energy impact, such as a bird strike or hail while in 

flight, results in a puncture and a severely damaged structure. 

In medium and high energy impacts, the damage is visible 

to the eye, but low energy impact is difficult to detect. 

[Figure 2-16]
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Figure 2-17. Composite materials in aircraft, such as Columbia 350 

(top), Boeing 787 (middle), and a Coast Guard HH-65 (bottom).

large amount of energy is delivered to the structure. Whether 

flying a light general aviation (GA) airplane or a large airliner, 

the basic principle of lightning strike protection is the same. 

For any size aircraft, the energy from the strike must be spread 

over a large surface area to lower the “amps per square inch” 

to a harmless level.

If lightning strikes an aluminum airplane, the electrical 

energy naturally conducts easily through the aluminum 

structure. The challenge is to keep the energy out of avionics, 

fuel systems, etc., until it can be safely conducted overboard. 

The outer skin of the aircraft is the path of least resistance.

In a composite aircraft, fiberglass is an excellent electrical 

insulator, while carbon fiber conducts electricity, but not 

as easily as aluminum. Therefore, additional electrical 

conductivity needs to be added to the outside layer of 

composite skin. This is done typically with fine metal meshes 

bonded to the skin surfaces. Aluminum and copper mesh 

are the two most common types, with aluminum used on 

fiberglass and copper on carbon fiber. Any structural repairs 

on lightning-strike protected areas must also include the mesh 

as well as the underlying structure.

For composite aircraft with internal radio antennas, there 

must be “windows” in the lightning strike mesh in the area 

of the antenna. Internal radio antennas may be found in 

fiberglass composites because fiberglass is transparent to radio 

frequencies, where as carbon fiber is not.

The Future of Composites

In the decades since World War II, composites have earned 

an important role in aircraft structure design. Their design 

flexibility and corrosion resistance, as well as the high 

strength-to-weight ratios possible, will undoubtedly continue 

to lead to more innovative aircraft designs in the future. 

From the Cirrus SR-20 to the Boeing 787, it is obvious that 

composites have found a home in aircraft construction and 

are here to stay. [Figure 2-17]

Instrumentation: Moving into the Future 

Until recently, most GA aircraft were equipped with 

individual instruments utilized collectively to safely operate 

and maneuver the aircraft. With the release of the electronic 

flight display (EFD) system, conventional instruments have 

been replaced by multiple liquid crystal display (LCD) 

screens. The first screen is installed in front of the left seat 

pilot position and is referred to as the primary flight display 

(PFD). The second screen, positioned approximately in 

the center of the instrument panel, is referred to as the 

multi-function display (MFD). These two screens declutter instrument panels while increasing safety. This has 

been accomplished through the utilization of solid state 

instruments which have a failure rate far less than those of 

conventional analog instrumentation. [Figure 2-18] 

With today’s improvements in avionics and the introduction 

of EFDs, pilots at any level of experience need an astute 

knowledge of the onboard flight control systems as well as 

an understanding of how automation melds with Aeronautical 

Decision-Making (ADM). These subjects are covered in 

detail in Chapter 17, Aeronautical Decision-Making. 
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Figure 2-18. Analog display (top) and digital display (bottom) from 

a Cessna 172. 

Whether an aircraft has analog or digital (“glass”) 

instruments, the instrumentation falls into three different 

categories: performance, control, and navigation.

Performance Instruments 

The performance instruments indicate the aircraft’s actual 

performance. Performance is determined by reference to the 

altimeter, airspeed or vertical speed indicator (VSI), heading 

indicator, and turn-and-slip indicator. The performance 

instruments directly reflect the performance the aircraft 

is achieving. The speed of the aircraft can be referenced 

on the airspeed indicator. The altitude can be referenced 

on the altimeter. The aircraft’s climb performance can be 

determined by referencing the VSI. Other performance 

instruments available are the heading indicator, angle of 

attack indicator, and the slip-skid indicator. [Figure 2-19] 

Control Instruments 

The control instruments [Figure 2-20] display immediate 

attitude and power changes, and are calibrated to permit 

adjustments in precise increments. The instrument for attitude 

display is the attitude indicator. The control instruments do 

not indicate aircraft speed or altitude. In order to determine 

these variable and others, a pilot must reference the 

performance instruments. 

Navigation Instruments 

The navigation instruments indicate the position of the 

aircraft in relation to a selected navigation facility or fix. 

This group of instruments includes various types of course 

indicators, range indicators, glideslope indicators, and 

bearing pointers. Newer aircraft with more technologically 

advanced instrumentation provide blended information, 

giving the pilot more accurate positional information. 

Navigation instruments are comprised of indicators that 

display GPS, very high frequency (VHF) omni-directional 

radio range (VOR), nondirectional beacon (NDB), 

and instrument landing system (ILS) information. The 

instruments indicate the position of the aircraft relative to a 

selected navigation facility or fix. They also provide pilotage 

information so the aircraft can be maneuvered to keep it on a 

predetermined path. The pilotage information can be in either 

two or three dimensions relative to the ground-based or spacebased navigation information. [Figures 2-21 and 2-22] 

Global Positioning System (GPS)

GPS is a satellite-based navigation system composed of a 

network of satellites placed into orbit by the United States 

Department of Defense (DOD). GPS was originally intended 

for military applications, but in the 1980s the government 

made the system available for civilian use. GPS works in all 

weather conditions, anywhere in the world, 24 hours a day. A 

GPS receiver must be locked onto the signal of at least three 

satellites to calculate a two-dimensional position (latitude 

and longitude) and track movement. With four or more 

satellites in view, the receiver can determine the user’s threedimensional position (latitude, longitude, and altitude). Other 

satellites must also be in view to offset signal loss and signal 

ambiguity. The use of the GPS is discussed in more detail in 

Chapter 15, Navigation. Additionally, GPS is discussed in 

the Aeronautical Information Manual (AIM). 

Chapter Summary

This chapter provides an overview of aircraft structures. 

A more in-depth understanding of aircraft structures and 

controls can be gained through the use of flight simulation 

software or interactive programs available online through 

aviation organizations such as the Aircraft Owners and Pilots 

Association (AOPA). Pilots are also encouraged to subscribe 

to or review the various aviation periodicals which contain 

valuable flying information. As discussed in Chapter 1, the 

National Air and Space Administration (NASA) and the FAA 

also offer free information for pilots. 
2-13

Figure 2-19. Performance instruments.

XPDR 5537 IDNT LCL23:00:34

VOR 1 

270°

2

1

1

2

4300

4200

4100

4000

3900

3800

4300

60

20

4000

4000

130

120

110

90

80

70

1

100

9

TAS 100KT

ALERTS

NAV1 108.00 113.00 NAV2 108.00 110.60134.000 118.000 COM1

123.800 118.000 COM2

WPT _ _ _ _ _ _ DIS _ _ ._ NM DTK _ _ _°T TRK 360°T

XPDR 5537 IDNT LCL23:00:34

130

ALERTS

NAV1 108.00 113.00 NAV2 108.00 110.6018.000 COM1

18.000 COM2

WPT _ _ _ _ _ _ DIS _

500

270°

Slip/Skid indicator

Turn rate trend vector

Altimeter and

Altitude trend tape

Vertical speed and 

tape indicator

Airspeed indicator Altitude indicator Altimeter indicator

Turn coordinator Heading indicator Vertical speed indicator 

Airspeed tape indicator

Turn rate indicator
2-14

Figure 2-20. Control instruments.
2-15

Figure 2-21. A comparison of navigation information as depicted on both analog and digital displays.

OBS

N

E

S

W

3

33

24

21

15 12

30

6

TO

33

30

24 2I

I5 I2

6 3

2

33 30

24

2I

I5 I2

6 3

OBS

N

E

S

W

3

33

24

21

15 12

30

6

TO

1

160°

Present position, inbound on 

160° radial.

HDG 360° CRS 340° 340°

1

The aircraft illustrated is heading 

020°, but the course is to the left.

HDG 360° CRS 340°

VOR 1

020°

2 A

OBS

N

E

S

W

3

33

24 12

15 12

30

6

NAV

GS

View is from the pilots

perspective, and the 

movable card is reset 

after each turn

Figure 2-22. Analog and digital indications for glideslope interception.

Fly up

OBS

FR

TO

N

E

S

W

3

33

24 21

12 15

30

6

NAV 1

20 20

I0 I0

2 0

I 0

4200

4100

3900

3800

20

80

4000

Glideslope needle indicates “fly up” to intercept glideslope

Fly down

OBS

FR

TO

N

E

S

W

3

33

24 21

12 15

30

6

NAV 1

20 20

I0 I0

2 0

I 0

4200

4100

3900

3800

20

80

4000

Glideslope needle indicates “fly 

down” to intercept glideslope

25

Glideslope
2-16