File #1339: "Airports Airways and Electronics (1958).pdf"

Airports Airways and Electronics (1958).pdf

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Copyright 1956 by Civil Air Patrol, Incorporated
(Revised 1958)

By

Harold E. Mehrens

William E. Rowland
Art Director

AIRPORTS,
AIRW YS,
A
EE T O I S
L C R NC

C NE T
O T NS
Page

OE
N

Growth and Development ................................................

1

T W O

The Charting of Airports and Airways ..................

7

T RE
HE

Electronics In Aviation ...................................................... 1 3

FU
OR

Why Airports Are Necessary .......................................

23

FIVE Airways. and the Facilities of Air Traffic Control 29

The Regulation of Air Traffic I .......................................

SVN
EE

37

The Regulation of Air Traffic II ....................................

45

Summary .................................................................................... 55

Foreword
Current technological developments in
different but related areas appear to be
interdependent. Such is certainly the case
with respect to progress in the field of aviation
radio and that in the field of electronics
generally. The use of radio in aviation has
enabled aviation to broaden the scope of its
operation and services. In order to handle the
air traffic thus generated, new electronic
devices had to be invented. Research
directed toward this problem discovered not
only new devices for air traffic control but also
new devices whose application is destined to
affect many other industrial fields.
New machines based upon electronic
principles, whether used in aviation or
elsewhere, have implication for broad
education and training. A machine may
surplant the man who does a menial task;
man still remains the master of such machine.
But men are not born with the knowledge and
the skills such mastery requires. These must
be learned. The foundations for their learning
are the fundamental scientific ideas
underlying the operation of the electronic
devices which it appears will soon dominate
the socio-economic scene.
The Civil Air Patrol seeks to assist those who
would help youth bring both understandings
and skills to the solution of the sociological
problems which our scientific and
technological advancement appears to have
created.
To this end it offers its services to organized
formal education, and it supports a national
youth movement. Its aviation education series
will serve well to acquaint the high school
student and the Civil Air Patrol cadet with
information essential to the general
understandings in aviation and many of its
related fields.
WALTER R. AGEE
Major General, USAF
National Commander
Civil Air Patrol

Preface
Airports, Airways, and Aviation Electronics is one of a
series of seven books prepared for use in the aviation
education program of the Civil Air Patrol. It is to be used
with an instructional 35 mm. color, sound filmstrip which
illustrates the concepts which it introduces.
The purpose of his book is to describe in terms of
secondary-school student understandings the scientific
principles basic to airport and airway operations. The
growth of airports and airways is reviewed. The
aeronautical chart is presented as a source of
information about airports and airways. A simple
explanation is given of the radio circuit as the basic
operational principle of the facilities of airway marking
and of airport and airway traffic control. The nature of
those electronic devices, which make possible the
dispatch of airport and airway functions, is explained.
Finally, the methods employed at airports and along
airways to prevent air traffic conflicts are discussed. The
book's treatment of the several areas with which it is
concerned is sufficiently general to be of basic
importance to all aviation career objectives. Yet, its
content is detailed enough to challenge the interest of
students and adults alike.
Although its first use will be with Civil Air Patrol cadets, it
will be found of considerable value in any science class
or any other class that stresses the role of scientific
concepts in relation to airports and airway operations.
The books and filmstrips of this series ore not limited to
use with Civil Air Patrol cadets only. They will be found
of value to students and teachers in any aviation
program. Those working with adults may also find this
material helpful, if the instruction or information goal is
general education as it relates to aviation.

The advice on technical matters received from the
Civil Air Patrol Headquarters, Office of Operations and
Training, and the advice on educational matters provided
b y t h e Av i a t i o n E d u c a t i o n P e r s o n n e l h e l p e d m a t e r i a l l y
in the preparation of this book. Special acknowledgem e n t i s a l s o d u e t o M r. I r v i n g R i p p s o f t h e C i v i l A i r
Patrol, Office of Information Services, and to contributing
members of the Civil Air Patrol National Commander's
Aviation Education Committee for suggestions and advice
offered. The names of the members of the three groups
mentioned above appear elsewhere in this booklet.

MERVIN K. STRICKLER, JR.
D i r e c t o r o f Av i a t i o n E d u c a t i o n

CHAPTER
OE
N

Airport growth and Federal Government aid.

G O T A DD V L P E T
R WH N EEOMN
You have heard it said that an airplane can travel from any point
on the surface of the earth to any other such point. In theory, this is
t r u e . I n t h e o r y, s u c h t r a v e l c a n f o l l o w a d i r e c t r o u t e . T h e r e a r e n o
surface obstacles such as mountains and deserts in the path of an
airplane's flight.
The need for airports.
In practice, however, aircraft need airports and air lanes, just as
ships need seaports and sea lanes. In the days of the pioneer pilot
when aviation was very young, airplanes were all small and any
c o w p a s t u r e c o u l d b e u s e d a s a " fl y i n g fi e l d . " H o w e v e r, w h e n t h e
value of aircraft as a practical mode of transportation was recog-

It is likely that airport growth would have proceeded at a less
rapid rate had not the Federal Government aided in the construction
of airports. Such aid was first made available in the early 1930's.
H o w e v e r, i t w a s n o t u n t i l t h e p a s s a g e o f t h e F e d e r a l A i r p o r t A c t o f
1946 that a substantial aid program was set up. This act authorized
a maximum expenditure of one-hundred-million dollars per year for
airport improvement and construction. The Federal Aid Airport
Program for the fiscal year 1958 involved 334 construction and
improvement projects and the expenditure of over $55 million of
Federal funds. The Federal Aid Airport Program of the fiscal year
1959 included 358 construction and improvement programs and an
increase in Federal Government expenditure to approximately $64
million.

nized and when the military potential of aviation was understood,
large airplanes began to be built. Airport growth had to keep pace
with aircraft development.

Airport growth.
Early airports were merely sod fields designated as airplane
landing areas. When these proved inadequate, cinder and gravel
runways up to 1500 feet in length were constructed. These were
laid out so that aircraft had a choice of landing direction and
could land into the prevailing wind. As larger aircraft were built
and these airports, in turn, became inadequate, runways were
lengthened and surfaced with concrete.
In 1928, only twenty-five years after the historic flight at Kitty
Hawk, there were 1,300 airports of all types in the United States.
Two years later, there were 1,800 airports of all types. However,
only 600 of these were lighted and could accommodate night
time, flight operations. Today there are about 7,000 airports of all
types serving the cities and towns of this country. As a matter of
fact, an airport of some type can now be found at intervals of 15
to 30 miles along most designated airways.
A modern airport.

Continued airport growth and development.
Airport growth is likely to continue for many years to come.
Modern transport planes, powered by reciprocating engines, land
at speeds up to 100 mph. In a few years, after jet propulsion comes
into greater use, landing speeds of transport aircraft are likely to
increase to 125 mph. A 100 mph landing speed requires a runway
of approximately 6,000 feet. A landing speed of 125 mph will
require a runway between 8,000 and 10,000 feet long depending,
of course, upon whether or not improved changes are made in
the landing and braking techniques now employed.

Lighting of airways by powerful electric searchlights was begun the
n e x t y e a r. B y 1 9 2 6 , a l i n e o f t h e s e a i r w a y b e a c o n s , p l a c e d e v e r y
ten or fifteen miles along the airways, extended from coast to coast.
On a clear night, a pilot could see three or four lights ahead and,
by keeping these in line, could stay on his course.
There were also route identification and course lights installed on
the beacon towers. Identification lights, using the international
Morse code, flashed an assigned signal at regular intervals. Course
lights were fixed so that one pointed one way along the airway and
another pointed in the opposite direction.

The purpose of airways.

The radio signal and airway marking.

Designated airways lead from airport to airport. They are marked
to help the pilot or the navigator keep the aircraft on its course.
Improvements in airway marking are made necessary by developments in aircraft and airport construction.

In 1919, two years before the first night flight of air mail, a
research project in the use of radio in airplanes was undertaken
b y t h e U n i v e r s i t y o f M a r y l a n d . H o w e v e r, i t w a s n o t u n t i l 1 9 2 7
that radio ranges were used to supplement light beacons. The early
use of these radio range stations was limited. Only seventeen were
o p e r a t i n g b y t h e c l o s e o f 1 9 2 7 . To d a y, o v e r 4 0 0 a r e i n o p e r a t i o n .
The marking of airways with radio broadcasting facilities progressed
until by 1940 there were about 40,000 miles of air routes defined
by four-course radio ranges.

Early methods of airway markings.
In 1921, when Jack Knight made the first scheduled night flight,
carrying mail from North Platte, Nebraska, to Chicago, Illinois, the
flight path he followed was marked by bonfires on the ground.

Lowfrequencyomni-directionalrange
(Intercontinental Navigation)

BURMUDA
AZORES

The growth of air marking.

A new system of airway marking.
The early 1940's saw the use of the four-course radio-range
station* increase until at the end of the year, 1946, there were
about 346 of these in operation. However, the number of aircraft
using the airways also increased. Consequently, beginning with
1946, serious air-traffic problems began to emerge. After careful
study was made of airway traffic problems and research and
experiment had proven the capabilities of new electronic devices,
a new system was put into use.
The new method of marking the airways of our nation could not
replace the old methods all at once for some very practical
reasons.
One of these reasons was economic. Vast sums of money would
be required to purchase new radio transmitting equipment for
airways and radio receiving instruments for aircraft. Another
reason was educational. Those using the airways had become
accustomed to one system; they would have to learn to use a new
one. The advent of jet-propelled aircraft and airway congestion
resulting from increasing growth of both military and civil aviation
made imperative adoption of modernized navigation and airway
control procedures.
Rotating beacon lights still mark some airways; four-course
airways are still marked by four-course range stations; yet, new
designated air routes marked by radio-range stations which use
equipment based upon the very high-frequency part of the radio
spectrum are rapidly coming into use. About 500 VOR stations
had been installed by 1956 and the Federal Government's airtraffic program called for installing several hundred VORTAC
stations during the three years following.
" S e e C h a p t e r V, p . 3 1 .

CHAPTER
TWO

Land Civil

Land Joint
Civil & Military

T EC A TN
H H RI G
O AR O T
F I P RS
ANDAR A S
I WY

Land Military

The importance of chart symbols.
Aeronautical charts reveal much about airports and airways. For
this reason and others, they are indispensable to pilots. However,
before a pilot can use a chart efficiently, he must know how to read
the symbols which he finds upon it.
It is important that you understand about the growth and developments of airports and airways. It is of greater importance that you
are able, when the need arises, to learn other things about them
from an aeronautical chart. In order to do this, you, like the pilot,
must become able to interpret the symbols that the chart-maker uses.

Wafer Civil

Classification of airports.
The chart-maker calls airports and airbases (military airports)
aerodromes. In addition to the method of airport classification which
divides airports into public airports and limited airports, there are
other methods of classifying aerodromes. (1) Airports or aerodromes
are either for land planes, or sea planes. (2) They are civil, military,
or joint (used by both civilian and military aircraft). (3) They are

Water Joint
Civil & Military

Water Military

Radio Range

0

for heavier-than-air craft, lighter-than-air craft, and helicopters. (4)
They are equipped with traffic control and service facilities, or they
are emergency aerodromes with no facilities. (See page 9.)
The chart and airport information.
You will remember that there is a relationship between air density and the
altitude at which an airplane operates. At sea level airports the air is more dense
than it is at airports having elevations of 500 feet, or more; for, as you
remember, the higher the altitude, the less dense the air. You will also remember
that air density has a relationship to lift, and that the less dense the air, the
longer the "take-off" run or "landing-roll" required for aircraft operation. For this
reason the pilot needs to know the elevation in feet of the airport he approaches.
There are other things he must know about the airport. His chart will tell him
about most of these. His flight information manual, a Federal Aviation Agency
publication, will give him information about the others, such as for example,
recent changes in airports and airways facilities. The pilot can learn from his
chart whether or not an airport is adequately lighted for night flight operations.
He can learn the length of its longest runway, whether or not it has low visibility
approach systems, and whether or not there is a direction finding station at the
airport. (See page 30 for explanations of these traffic control facilities.) He can
also learn from his chart the transmitting frequencies of the control tower, so that
he can "tune-in" the proper frequency and receive information and instructions
from the tower.
The symbols used to disclose airport information are presented in the following
illustration. You will note that runway length is given in hundreds of feet and that
a dash in the place of a symbol means that the facility in question is not
available.

908

Elevation in feet

L

Minimum Lighting

H

Hard Surfaced runway

41

The chart and airway information.
The radio ranges, beacons, communication stations and
broadcasting stations are also shown on charts. Generally such
stations are located near airports. If a radio facility uses the low/
medium radio frequencies (LF/MF) the chart symbol will be printed in
red; if it uses the very high radio frequency (VHF) the symbol will be
printed in blue. If voice broadcast is included in the service provided,
the word, "Radio," will follow the name of the airport near which the
station or beacon is located.
Airways marked by LF/MF range stations are designated by the
colors, Red, Green, Blue, and Amber and a number. Such an airway
might be designated as Green 5 or Red 68. Airways marked by a
VOR (Very High Frequency Omnirange) are called Victor airways.
They are designated by the letter V and a number. Victor airways
which extend in an easterly-westerly direction employ even numbers;
those in a northerly-southerly direction, odd numbers. Other
controlled civil airways indicate direction by use of names of colors.
Red and green airways extend easterly-westerly; blue and amber,
northerly-southerly.

Length of Longest runway
in hundreds of feet

Chart symbols of surface obstacles and caution areas.
A l t h o u g h t h e p a t h t h a t t h e a i r c r a f t fl i e s i s t h r o u g h t h e a i r, t h e
airplane pilot must still pay heed to many kinds of surface obstruc-

tions. Some of these are natural obstacles to safe flying, such as
mountains; others may be obstructions which are man-made,
such as high towers and transmission lines. The chart symbols
for these must be readily recognized. Caution areas, danger,
restricted, and warning areas, and prohibited areas are all
significant in terms of airway mapping. (See illustration below.)

Points along the airways at which aircraft in flight may report their
position to radio communication systems are indicated by a
triangular marking; such points at which reports are compulsory
are indicated by a solid color, triangular symbol.
Moreover, an aeronautical chart indicates by blue tint all areas
where air traffic is controlled. The rules for such control are
explained in Chapter 6.
In view of the fact that omni-directional ranges reveal to the pilot
the directions (magnetic bearing) of his aircraft from omni-range
stations, a pilot can easily find his position in relation to the airport
he wishes to reach. (See page 32). Such a circumstance might
appear to lessen the need for designated airways. This is not the
case. In order to prevent air traffic conflicts, scheduled air traffic
must be controlled. This can be done only by means of the
electronic facilities of airways and airports. The following chapters
discuss some of the scientific principles upon which these
facilities are based, briefly describe the operation of them, and
outline the traffic rule enforced by means of them.

PROHIBITED AREA
Flight of aircraft
prohibited except by
specific authority of
using agency.

DANGER.
RESTRICTED OR
WARNING AREA
Invisible hazards to
air navigation.

CAUTION AREA
Visible hazards
to air navigation.

ELECTRONICSINA TION
VIA
The principles of radio make modern aviation possible. Radio
signals mark the airways. By means of radio, airplane pilots are
g i v e n t a k e - o ff a n d l a n d i n g i n s t r u c t i o n s . W h e r e v e r a i r c r a f t fl y, r a d i o
provides a means of ground-to-air and air-to-ground communication.
Flight plans are filed, weather information is dispersed, and traffic
is controlled---all by means of radio. Radio signals help the pilot
k e e p t h e p r o p e r c o u r s e , a n d r a d i o , r a d a r, a n d i n s t r u m e n t s w h i c h
make use of radio signals guide him to safe landings, although
clouds and fog may hide the airport and obscure his vision.

How electrical energy gets from transmitter to receiver.
You remember from your study of magnetos that to open and
close a switch in the primary circuit would cause a magnetic
field to build up and collapse around the primary coil of this
circuit. The movement of the lines of force that occurred as a
consequence, induced voltage and alternating movement of
electrons within the secondary circuit. In the magneto, the
primary and secondary coils were adjacent one to the other.
However, the effect would have been the same had the coils
been placed a considerable distance apart.

The basic principle of induction

Magnetic lines of force.

The principles upon which radio is based are really quite simple.
Electronic devices seem hard to understand chiefly because the
t h i n g s w h i c h m a k e t h e m w o r k a r e i n v i s i b l e . We c a n n o t s e e a t o m s

The magnetic lines of force around a bar magnet or a coil take
a c i r c u l a r p a t h f r o m o n e o f t h e p o l e s o f t h e m a g n e t t o t h e o t h e r.
H o w e v e r, w h e n t h e fl o w o f e l e c t r o n s i s t h r o u g h a l i n e a r c o n d u c t o r

and electrons; neither can we see electric currents nor radio waves.
Yet, even if we cannot see them, we can get some ideas about them
f r o m t h e t h i n g s t h e y d o w h i c h w e c a n s e e , f e e l , o r h e a r.
Through applying the principle of induction (see Power for Air.
craft, page 30), a radio transmitter changes sound waves to audio
waves. It then causes radio-frequency waves to carry the audio
impulses (waves) through space, enabling a radio receiver tuned
to the proper frequency to pick up the carrier wave and the audio
impulses it carries, and to change these again into sound waves
which you, the listener, may hear. Other electronic appliances make
similar use of radio-frequency waves (electro-magnetic oscillations)
which induce voltage within a secondary circuit.

s u c h a s t h e a n t e n n a o f a r a d i o t r a n s m i t t e r, t h e l i n e s o f f o r c e a r e
circular and radiate outward in the form of waves, much as surface ripples move when a stone is dropped into a lake.

The radio in modern aviation.

The electronic flow induced into a radio receiver has a very
s p e c i a l t a s k t o p e r f o r m ; c o n s e q u e n t l y, t h e p r i m a r y c i r c u i t w h i c h
transmits the radio-frequency waves modulated by the audio waves
must be provided with some special kinds of controlling devices.
Among these devices are found resistors, induction coils, condensers, transformers, and electron tubes. Resistors offer resistance
to the alternating flow of electrons in a circuit. (An electric light in a
circuit is one kind of resistor.)

GENERATOR
(Pressure)

COIL

RESISTOR

(inductance)

(Resistance)

non-conductor between them. Such a condenser may be of the fixed
or variable type. The area of its plates determine the capacitance
value of a condenser. The greater the capacitance of a circuit, the
lower the frequency of its current's oscillations. Consequently, if the
variable condenser of a circuit is turned so that it has a large plate
area, the circuit will be tuned to a correspondingly low frequency; if it
is turned so that it has a small plate area, the circuit will be tuned to
a correspondingly high frequency. A condenser acts as a broken
circuit to a direct current and as a low resistance resistor to a lowfrequency, alternating current. To a high-frequency, alternating
current, the condenser appears to have no effect. The frequency of
the radio waves of a circuit is controlled by the relationship of
inductance to capacitance.

VARIABLE CONDENSER

Important among the elements of electronic circuits are transformers
and electron tubes. A transformer is a form of induction coil. You
remember that the ratio between the number of turns of wire in a
primary and the number in its secondary coil determines the change
in voltage between the two coils. The function of transformers in the
electronic circuit is to increase voltage.

(Capet;fence)

The scientific device which is perhaps the most important in modern
radio communication and which has made possible many developments in electronic aids to aviation is the electron (or vacuum) tube.
Yo u k n o w t h a t r a d i o w a s s u c c e s s f u l l y u s e d b e f o r e e l e c t r o n t u b e s
were invented, but without the electron tube, its use was quite limited.
The induction coil provides inductance in a circuit. Inductance
is the property of a circuit that tends to oppose a change in the
direction of an existing current. It is present only when the current
is changing. The induction coil acts as a high-resistance resistor to
an alternating current, as a low-resistance resistor to a direct current, and as a break in the circuit to very high frequencies.
The condenser provides capacitance in a circuit. Capacitance is
the ability of a circuit to store up an electrical charge. In an electrical circuit, a condenser is composed of two sets of plates with a

The fundamental principle of the electron tube is that the heating
of a metal causes it to give off electrons. Electrons appear to
escape from heated metal much as water molecules evaporate when
w a t e r i s h e a t e d . E d i s o n i n 1 8 8 3 w a s t h e fi r s t t o n o t e t h e e ff e c t o f
e l e c t r o n s g i v e n o ff b y a h e a t e d w i r e .
Following Edison's observation, an Englishman named Fleming discovered that if he placed a filament (a loop of very fine wire) and a

N E G AT I V E

DIODE
metal plate within a vacuum tube, electrons would flow when the
fila- ment was heated and the plate was positive. He also
discovered, when he made the plate negative, that although
electrons still boiled off the heated filament they did not flow but
gathered around the filament. (Unlike attracts, like repels.) Thus
Fleming had found both a third method* by which electrons can
be made to flow and also a method of changing alternating
electrical currents to direct electrical currents.
The nature of the triode electron tube.
In 1906, an American, Lee De Forest, added a third element to
the vacuum tube. This element is called a grid. The three-element
electron tube is called a triode. The development of the modern
triode electron tube makes possible long distance radio
transmission and many other electronic miracles. In the interest
of refining the operation of the triode, other elements have been
introduced into the vacuum tube: a four-element tube is called a
tetrode; a five element tube, a pentrode.
The grid.
The grid is a small coil of wire placed between the filament and
the plate of the electron tube. When it is given a small positive
charge, the flow of electrons from heated filament to plate is
increased; consequently, the plate current is increased. When it is
given a negative charge, the electrons will be repelled and remain

* The other two methods are induction (as in a generator, magneto, and induct i o n c o l ! ) a n d c h e m i c a l a c t i o n a s i n a n e l e c t r i c b a t t e r y.

TRIODE
c l u s t e r e d a r o u n d t h e fi l a m e n t ; c o n s e q u e n t l y, t h e p l a t e c u r r e n t i s
decreased. These properties of the of the grid make it a control
device.

W h y t h e g r i d c a n b e u s e d a s a c o n t r o l o f e l e c t r o n fl a w.
By regulating the positivity or negativity of the grid, the number
of electrons entering the plate circuit can be controlled. A small
change in voltage will cause a large change in the plate current.
These properties of the grid make it a control device and enable
the "electron tube to perform its three most important functions in
radio: oscillation, amplification and detection.

The stages of sound transmission.

In the first stage of sound transmission, sound waves strike the
diaphragm i of a microphone causing varying degrees of pressure
upon carbon granules through which flows an electric current.
The degree of flow of this current is directly proportional to the
degree to which the granules are compressed. The more they are
compressed, the more the current flow; the less they are
compressed, the less the current flow.
It is this effect of the sound waves upon the microphone circuit
that changes sound waves into audio waves. The voltage of the
microphone circuit is now "stepped-up" by a transformer (see
page 17) and fed into the grid circuit of the electron tubes.

HERTZ
OSCILLATOR

MODULATED RADIO FREQUENCY WAVE

RADIO FREQUENCY
OSCILLATOR TUBE

AMPLIFIER TUBE

In the second stage of sound transmission, the voltage variations
of the microphone circuit, which correspond to the sound frequency
(the audio wave) reaching the grid circuits of the oscillation tubes,
cause variation in the amplitude of the radio-frequency wave (the
carrier wave). These variations, obviously, correspond to the shape
of the audio wave. Thus, electrical impulses representing sound
waves are impressed upon the radio carrier waves. This process
is called modulation. When the amplitude of the radio frequency
wave is varied, the process is called amplitude modulation (AM.)*

Heinrich Hertz (1889) developed the first device that would produce
electro-magnetic waves that could be detected. This device called
an oscillator consisted of an induction coil and a spark gap.
When the induction coil was energized, the electrical energy would
discharge across the gap. It was this discharge that could be
detected. Radio frequencies are generated by electron tubes in
much the same way that electric fluctuations were produced by the
Hertz oscillator.

Between 1935 and 1940, Molar E. H. Armstrong devised a radio transmitting system
which depends not upon amplitude changes, but upon frequency changes in the carrier
wave. When the frequency of the radio wave is changed by the effect of the radio wave
upon it, the process is called frequency modulation (FM). (See illustration page 21)

You remember that radio waves lose strength after they leave the
transmitting station. Consequently, it is necessary that they be amplified by the radio receiver. This is accomplished by connecting the
receiving antenna to the grid of an electron tube known as the
a m p l i fi e r. T h e a l t e r n a t i o n s o f t h e s i g n a l s c o m i n g t h r o u g h t h e
antenna will be amplified as the grid voltage of the amplifying tube
a ff e c t s t h e c u r r e n t o f t h e p l a t e c i r c u i t . T h i s t y p e o f a m p l i fi c a t i o n
is called radio-frequency amplification.

When an electron tube is operated at a proper, negative, grid
voltage, vibrations are caused in its plate current corresponding to
the audio wave carried by the radio frequency waves. This process
which separates the audio wave from the carrier wave is called
demodulation.
After the radio frequency wave has been demodulated, the audio
wave obtained is put into the grid circuits of audio amplifying tubes.

AMPLITUDE MODULATION
(Constant frequency. Changing
amplitude) VOICE SIGNAL

LOW VALUE Small change in
frequency

HIGH VALUE Large change in
frequency

FREQUENCY MODULATION (Changing frequency, Constant amplitude)

AMPLIFIER
DETECTOR
Audio Signals

These tubes amplify the audio frequency waves

just

as the radio

frequency amplifying tubes amplify the radio frequency waves. The
diaphragm of loud speakers or pilots' headphones then convert the
amplified audio waves into sound waves.
Electronics is a young and fast growing science.
Each day sees new electronic appliances developed and new
industrial uses made of these. One school of thought is of the
opinion that within a comparatively short period of time, aggressor
nations will be deterred, not by fighter aircraft manned by pilots,
but by electronically guided missiles that always reach their target.
Recently electronic research has made discoveries which are helping to improve electronic instruments now in use. For example, from
a substance called germanium, very small devices called transistors
can be made. When installed in an electronic circuit these will do
much of the work previously done by the electron tube. From substances such as quartz and barium titanate, other small devices
called transducers are made. These have made it possible to create
an entirely new technological field--ultrasonics*. Should you in
the future acquire a special interest in electronics, the study such an
interest motivates will open for you the doors to a world of fascinating marvels.
* U l t r a s o n i c s fi n d s m a n y u s e s i n i n d u s t r y. O n e o f t h e m o s t i m p o r t a n t o f t h e s e i s
to drive the drills and cutters which shape brittle and hard-to-machine materials.

CHAPTE
R FOUR

Why Airports Are Necessary
The modern airport terminal building.
The departing airline passenger arriving at the airport of one of our
larger cities finds much activity there. His airport limousine may be one
of half dozen or more unloading passengers at one entrance while still
other limousines and taxis load incoming passengers at another. An
attendant takes his luggage and guides him to the proper airline
counter, where a courteous and uniformed ticket agent verifies his
reservations, checks his baggage, and places it on a conveyor which
whisks it rapidly out of sight.
After receiving his "gate-pass" and information about the departure time
of his flight and how to reach the proper aircraft, the passenger may
look around him. He will be in the airport terminal building. That which
he sees will have been arranged to accommodate him and other air
travelers. He will find vending machines which sell a variety of things
from hosiery to insurance. He can purchase his stamps from a vending
machine, or at some airports he may patronize the Post Office which
will be found within the building.
In the most modern of airport terminal buildings, he will find a news
stand, a book store, a drug store, a gift shop, a barber shop, a beauty
shop, a snack bar, a restaurant, clothing stores for men and women, a
cocktail lounge, and perhaps a club room. At any airport terminal
building he will find ample waiting room and passenger
accommodations.
The departing passenger will hear announcements of incoming and
departing flights. If he looks out of the window onto the parking apron,
he will observe these incoming and departing aircraft. Lines of
passengers will be leaving, others will be boarding the aircraft which he
sees. He will see luggage unloaded and loaded as planes discharge
their passengers or are made ready for take-off. He will observe people
fueling aircraft. He will see still other people performing still other tasks.
Some of these tasks he may not understand. After his flight is
announced, he surrenders his ticket or gate pass to an airline attendant
and boards his plane. He is greeted by a smiling stewardess and
departs the airport unaware of many airport activities designed to make
his flight as safe as his departure was pleasant.

Some of these "behind-the-scenes" airport activities are called
airport operations. These operations may be conducted by a
commercial aviation business, such as a scheduled airline, a
charter service, a flight school, or a maintenance and repair service
based at the airport. Aviation businesses conducted at the airport,
other than those of the airlines, are called fixed base operations.

Behind the scene, airport activities include the work of administering
the business affairs of the airport itself. Many business details
engage the attention of the airport administrator and his staff. These
range from the collection of rentals from those who use the airport's
facilities to the construction of airport buildings and runways.

When the airline passenger requests a reservation, he is told
either that space is available and that his reservation can be
confirmed or that no space is immediately available and that if he
chooses he may obtain a reservation on an alternate flight. Before
flight information can be given to passengers, rather detailed and
exact records Must be kept by the telephone-sales section of the
airline. Sometimes this service is located at the airport. When
such is the case, like many other airline tasks which keep
transport aircraft flying and passengers happy, it escapes notice.
Generally, when the airline passenger thinks about the work of
the airline, he thinks of the pilot and perhaps the mechanic. Yet,
fewer than 10% of airline employees are pilots, and fewer than
25% are mechanics.
(See Aviation and You, pp. 19-21 .)
“Behind the scene'' airport activities.

Government airport eperations.
Behind-the-scene, airport activities include certain services provided to all types of aviation by the Federal Government; for aviation in the few short years of its existence has become very essential
both to our national economy and to our national defense. Without
the Federal Government services housed in airport buildings, modern
air transportation could not exist. These services are made available by the Weather Bureau, by the Federal Aviation Agency (FAA),
and at military air bases by the U. S. Air Force and Naval air operations. They range from the briefing of pilots planning a cross-country
flight to the installing and operating of radio-range stations and
approach control and aircraft landing systems.
The pilot and flight assistance service.
Imagine that you are a pilot planning a flight to a distant airport.
Under such a circumstance you will need to know certain things.
F i r s t o f a l l , y o u w i l l n e e d t o k n o w a b o u t t h e w e a t h e r.
W i l l i t o ff e r a n y o b s t a c l e s t o y o u r fl i g h t , r e q u i r i n g y o u t o fl y a
longer route or to use special flight instruments en route? Then,
you will want to know what radio facilities along your roule are
operating; what runway conditions you may expect at your destination, and what service facilities are available at the airport of your
destination. Flight assistance service will give you answers to all
these questions and even help you prepare and file your flight plan.
The radio and flight assistance service.

A i r Tr a f fi c C o n t r o l ( AT C ) .
A i r Tr a f fi c C o n t r o l f a c i l i t i e s a r e l o c a t e d a t m a j o r a i r p o r t s . O n e
o f t h e s e i s t h e a i r p o r t t r a f fi c c o n t r o l t o w e r. T h e o t h e r i s t h e a i r
route traffic control center.
Airport traffic control towers are often provided with approach
control facilities for instrument landings. They are designed so that
on clear days nothing obstructs the control tower operator's view of
the airport or the sky above it. Approach control radar provides
the controller with a means of observing the position of aircraft
when clouds cover the sky. Consequently, the men or women working in the control tower are able to tell the pilot when, without fear
o f c o l l i d i n g w i t h o t h e r a i r c r a f t , h e c a n t a x i i n t o t a k e - o ff p o s i t i o n ,
t a k e - o ff , l a n d , a n d a f t e r l a n d i n g , t a x i t o t h e r a m p o r h a n g a r.
Approach Control.
On days when the weather is such that visibility is obstructed,
A p p r o a c h C o n t r o l i s o p e r a t e d b y t h e c o n t r o l t o w e r. A p p r o a c h C o n t r o l m a y u s e s e v e r a l d i ff e r e n t k i n d s o f r a d i o a n d o t h e r e l e c t r o n i c
devices. Each of these serves a specific purpose and is used in
c o o r d i n a t i o n o n e w i t h t h e o t h e r t o h e l p t h e p i l o t l a n d s a f e l y.
Airport Surveillance Radar (ASR) is an electronic device used by
Approach Control to learn the bearing (direction) and distance from
the airport of the incoming aircraft before it begins its "let-down"
approach. This information is given to the pilot by" radio. Precision
approach radar (PAR) is an electronic device that shows to the PAR
operator in the tower the position of an incoming airplane on its
glide path. By the use of radio, the operator on the ground gives

Once you are en route, you may still use the flight assistance
service, if your airplane is equipped with a radio in operating condition. Twice each hour, at 15 and 45 minutes past the hour, weather
r e p o r t s a r e b r o a d c a s t w h e t h e r y o u a s k f o r t h e m o r n o t . Yo u m a y,
h o w e v e r, u s e y o u r r a d i o t o a s k f o r w e a t h e r i n f o r m a t i o n a t o t h e r
t i m e s . Yo u m a y a l s o r e q u e s t o t h e r i n f o r m a t i o n , s u c h a s h e l p i n

proper landing directions to the pilot of the incoming aircraft. The
pilot of the landing aircraft will also use electronic devices called
I o c a l i z e r, g l i d e s l o p e f a c i l i t y, a n d I L S ( i n s t r u m e n t l a n d i n g s y s t e m )
markers. (See p. 35.)

establishing your position, the condition of radio aids along your
route, and the general conditions at the field where you are to land.
It is best not to call for information too near the time of the regular
weather report broadcasts. But at all times, flight assistance is no
further from you as a pilot than is your radio transmitter-receiver.

Air route traffic control centers.
The air route traffic control centers supervise air traffic within
control areas. The control area proper is a designated civil airway
which has necessary control facilities. It is generally defined as an
airspace ten miles wide extending upward from 700 feet above the

surface. However, control areas may be expanded to include areas
near airports not on the airways over which aircraft fly on IFR (instrument flight rules). The control facilities administered by air route
traffic control centers, like the facilities of communication stations and
the airport traffic control tower and its approach control, employ
electronic devices adapted to special purposes.

Airline communication services, which supplement some of the
Federal Government services, may be found at many airports.
Every effort is made by both industry and government to make
flying safe and to provide for the growth and development of
aviation. Just as the FAA and the Weather Bureau in the interest of
aviation safety use both electronic and land-line communication
systems, the airlines also use such systems. The airport is the
center from which radiate all of these services, whether provided by
government or industry. The fact that the airport is an aviation
administration center employing many devices based upon the
discoveries of modern science may escape the attention of the
casual visitor and the airline passenger.
Yet, these activities are most significant. The airport, whatever its
size and whatever activities are found there, has as its primary
purpose to provide a take-off and landing area for aircraft. Yet,
were it not also used as a center from which services, in the
interest of safe take-off, sale flight, and safe landing, are available,
civil and military aviation as we know it would not exist; there would
be little incentive for practical aviation pursuits; and our national
defense and economy would suffer.

AR A S
I WY
and the facilities of air traffic control
All-weather flight,
If aircraft could fly safely only when the pilot could see the ground
below him and other aircraft in the sky around him, airways and the
methods used to control the traffic along them would be comparat i v e l y u n i m p o r t a n t t o a v i a t i o n o p e r a t i o n s . H o w e v e r, t h e p i l o t o f a n
airplane must be prepared to guide his aircraft when he can see
nothing ahead of him nor below him except cloud or fog.
Under these and similar conditions, he must avoid other aircraft
in flight and keep a safe distance from mountains, radio towers, and
other such obstructions. He must also fly directly to his destination,
know when to begin his descent so that he can reach the airport
runway, and complete his landing safely. Radio and electronic aids
make these things possible.
Radio aids and electronic devices.

Radio aids have helped make flying safe since they were first
introduced in aviation~ You have learned, however, that the
increase in number of modern aircraft in operation has made it
necessary to adopt improved radio devices. (See page 6.)
Although older radio aids are still used, modern aircraft make
increasing use of modern electronic devices. Such devices are
installed and maintained by the FAA, Office of Air Navigation.

megacycles (very high frequency). Some military radio communication facilities operate on frequencies above 200 megacycles.

Radio ranges and airways.
Radio ranges, you remember, actually tell the pilot where the airway
is located. The low frequency, range station transmits signals in such
a way that overlapping of signals defines four courses. The high
frequency, range station transmits signals in such a way that, no
matter what the flight position of an aircraft is, the pilot can always
tune in on a direct course to the station near his destination.

The low frequency, range station.
The low frequency, range station (LF/MF) has one advantage
over the high frequency, omni-range station (VOR) which will
likely keep many LF/MF range stations operating for many years
to come. Low frequency broadcasts can be tuned in at low
altitudes and can be received on the ground. High frequency
broadcasts like television broadcasts follow a "line-of-sight"
direction. The curvature of the earth obstructs VOR signal
reception by aircraft flying at low altitudes.
LF/MF radio range signals are broadcast on frequencies from 200
to 415 kilocycles. Each range station is assigned a three-letter
identification signal which is transmitted at 30 second intervals.
Weather information is also broadcast by these stations.
Through the use of special antennas the Morse code for A (- —,
dit da) is directionally broadcast into two opposing quadrants of

Air-ground communications.

As a matter of fact, air-ground communication made possible by
the two-way radio is indispensable to modern aviation. By means
of radio the pilot may, through the communication stations along his
route, ask for and receive instructions from traffic control centers
and from weather service. Aircraft radios which have been put into
most recent use, operate on frequencies between 108 and 150

The four-course radio range.

the range and the Morse code for N (— -, da dit) is broadcast into the
other two quadrants. These signals are of equal amplitude and
interlock along the quadrant boundaries to make a steady tone which
defines the course. This tone is continuous except when broken to
accommodate station identification signals and weather reports. In
order to receive the signals transmitted by these stations, the radio
receiver must be tuned to the proper frequency. This frequency can be
found by referring to the appropriate aeronautical chart. (See page 11.)
As the distance from an LF/MF radio range station increases, the beam
it transmits fans out and its signal begins to fade. However, as the
distance from the range decreases, its beam narrows and its signal
becomes stronger. Directly over a station is a cone of silence.
An aircraft entering a cone of silence will receive no course signals.
Voice broadcasts, however, will still be received.

The omni-range station (VOR) sends out two signals in the
108-118 megacycle radio-frequency band. One of these, from the
center antenna of the five antenna group, is non-directional. It
radiates in the form of a circle expanding and contracting exactly
30 times each second. The second signal, from the four corner
antennas, forms a figure eight pattern that rotates clockwise 30
times each second with the center antenna as its center of
rotation. The first signal is called the reference-phase signal; the
second, the variable phase signal.
The VOR range is set so that the variable-phase signal pattern
points toward magnetic north at the exact time the referencephase signal reaches its maximum expansion. At this time, on a
line (or radial) extending between the VOR and magnetic north,
the two signals are in phase (match) with each other. All other
radials (lines extending from VOR) are in different phase. The
VOR receiver is able to measure the degree of this difference and
indicates it on the dial of a visual indicator on the aircraft
instrument panel.

So that you can better understand how VOR works, let us compare it with a special kind of airport beacon which does not actually

Ve r y h i g h f r e q u e n c y o m n i - r a n g e .

exist. However, suppose a fixed, green, beacon light flashed every
time that the rotating light beam of this imaginary beacon sweeps
past magnetic north. Assume that the beam rotated once each 10
seconds. In that event, since there are 360° in a circle, it will pass
through 36° each second
It is possible for a pilot to
find his direction from this beacon (1) by starting a stop watch upon
seeing the green light and stopping it when the beam sweeps past
him and (2) by then multiplying the number of seconds shown on
the stop watch by 36% If the pilot's stop-watch shows 7.5 seconds,
his position with reference to the beacon will be 270. That is, he will
be directly west of the beacon. The VOR receiver is a kind of
combination stop-watch and calculating machine which uses radio
signals rather than light signals as a basis for its computations.

Although the radio range stations tell a pilot of an aircraft in flight
where the airways are located and the direction of airport from his aircraft, they give him no information as to his distance from the airport.

He must depend for this information upon his skill in navigation
and the knowledge he has of the direction and strength of winds
which may affect his speed over the ground. Or, if his aircraft is
properly equipped, he may use electronic devices called markers.
There are two principal types of markers used for this purpose—
fan and Z markers. Both types of markers operate on the
frequency of 75 megacycles. The fan marker is 3 miles wide and
12 miles long across the airway near the earth's surface.
However, at higher altitudes, the signals spread out until their
effect covers a correspondingly greater area. The Z marker is
always located at the site of an LF/MF radio-range station. It
radiates a cone shaped signal vertically above the range station.
(See page 32.) If the marker receiver of the aircraft is tuned to 75
megacycles, the pilot receives a signal when his aircraft flies over
a marker. He will hear a tone, and the marker light will glow.

Flight position.

The pilot who uses a VOR receiver can use this to find his
position along his route. He knows the bearing he is flying. He
can tune his receiver to another VOR station to the right or left of
his course and take a bearing on this VOR station. If he plots the
two bearing lines on an aeronautical chart, they will intersect.
(See illustration page 9.) The point at which they intersect is his
chart position. From this and other chart information the distance
of the aircraft's position from the first VOR station may be found.

Within the last few years an electronic speedometer for aircraft
has been developed. This device is called Distance Measuring Equipment (DME). In the cockpit of an aircraft using DME, the pilot can
learn from an indicator his distance to or from a VOR station.
DME requires receiver-transmitter combinations on both ground
and in the aircraft. It operates at 1,000 megacycles. The ground
equipment is called a Transponder; the airborne equipment is called
a n I n t e r r o g a t o r. D M E e q u i p m e n t i s v e r y i n t r i c a t e . W h e n o n e t h i n k s
of the speed at which ultra-high frequency radio waves travel (approximately at the speed of light--186,000 miles per second) it
seems incredible that DME can measure the time required for the
radio impulse to travel a few miles; yet it is able to do this task.

VORTAC.
When TACAN (tactical air navigation) is combined with VOR, the
installation is termed a VORTAC system. TACAN is an air navigation
system developed by the military services. The VOR feature of
VORTAC will provide directional information to civil aircraft. The
TACAN feature will provide directional information to military aircraft
and distance information to both civil and military aircraft.
Eventually the TACAN feature of VORTAC stations will replace DME
facilities.
The Course Line Computer.
Should a pilot wish to fly his aircraft on a path that does not pass
directly over a VOR or VORTAC station, he can still make use of the
station. The device that makes this possible is called the Course
Line Computer.
The Course Line Computer by means of information fed to it
electronically by VOR ground equipment actually solves a pilot's
navigational problems. The pilot simply sets the proper information
into the computer, then flies the needle [heads the aircraft so that
the indicator needle is properly centered) to his destination. For all
practical purposes, he has apparently moved the VOR station from
its location to the point of his destination. One type of course line
computer, called a pictorial computer, shows the pilot his position
and progress by means of an indicator which moves across a chart
as the aircraft moves through the air.
Other electronic instruments.
If low-visibility weather conditions prevail at the time an incoming
flight reaches an airport, other electronic equipment the aircraft
carries may be put into use. Localizers, glide slope indicators, and
marker detectors receive and interpret signals from ground
installations. (See page 27.) The Iocalizer gives the pilot of an
incoming aircraft lateral (sideways) guidance during his landing
approach.
The glide slope indicator gives him vertical (up and down) guidance.
The Instrument Landing System (lLS) markers tell him how far he is
from the airport.
The airways modernization program of the Federal Aviation Agency
in its efforts to facilitate airway traffic control within high density
control areas has sponsored the development of navigation systems
contained within the aircraft itself and which do not depend
completely upon ground-controlled facilities. Such systems are
typified by HIDAN (High Density Air Navigation) manufactured

H O L D I N G PAT T E R N

HOLDING
MARKER

RADAR SCOPES

RADAR AID TO AIRPORT TRAFFIC CONTROL

by General Precision Equipment Corporation. HIDAN consists of
an airborne automatic navigation component which reports
continuous ground speed and drift angle and a position
component which instantaneously indicates divergence of the
aircraft from its planned position along its flight course.

The devices that help airports and airways do their most
important tasks and help the pilots guide their aircraft safely, all
are based upon the science of electronics. Electronic devices
mark the thousands of miles of airways that cover continental
United States. They make possible the control of the traffic along
these airways. They make possible landing and take-off
operations at airports during low visibility weather conditions.
You may follow, as a career, fields of interest other than aviation
or electronics. In that event, the general understandings of
electronic circuits which you now have gained is sufficient. You
have learned about principles which are basic to the tools men
use to prevent air traffic conflicts and keep air travel safe.
Should you choose to follow a career in aviation electronics, you
will need to learn in detail how each of the devices based upon
the electronic circuit operates. Someday, you may need to learn
how to operate or repair these and other electronic devices that
are now in process of development. When that time comes, the
understanding of the basic electronic principles which you have
gained from your present studies will be of great help to you.

THEREGULA ONOFAIRTRAFFIC,I
TI
The need for air traffic control.
Since air space is three-dimensional, it would almost appear that
t h e r e g u l a t i o n o f a i r - t r a f fi c i s u n n e c e s s a r y. U n l i k e t h e a u t o m o b i l e
w h o s e t r a v e l r o u t e i s a r o a d w a y, t h e a i r c r a f t i n t r a n s i t n e e d n o t
r e l y u p o n a n a r r o w p a t h . M o r e o v e r, a i r c r a f t i n fl i g h t h a v e a n
advantage over the automobile in highway traffic, in that one aircraft
may pass around, over, or under another. However, there are certain
things an automobile on the highway can do that an airplane in
flight cannot do. An automobile can stop at an intersection signal,
permitting a flow of cross-traffic. Since an aircraft to stay aloft must
keep in continuous motion, there can be no accommodating pauses
in the flow of air traffic. Moreover, during instrument flight conditions
(when clouds or other obstructions to vision prevail) the advantages
o f t h r e e d i m e n s i o n a l t r a v e l a c t u a l l y b e c o m e h a z a r d s . F o r, i f o n e
aircraft were permitted to fly at any altitude and along any course,
unrestricted and uncontrolled, another aircraft would have the same
privilege, and collision between the two during low-visibility weather
c o n d i t i o n s w o u l d b e c o m e l i k e l y.
The purpose of air traffic regulation.
Air traffic regulation is important in the interest of safe operation
of aircraft. In addition, it is essential to modern aircraft operation
because the flow of air traffic in and out of modern airports is so
heavy that without it, maximum use cannot be made of airports
and airways, and serious traffic conflicts will occur.
The facilities of communication.

Both radio signals and light signals are used to convey air traffic
information from ground communicators to the aircraft flight crew.
Chapters IV and V offer general explanations of the devices and
appliances of radio and the uses of some of these in communications.
When you study the operational procedures of radio, you will learn in
detail how to use radio communications in aviation. Before you

learn this procedure, however, it is necessary to learn in a general way
the purposes for which the procedure has been established. It also
should be remembered that some light aircraft may be equipped only
with the essential instruments for clear weather flying. Consequently,
these aircraft will have no radio transmitter nor receiver.
Under such a circumstance, light signals, each of which has a specific
meaning understood by pilot and communicator are directed toward
the maneuvering aircraft by means of a "light gun" which shoots either
a red, green, amber, or white beam.

The scope of air traffic regulation.
Air traffic, both at airports and along airways, is governed by
regulations. At the airport, the movement both of aircraft on the
ground and of those departing and arriving is in accordance with
established rules. Along the airways, the movement of aircraft also
must be in accordance with prescribed procedures. Moreover, regulations formulated in the interest of the safe and expeditious flow of
air traffic are concerned not only with air traffic rules but also, under
certain flight conditions, with proper aircraft equipment and special
pilot skills.
The methods of flight regulation and control.
Air traffic regulation
employs a system founded
upon three things:
a. A body of regulations, based upon analyses of practical flight
situations, prepared by
the Federal Aviation
Agency.
b. Services which use
electronic and other communication facilities by
means of which pilots of
aircraft receive advice, instruction, and flight information.
c. The pilot's good

judgment and his use of good operating practices.

An aircraft's position in the air space above a certain kind of
surface area governs the air traffic rules under which it operates.
For example, when an aircraft is flying over or near an airport, it
is governed by rules that may not apply when it is flying along an
airway.

For the sake of convenience, traffic regulations are classified in
terms of control zones, control areas, and elsewhere. A control
zone is an air space extending upward from the surface of the
earth. Its dimensions have been established by the FAA
Administrator. It includes at least one airport and the air space over
land adjacent to the airport.
A control area is an air space extending upward from an altitude
700 feet above the surface of the earth. It generally includes the air
space used by aircraft flying along civil airways. You remember that
although a civil airway is a path through air space which can be
navigated and which the FAA Administrator has approved as
suitable for air commerce, it is not necessarily within a control area.
It is only within a control area when it has facilities, such as air-toground radio communications, radio-range stations and marker
beacons, which make traffic control possible.
On December 1, 1957, the Federal Government instituted a
Continental Control Area including airspace within continental
United States at and above 24,000 feet. Within this area FAA
control is optional with the pilot during clear weather but
compulsory during periods when visibility is less than 5 miles.
Aircraft operating within this area VFR may not fly nearer a cloud
formation than 1,000 feet vertically nor one mile horizontally. Series
of routes for jet aircraft operating at high altitudes were also
designated, and on June 15, 1958, three new transcontinental
routes linking New York and Washington, D. C., with Los Angeles
and San Francisco were inaugurated.
These routes are 40 miles wide, begin at an altitude of 17,000 feet,

and extend to an altitude of 22,000 feet. Civil and military aircraft
must file IFR flight plans (see p. 49) before either type is permitted
to use these routes. Also, in the interest of avoiding traffic conflicts,
air carrier companies have voluntarily agreed to file IFR flight plans,
regardless of weather conditions, before operating above 10,000
feet along any air route.

Sometimes aircraft arriving at an airport during low-visibility weather
conditions cannot be cleared for an immediate landing.
1he pilot in such instances is instructed to hold (fly a circular course
at a designated altitude). Since flight during the time an aircraft
awaits landing instructions must also be controlled, the air space
within which this kind of maneuvering takes place is called a control
area extension.
As a matter of fact, certain flight regulations establish some controls
over flight elsewhere than in control areas and control zones.
The difference between the types of control exercised in control
zones and areas and that exercised elsewhere, results chiefly from
differences in the amount of air traffic. For, where air traffic is
sufficiently heavy so that hazards to safe flight operation threaten,
control areas are extended and facilities for air traffic control are
soon established.

At airports which have control towers in operation, airport traffic
control is exercised by such towers during VFR weather conditions.
At such airports all movement of aircraft within the airport's control
zone must be cleared either to taxi, to take off, or to land. Clearance
in these instances may be communicated to the pilot of an aircraft
by means of either radio or light signals.

When his aircraft is equipped with an aircraft radio, the pilot
simply tunes to the proper channel i and asks instructions. He
receives these by tuning his radio receiver to the proper channel1
and operates his aircraft accordingly.
Light signals and their meaning.
When his aircraft is not equipped with radio, the pilot depends
on light signals for his information. The following chart lists these
signals and gives their meanings:

Signals from a portable, traffic control light and their
meanings

Color and Type
of Signal
STEADY
GREEN
FLASHING
GREEN
STEADY RED
FLASHING
RED
FLASHING
WHITE

On the Ground
Cleared for takeoff Cleared to taxi

Stop

Taxi clear of landing
area (runway) in use

In Flight
Cleared to land
Return for landing (to
be followed by steady
green at proper time)

Give way to other
aircraft and continue
circling
Airport unsafe do not
land

Return to starting point
on airport
ALTERNATING
RED AND GREEN

General Warning Signal
Exercise Extreme Caution.

Pilots should acknowledge light signals during hours of daylight by rocking the
wings of their aircraft in flight and during hours of darkness by blinking their landing
or navigation lights. At night in order to attract the attention of the control tower, the
pilot of an
1 Radio transmitters used on light, private aircraft have fixed reception positions.
Those used on commercial aircraft may have several positions; the pilot selects
the one appropriate, such as you select a channel on your television set.
2 Taxi information is communicated on frequencies of 121.7 and 121.9 mc. Air
Trafic Control communications use frequencies in the 118.1 through the 121.3 mc
range and 123.7 through the 126.5 mc range. Frequencies used by airport and
airway radio facilities may be found on the aeronautical charts.

aircraft not equipped with radio should turn on his landing lights
and taxi into a position which makes these visible to the tower
operator.
If the rotating beacon of an airport is lighted during hours of
daylight, it means that weather conditions are below VFR minimums
a n d t h a t a n A i r Tr a f fi c C o n t r o l c l e a r a n c e i s n e c e s s a r y f o r a i r c r a f t
operation. During the hours of darkness, flashing lights outlining
the traffic direction indicator (wind tee, tetrahedron, or other such
device) has the same meaning.
Airport traffic patterns.
An aircraft not equipped with radio, upon arriving at its destination, makes at least one complete circle of the airport, conforming
meanwhile to the proper traffic pattern and watching both for other
a i r c r a f t a n d f o r a s i g n a l f r o m t h e c o n t r o l t o w e r. A t s o m e a i r p o r t s ,
a segmented circle is used to convey traffic pattern information (See
illustration page 44 ). A pilot should observe the flow of traffic
to determine the runway in use and, in the absence of traffic, use
the runway which will enable him to land his aircraft into the wind.
A wind tee, tetrahedron, or other wind indicator will give him the
wind direction. Upon its final landing approach, a pilot, when practicable, should keep his aircraft on a straight course for the last
1,000 feet before crossing an airport boundary.

TRAFFIC PATTERN INDICATOR
LANDING STRIP INDICATOR

WIND INDICATOR

CITY

Segmented Circle

CHAPTER
SVN
EE

4,500 FT.
6,500 FT.
8,500 FT.
10,500
FT.

NORTH


Ceiling, Visibility and Cloud Clearances; VFR Minimums In Control Zones
In Control Areas
Elsewhere Visibility
3 Miles
3 Miles
1 Mile

WEST

EAST

SOUTH

180°

At altitudes
at or under
700’ above
the surface.

Distances
from clouds

500' under
1000' over
2000' horizontally
1000' ceiling

Visibility

At altitudes
over 700'
above the
surface.

3 Miles

Distances
from clouds

500' under
1000' over
2000' horizontally
1000' ceiling

500' under
1000' over
2000' horizontally

500' under
1000' over
2000' horizontally

1 Mile:
Clear of clouds

The VFR odd-even altitude rule.

THEREGULA ONOFAIRTRAFFIC,I
TI
The flight rules.
The rules under which an aircraft operates depend in part upon
the weather which prevails. If the rules followed are those practiced
when the weather is comparatively clear, it is customary to say that
the flight is a VFR flight. If the weather is such that the cloud ceiling
is quite low and flight visibility is obstructed, aircraft operating
legally will be governed by IFR (Instrument Flight Rules) procedures.
Visual flight rules.
The flight of an aircraft operating on VFR procedures is restricted
by the location of an aircraft's operation and the weather conditions
a t t h e t i m e o f i t s fl i g h t . To h e l p y o u u n d e r s t a n d t h e r e l a t i o n s h i p
between the location of a flight and the corresponding VFR procedure, the following table is used:

"On the top flight" (the operation of aircraft above a well-defined
cloud formation) may be undertaken by aircraft operating VFR, provided that the climb to and descent from such flight can be made
in accordance with visual flight rules. Under such circumstances,
the required horizontal, visibility distance is still three miles. Moreo v e r, a s h e c l i m b s t o o r d e s c e n d e d f r o m h i s c r u i s i n g a l t i t u d e , t h e
pilot must still keep a horizontal separation of 2,000 feet between
his aircraft and the cloud formation.

VFR altitude rules.
You remember from the chapter on Aeronautical Charts (see page
11 ) that airways are identified as either colored or Victor airways
depending upon whether the ranges which define them are fourcourse or omni-directional. Green, red, and even numbered Victor

1 When visibility conditions are below minimum, if IFR traffic conditions permit, authorization to
conduct local VFR operations, such as practice take offs and landings, may be given by ATC (Air
Traffic Control) to an airport operator.
2 Helicopter operation at reduced speeds is not governed by this minimum.

airways are east-west airways. When aircraft are flown on
magnetic courses between 0° and 179° upon such airways at an
altitude of or over 3,500 feet above the surface, they must
maintain an indicated altitude above sea level which is at the "oddthousand-plus-500-foot levels," such as 3,500 feet, 5,500 feet,
7,500 feet, 9,500 feet, etc.
When aircraft are flown between 180° and 359°, they must
maintain an indicated altitude which is at the "even-thousandplus-500 foot levels," such as 4,500 feet, 6,500 feet, 8,500 feet,
10,500 feet, etc.
Amber, blue, and odd-numbered Victor airways are north-south
airways. Along these airways, aircraft operating between 0° and
179° and at levels of 3,000 feet or over are flown at odd-thousandplus-500-foot altitudes. Aircraft operating between 180° and 359°
under like conditions are flown at even-thousand-plus-500-foot
altitudes. Aircraft more than 3,000 feet above the surface within a
control zone are also governed by the odd-even flight altitude rule.
When in level cruising flight, the flight-path altitudes of aircraft
operating elsewhere than within a control zone or area and flown
at 3,000 feet or over 3,000 feet and under 29,000 feet above the
surface are governed by the magnetic course the aircraft follows.
Aircraft on magnetic courses between 0° and 179° cruise at oddthousand-foot levels plus 500 feet (3,500'; 5,500'; 7,500"; etc.);
between 180° and 359°, at even-thousand-foot levels plus 500 feet
(4,500'; 6,500'; 8,500'; etc.) The flight-path altitudes of aircraft
flown above 29,000 feet are separated vertically by 4,000-foot
intervals. For flights on magnetic courses between 0° and 179°
such separations begin at 30,000 feet; for flights on magnetic
courses between 180° and 359° such separations begin at 32,000
feet.

Instrument flight rules.

Flight plans may be filed for aircraft operating VFR. In fact, it is a
safe practice always to file a flight plan. It is required that flight
plans be submitted for aircraft operating when weather conditions
are below VFR minimums. The filing of an instrument flight plan
assures Air Traffic Control that the aircraft is equipped for
instrument flight and that the pilot holds instrument ratings. Before
leaving or entering a control zone or area, aircraft operating IFR
must submit a flight plan and receive an air traffic clearance.

Air Traffic Control Communications

The flight plan.
The flight plan may be filed with the nearest airway communication
station, airport traffic control tower, or air-route traffic control center
by person, telephone, or radio. It is by means of information given
on the flight plan that the air traffic controller can plot a flight
accurately and plan the flight's movement in terms of the prevailing
flow of air traffic. The flight plan contains information about the type
and identification of the aircraft and the name, address, and
certificate number of the pilot in command. In addition to the airport
of intended destination, the flight plan names alternate airports
within the aircraft's radius of flight. The flight plan also states in
terms of hours and minutes the amount of fuel on board.
Of special significance to Air Traffic Control are the following items
which the flight plan must contain: the point of departure, or when
the flight plan is filed enroute, the position of the aircraft; the
proposed time of departure; the route to be followed; the cruising
altitude

IFR altitude rules.
or altitudes; the proposed true air speed at cruising; the radio transmitting and receiving frequencies to be used; the point of the first
intended landing; and an estimation of the time which will elapse
between departure and arrival over the airport of first intended
landing.
The filing of a flight plan also has a very important secondary
purpose. In the event a pilot is forced to make an unscheduled
landing in an area not equipped with communication facilities and
fails to make scheduled position reports, an air search for the aircraft
is made. Civil Air Patrol senior members play an important role in
search and rescue. The information in the flight plan helps the Civil
Air Patrol search team quickly locate the overdue aircraft.
IFR altitude rules.

In stating his proposed flight altitude in his flight plan, the pilot
should indicate odd altitudes for easterly-northerly flights and
even altitudes for westerly-southerly flights. Except where other
altitude minimums have been established, IFR flights must
operate so as to clear, by at least 1000 feet, the highest obstacle
within five miles of the center of the intended course. In
mountainous regions, a clearance of 2000 feet is required.

Rules cannot replace pilot judgment.

W i t h i n c o n t r o l z o n e s a n d a r e a s , AT C k e e p s a i r c r a f t a c e r t a i n
d i s t a n c e f r o m o n e a n o t h e r. Ve r t i c a l l y ( u p - a n d - d o w n ) s e p a r a t i o n i s
maintained by assigning different altitudes to different aircraft; longitudinally (forward-and-backward) separation, by establishing minimum time separation between different aircraft on the same course;
and laterally (right side-and-left side) separation, by assigning parallel courses to different aircraft enroute to the same destination. IFR
flights cruising outside control zones below 29,000 feet on magnetic
courses between 0° and 179°. shall fly at odd-thousand-foot, indicated, sea-level altitudes (1,000'; 3,000'; 5,000'; etc.). When they
cruise on magnetic courses between 180° and 359°, they shall fly
at even thousands (2,000t; 4,000'; 6,000'; etc.). For IFR flights
above 29,000 feet cruising altitudes are separated vertically by
4,000 feet. For flights on magnetic courses between 0° and 179°,
separations begin at 29,000 feet. For flights on magnetic courses
between 180° and 359°, separtions begin at 31,000 feet.
The air traffic clearance.
The air traffic clearance issued by ATC authorizes the operation of
a n a i r c r a f t w i t h i n a c o n t r o l z o n e o r a r e a . H o w e v e r, s u c h o p e r a t i o n

must be conducted under the specific conditions of the clearance.
Otherwise the purpose of the clearance would be defeated, since
its principal object is to help the controller know at all times the
position of the aircraft in flight.
In this respect, it is important for the pilot to remember that only
during IFR weather conditions is it possible to keep standard
separation between aircraft. During VFR weather conditions, VFR
flights can be operating without the knowledge of ATC.
Generally, traffic clearances are issued for the altitude and route
requested on the flight plan. However, traffic conditions sometimes
make it necessary to assign altitudes and routes other than those
requested. Such may be the case in areas of air traffic congestion.
Flow patterns are established for these areas in order to increase
the capacity of the airway. Under such conditions, ATC may
change a requested route to conform to the flow pattern.
Sometimes after a clearance has been issued, it becomes
necessary, in order to avoid possible conflict between aircraft in
flight, for ATC to amend the clearance. Such action may be taken
by an air traffic controller any time he believes it necessary.
The pilot should give close attention to the clearance when issued
and not assume that the route and altitude for which his flight is
cleared are those requested. When receiving clearance by radio,
he should ask for a "repeat-back" of any instructions that he does
not clearly understand. He always has the privilege of requesting a
clearance different from the one issued by ATC when for some
reason compliance with the clearance issued would not be
practicable.
A pilot may also cancel an IFR flight plan at any time he is
operating in VFR weather conditions. He does this by a message
to the controller or to the air-ground radio station with which he is
in communication. However, should subsequent IFR operation
become necessary, he must file a new flight plan immediately.
Pilot responsibilities under an air traffic clearance.
After an air traffic clearance has been obtained, its conditions
must be observed unless for some reason the clearance is
subsequently amended. The pilot should remember that any
change in altitude, route, or true air speed at cruising altitude is a
change of flight plan.

The pilot must notify ATC if the radio equipment of his aircraft
cannot receive omni-range signals when he is instructed to use
these to identify a radio fix. For pilots are required when operating
IFR to keep continuous listening watch on the proper radio
channel and to make a position report at the time they are over
any compulsory reporting point or any reporting point specified in
the flight plan.
Position reporting is very important and must be as accurate as
possible, because safe and effective traffic control depends upon
accurate position reporting. It is also necessary for each position
report to include an estimated time of passing the next reporting
point.
It is important that pilots in flight report any unforecast weather
condition encountered. They also must report, to ATC, hazardous
conditions when these are encountered, whether or not these
conditions have been forecast. Other reports that pilots must
make include the altitude of the aircraft when it reaches the point
to which it has been cleared, or a "holding point," and the time of
its arrival there. The pilot must report his departure from one
altitude for a newly assigned altitude, the time of leaving an
assigned holding point, and when such happens, the fact that an
approach has been missed. Still other reports may be requested
by Air Traffic Control, and it is the responsibility of the pilot to
conform to these requests.
Finally, it is mandatory that a pilot reaching his destination "close
out his flight plan."
Approach Control.
As you know, Approach Control is a service of Air Traffic Control
that supervises departing and arriving IFR flights. Unless he has
been instructed by ATC to do so enroute, the pilot waits until he
reaches his assigned holding point before requesting instructions
from Approach Control. Through the means of appropriate
instructions to a pilot based upon the controller's knowledge of the
positions of other aircraft within the control zone, an approaching
aircraft is directed to a safe landing. IFR landings depend upon
both the electronic facilities used by controllers to obtain and
impart information and those used by the pilot to receive this
information (See page 27 ).
Approaches which employ electronic devices and appliances
demand complete cooperation of controller and pilot.

The pilot's knowledge, skill, and judgment.
Important as are the rules of air traffic and valuable as are the
tools of communication and navigation, the quality of judgment
exercised by the pilot of the aircraft is the most significant factor in
successful air traffic regulation and safety of aircraft operation.
Rules and regulations can only define patterns of behavior.
Electronic devices can only convey information and instructions.
Unless the pilot disciplines himself to respect the rules of flight
and to follow them and unless he is capable of basing his flight
procedures upon the instructions and information he receives,
both rules and instructional guidance will have been wasted.

Good operating practices.
Obviously those who use the facilities of airports and airways
must observe carefully the air traffic rules established in the
interest of safe flying. However, quite frequently air traffic conflicts
occur when no rule has been violated. As a matter of fact, rules
cannot replace good pilot judgment in the planning and
conducting of a flight.
Pilots should be cautious about exercising the prerogatives of
VFR flight. For example, pilots operating VFR should use good
judgment and stay away from approach areas when visibility is
down to three or four miles.
Pilots should be alert at all times. Air collisions occur most often
when weather conditions are excellent and pilot's attention
becomes relaxed.
Good judgment dictates that a pilot, although he has the right of
way, should give way to approaching aircraft that get too close.
In the interest of best possible judgment, pilots whenever possible
should use the designated airways, operate near the center of the
Victor airways, follow IFR procedures even when operating VFR,
conduct IFR operations unless weather is well above VFR
minimums, and when conducting an IFR operation, make a written
record of the condition of his ATC clearance.

SUMMAR
Y

In theory, an aircraft can fly an infinite number of paths through
the air from any surface point to any other. In practice, the paths
of flight lead from airport to airport. In. practice, aircraft land and
take-off from land or water areas suitable for these purposes.
Aircraft not only need proper landing and take-off facilities, they
also need maintenance and repair facilities. Moreover, those who
use aircraft need services and accommodations which the airport
must provide.
Much can be learned about the nature of a specific airport or a
specific airway from an aeronautical chart such as pilots use. For
example, the chart reveals the type and size of an airport, the
radio facilities it uses, and its altitude and location. The chart also
shows the location of the civil airways, the magnetic direction they
follow, the type of radio beams and markers that they use, the
position of check points, and the geographical location of radio
facilities and communications stations along them.
Airway and airport developments are keeping pace with aircraft
and engine developments. These developments are mainly in the
fields of airport engineering and aviation electronics. In terms of
the relative importance of these two developmental areas,
progress in the latter is of the most significance. Consequently,
the student of airports and airways must reach some general
understandings of the scientific principles underlying the devices
which are used in airport and airway operation.
The radio signals, which are used in ground to air
communications and to mark airways, are made possible because
men have learned how to control the nature of radio wave
frequencies. The electron tube is one of the devices which helps
establish such control.
Through use of one type of electron tube, a faint radio signal can
be amplified by a radio receiver. When this is done, a very weak
electromotive force can be increased until it is strong enough to
activate the diaphragms of both the earphones used by a pilot
and the indicator of an electronic instrument. This is possible
because one of the functions of the electron tube is the same as
that of an electric generator or battery--to generate electricity.
Each day sees new discoveries made in the field of electronics.
The scientific application

of these discoveries will continue to bring about improvements in the
tools of aviation.
Airports are necessary for many obvious reasons. There are other
reasons for their importance that may be overlooked by the airplane
passenger or the airport visitor. Among these is the provision by the
airport of administrative centers for the control of the several
aspects of air traffic. Among the services administered by airport
centers are (a) Flight Assistance Service; (b) Air Traffic Control,
which includes Airport Traffic Control, Approach Control and Air
Route Traffic Control; (c) Radio Communications, and (d) Weather
Bureau Observation and Forecasting.
Some of the flight services whose administrative centers may be
housed at an airport extend along the civil airways. The use of
electronic devices to control air traffic along an airway is the factor
which makes an airway of a path of flight. It is true that through the
use of modern electronic devices, such as VORTAC a pilot, whether
or not he is on a civil airway, may fly an exact and direct course from
the airport of his flight's origin to the airport of its destination.
However, a flight which does not follow a designated civil airway
cannot profit by many ATC services that otherwise would be
available.
In the interest of aviation safety and air traffic assistance and
control, air traffic rules have been established. The rules relate
chiefly to weather minimums, flight altitudes, and traffic patterns to
be used under different circumstances. An aircraft observes one set
of rules when the weather permits flight by reference to objects and
landmarks on the ground (VFR), another set when the weather
requires the use of special instruments (IFR). More exacting rules
govern flight in control zones than govern Night in control areas.
While air traffic rules extend to airspace elsewhere than that at
control zones and areas, these in the main relate to procedures
which cannot be supervised by Air Traffic Control (ATC).
In the final analysis, effective air traffic control depends upon the
integrity and good judgment of the pilot who operates an aircraft.
IFR operations must always be cleared by ATC in terms of a flight
plan filed by the pilot in command. A wise pilot will not chance VFR
operations when weather conditions are doubtful. Moreover, he will

accept the responsibilities imposed on him by the ATC clearance he
received. His course of action will be in accord with these responsibilities.
Airports and airways are of principal importance because without
them and the equipment they employ, aircraft would have no system
of organized operation.

EDUCATIONAL ADVISORY COMMITTEE

Emmett A, Belts
Director. Betts Reading Clinic
Willis C. Brown
Specialist for Aviation Education
Division of State and Local School Systems Office
of Education
Leslie A. Bryan
Director. Institute of Aviation
University of Illnois
John H. Furbay
Director
Air World Education
Trans-World Airlines. Inc.
Georqe N. Gardner
Superintendent, Educational Services Pan
American World Airways System John L. Goodwin
Associate Professor
University of California
Department of Business Administration Dawson C.
McDowell
Director, Institute of Tropical Meteorology Universlty
of Puerto Rico
Merlin McLaughlin. Lt. Col. USAF 58 Gruber Street
Des Moines. Iowa
Raymond O. Mertes
Director, School and College Service United Air
Lines
Jordan L. Larson

Kenneth E. Newland
Occupations Division
Stephens College
Willoughby E. Sores. Consultant Aviation
Education
California State Department of Education
Harry C, Schmid
State Director
Vocational Division
Department of Education
State of Minnesota
Prank E. Sorenson
Professor of Education
Teachers College
The University of Nebraska
Roland H. Spaulding
Professor in Education in Charge of
Aeronautical Education New York
University
School of Education
Parker Van Zandt
International Staff, NATO
USRO Defense 1
Paul A. Wilkinson
Denver Public Schools
Harry Zaritsky
Audio-Visual Division
Naval Medical School
National Naval Medical Center

Superintendent of Schools
Mount Vernon. New York

AV I AT I O N

EDUCATION
PERSONNEL Charles J. Wood. Assistant
Mervin K. Strickler. Jr.
Director.
Chief of Aviation Education
Audio Visual Training Aids Division Everett E.
Charles W. Webb. Assistant Chief of Aviation
Collin, North Central Region
Education
Arthur I. Martin, Southwestern Region John M.
Harrold E. Mehrens. Director. Editorial andOgle. Rocky Mountain Region John E. Sims,
Curriculum Division
Southeastern Region John V. Sorenson. Pacific
William E. Rowland, Director. Audio VisualRegion
Training Aids Division