Having never been a sports aficionado, I have not spent much money
or time at baseball, football, or soccer fields, hockey rinks, bowling
alleys, curling sheets, or basketball courts. When an air show comes
to town, however, I'm there. I'll stand in line for 45 minutes to
tour the inside of a DC-3, B-25, B-17, PBY-5, or just about anything
that will admit me. What is particularly enjoyable is inspecting
the radio equipment racks and bays.
sight and smell (I consider it an aroma)
of the old UHF and VHF sets, recording equipment, power supplies,
generators, synchros, and the associated wiring and connectors is
something I never tire of experiencing. I always imagine the men
who operated and maintained everything doing their assigned duties
to keep those wonderful machines flying. Maybe you know of what
I write. This article provides a nice overview of the state of the
art for airborne electronics in the post WWII era. A couple photos
are included here.
the Zenith console radio that I found lashed into in a C-54!
Funny anecdote: A couple years ago Melanie and I were standing
in line at the Erie Airshow waiting to crawl
(literally) through a B-25. The fellow
in front of us was rather portly and I wondered whether he would
be able to squeeze through the area in the mid fuselage that separated
the front and rear portions of the airplane. Up the ladder he climbed
few photos of the cockpit area. Then, he turned to proceed to the
rear and I could see his visage fall as he was confronted with the
narrow passageway behind him. The poor guy had to back down the
ladder and give up his hopes of seeing the rear gunner position
and some of the bomb bay mechanisms. Fortunately for him, there
are plenty of pictures online of anything he missed.
One of American Airlines' postwar fleet of DC-6's.
By James Holahan*
A brief review of the field of aircraft radio, covering the nature
of the work, the necessary qualifications for entering this field,
and the available opportunities. The various types of radio equipment
found in modern commercial planes are also discussed.
In the few brief years that comprise the history of aviation
the expansion of air travel has been tremendous. The blockade of
Berlin has proven to the world that life in a modern metropolis
can, in an emergency, be sustained through the medium of air transport,
in spite of the hazards of weather.
Throughout the United States hundreds of airfields, built by
the military for training, are now serving in a commercial capacity
as municipal airports. Few populated areas in the country are farther
than a short drive from a local landing field. Foreign travel, too,
has expanded. Giant wings and powerful engines are shrinking distances,
formerly measured in weeks, into hours.
With this expansion of air travel new opportunities are opening
up for the radio technician and electronic specialist.
Practically every aircraft in use today, both privately and commercially,
carries some form of radio equipment ranging from a single receiver
to complete racks of electronic gear.
Modern day flying has placed radio in the "essential" class.
Improperly functioning radios are not only useless but hazardous
because such great dependence is placed upon them.
This was pointed out as far back as 1929 in an article by an
unnamed manager of the National Air Transport Company, when he wrote:
"There is no question as to the assistance that radio can give
the air transport pilot. The problem is to insure against failure
of the system. A catastrophe may easily be caused if the pilot relies
on radio and it fails him for one cause or another. It would be
far better if he had no radio at all."
These were the days when flying was in its infancy and many airmen
used radio with caution and distrust. Today our entire system of
instrument flying is built around radio. Complete radio failure
in an airplane could result in disaster not only to the occupants
of the affected aircraft but to other aircraft in the vicinity.
A few of the radio operators employed by Trans
World Air Lines. On these men fall the responsibility of transmitting
important weather and flying data which keeps the planes aloft and,
safe during bad weather and good, clear skies and overcasts.
The use of radio in aircraft has transformed the airplane from
a contraption reserved for daredevils to an instrument of commercial
utility. It permits the pilot to fly through the worst weather without
reference to the ground. It gives him a course to follow and enables
him to keep in contact with ground stations and other aircraft.
Aircraft radio may be divided into two categories, communications
and navigation. The radio gear carried on any aircraft is used for
one or both of these purposes.
The communications category includes voice transmission and reception
of pertinent messages, such as takeoff and landing instructions,
flight plans, weather information, emergency calls, etc. On long
trips away from ground stations, such as on transoceanic flights,
c.w. is used.
The aircraft receives these voice and c.w. messages on the aircraft
band (200-400 kc.) or on v.h.f. (100-150 mc. voice only) and transmits
on a variety of medium high frequencies between 3 and 9 mc. with
3105 kc. and 6210 kc. being most frequently used in the United States.
Communications from air to ground are also transmitted on any of
the allotted channels between 100 and 150 mc.
Air navigation by radio requires more of a discussion. An extensive
system of radio aids to navigation, which form the civil airways
of the United States, is maintained by the Civil Aeronautics Authority,
an agency of the Department of Commerce. The principal component
of this system is the radio range station.
The radio installation in the "Convair." This
300 m.p.h. 40-passenger plane has a built-in loading ramp, a cruising
range of 800 miles, and an air-conditioned cabin.
The v.h.f. transmitting and receiving equipment
installed in a Beach Bonanza baggage compartment.
A radio range station consists of a transmitter emitting a carrier
on an assigned frequency in the aircraft band. It has two independent
r.f. channels differing in frequency by 1020 cycles, controlled
by matched crystal oscillators. The antenna system is comprised
of two crossed Adcock antennas that are 90 degrees in space with
respect to each other, together with a center vertical antenna.
The central tower is constantly fed by the output of one r.f. channel
while the output of the other is switched from one Adcock antenna
to the other. First the code letter "A" is fed to one Adcock then
an "N" is fed to the other. A receiver monitoring these signals
will pick up the 1020 cycle beat note which corresponds to the difference
between the r.f. channels. A steady tone is heard whenever the energy
from the two antenna systems is received with equal intensity. This
is called the "on course" or beam. Each range station produces four
beams, the direction of which is controlled by the directional antenna
systems. These beams give the pilot a definite course to follow
toward or away from the station. In the sectors between the courses
either an "A" or "N" is received depending upon what quadrant the
plane receiving the signal may be in.
Actually the civil airways are aerial routes whose courses are determined
by these beams. While flying on the airways the pilot is aware of
his position with respect to the range station, which he can locate
on his map, from the type and intensity of the received signal.
The point directly over the range station is identified by the lack
of signal because the vertical antenna system used will radiate
negligible energy directly upward. This "no signal" area widens
with height and for that reason is called the "cone of silence."
On most ranges the cone is further identified by a "Z" marker, operating
on a frequency of 75 mc., which constantly keys the letter "Z".
A special receiver is necessary to pick up this signal.
An important piece of equipment for radio navigation is the loop
We all know the directional qualities of a loop. For instance,
a portable radio with a loop antenna will receive the maximum signal
when the plane of the loop is in line with the station and minimum
signal when the plane of the loop is at right angles to the station.
A 4-place Stinson, a private aircraft, showing
rotatable loop antenna.
This same principle is used in navigation. In its simplest form,
the loop is used with a receiver as an extra antenna which may be
switched into the circuit for direction finding.
More elaborate receivers employ amplifiers and balancing networks
which greatly increase the loop's sensitivity.
The loop is the heart of the radio compass or, as it is also
called, the automatic direction finder. The operation of an ADF
is simple - just tune in the desired station, identify it, flip
a switch and a needle, mounted on a 360 degree azimuth scale, points
to the angular direction of the station relative to the aircraft.
Formerly the radio compass, because of its weight, was confined
to large aircraft only. Now a manufacturer has come out with an
ADF weighing 24 lbs. including the power supply.
About the latest thing in radio navigation is the omnidirectional
range which will soon be in operation all over the country. These
ranges operate in the v.h.f. spectrum between 90 and 110 mc. In
contrast to the low frequency ranges which present four beams or
pathways to the station, the omnirange allows the aircraft to come
in "on the beam" in any direction. The aircraft having this equipment
installed will have a "To-From" indicator which tells the pilot
whether he is going toward or away from the station he is working.
On the low frequency ranges the pilot determines this from the build
or fading of signal strength which can often be difficult in times
of poor reception due to precipitation static and the like.
For navigating great distances by radio, such as in transoceanic
flying, a system called "loran" is used. The name is a coined word
derived from the words LOng RAnge Navigation. Developed during the
war, this system is composed of a receiver operating on a band just
above the broadcast frequencies. There is no audio. Pips appearing
on the face of the cathode-ray tube furnish visual information,
which when applied to special loran charts gives very accurate fixes.
The only installation required in the aircraft is the loran receiver.
Ground installations consist of several pairs of transmitting
stations operating on the same frequency. In each pair one is termed
the "master" the other the "slave" station. The system is based
upon the microsecond interval between the reception of the signals
emitted by the master and the slave.
A navigator, specially trained in the loran method of navigation,
is required to operate this system. It is possible to obtain accurate
fixes from stations up to 1500 miles distant by means of loran.
The instrument landing system may also be included in the navigation
category. Two receivers and an instrument panel indicator comprise
the main components of the ILS equipment as it is installed in the
One receiver, the localizer, operates on any of six crystal-controlled
frequencies located between 108 mc. and 110 mc. This receiver tells
the pilot, through movements of a vertical needle of a specially
designed micro ammeter, whether he is to the right or left of the
runway. Actually the needle is differentiating between an area of
150 cycle modulation and 90 cycle modulation.
The path of descent can be seen in the movement of a horizontal
needle of another micro ammeter utilizing the same meter face as
the vertical needle. A glide path receiver controls the movement
of the horizontal needle. It operates on three crystal-controlled
channels between 332 mc. and 335 mc. The glide path signal when
modulated by 150 cycles will indicate position above the correct
path while 90 cycle modulation is used to show position below the
Also used for blind landings is the much publicized GCA-ground
controlled approach. All the equipment that is needed in the aircraft
in order to use this system is voice communication between the pilot
and ground. The path of the aircraft is accurately plotted and the
pilot is literally talked into a landing by ground operators.
Non-directional radio beacons and marker beacons are two more
of the facilities offered by the Federal airways system. The former
is simply a station transmitting a continuous carrier in the aircraft
band interrupted by regular station identifications. It is used
only with direction finders as a homing aid.
The instrument panel of the DC-6. Much of the
responsibility for the safe operation of such sky giants rests with
Marker beacons are vertically directed signals on 75 mc. These
are located along the "on course" of low frequency radio ranges.
They enable the pilot to definitely establish his position while
flying the ranges.
Thus it may be seen that the present day aircraft is well equipped
with radio gear. In flying, radio is essential. Just as essential
is the personnel which maintains this equipment.
Commercial operators (airlines, flight schools, charter companies,
etc.) and most private owners who engage in regular and frequent
flights away from their home airport have their radios checked periodically.
Scheduled airlines probably have as complete a radio maintenance
system as can be found in the aviation industry. Here all radio
equipment is given periodic checks according to the number of hours
which the ship has flown. There are checks approximately every 25
hours. Each succeeding check involves more operations until, at
the end of a set period of time, say 2000 hours, all the radio gear
is removed for overhaul and bench checks.
Aviation companies have found that it doesn't pay to wait until
a unit breaks down before servicing it - preventive maintenance
is the watchword. Those used to servicing home sets would find aircraft
radio quite different in this respect - less than half the time
is spent repairing defective units; more time is consumed looking
for troubles before they cause the delay of a flight.
The more technical aspects of an aircraft radioman's job involve
making new installations and modifying certain types (especially
surplus) radio equipment, either to improve performance or to conform
with government regulations. In large companies very little of the
installation or conversion work is left to the initiative of the
technician. In small outfits the success of the installations and
conversions is largely dependent upon the skill of the technician.
An over-all view of American Airlines' radio
overhaul shop. As mentioned in the article, the large commercial
outfits stress preventive maintenance and subject all radio equipment
to periodic and thorough checks.
The qualifications necessary for an aircraft radio technician
can be summed up in five basic requirements:
1. Technical knowledge. He must possess a high degree of widely
diversified theoretical and practical knowledge of radio.
2. Mechanical ability. He must be able to work well with tools
and be fairly skillful in designing and building things of a mechanical
33. Familiarity with aircraft. Since most of the equipment is
installed in aircraft, familiarity with the location of the power
sources, switches, controls, etc., are of vital importance.
4. Physical fitness. This work involves a good deal of climbing
in and out of tight places, much walking, and lugging of heavy equipment.
5. FCC license. A 2nd class phone is the minimum license requirement.
If a man has the proper technical knowledge all the preparation
needed to pass this exam is a study of the FCC regulations governing
the holders of commercial licenses. However, a review of a "question
and answer" book (sold at most technical book stores) for this exam,
might prove helpful to those who might be rusty on theory.
Now, providing a man has the proper qualifications, the next
step would be applying for a position. Let us consider the main
sources of employment. There are five.
1. Scheduled airlines
2. Non-scheduled airlines
3. Maintenance and overhaul shops
4. Civil service
5. Own business
The scheduled airlines hire the bulk of the personnel, each major
airline having in the neighborhood of 50 to 175 men over-all, doing
radio work. The starting pay is fair - about $1.52 per hour with
periodic increases governed by the length of service. The work is
shift work. Working conditions and equipment are excellent. There
is a strict union shop with its attendant seniority regulations.
The radio operator's station in a Douglas DC-4.
Note the many different types of radio equipment used in the operation
of this craft.
Non-scheduled airlines pay on a par with the scheduled lines
but do not offer the same security since the "non-scheds" have not
as yet attained any form of inherent stability.
The working conditions are just fair while ofttimes the equipment
is worse. Here more skill and versatility is required. Hours are
frequently long and irregular. However, experience is plentiful
and the opportunities of high gains that go along with new enterprise
are ever present.
Maintenance and overhaul shops usually pay scheduled airline
rates to the regular technicians but the rates of the leadmen and
foremen are somewhat higher. Here the hours and working conditions
depend upon many factors, such as the size and policy of the particular
Civil service positions are sometimes available in the aircraft
radio field as civilian technicians with the armed services and
with the Civil Aeronautics Authority.
With the latter the work is for the most part installing and
maintaining the ground stations of the radio aids to aerial navigation
that span the entire country. Such positions for technicians pay
between three and four thousand per year, depending upon the classification
of the work. Announcements of examinations for these positions appear
in the Civil Service periodicals.
Operating one's own business will offer opportunity only to those
experienced in aircraft radio as well as business techniques.
Not unlike similar ventures in the home sales-service field it
offers advantages, such as being your own boss, on the one side
and disadvantages, like a seven day-ninety hour week, on the other.
For the benefit of those considering an entry into this phase
of electronics let me outline briefly the future of aviation radio.
The radio panel of the four-engined "Constellation."
Because the "Connie" is used for long, over-ocean flights the radio
equipment includes c.w. as well as phone equipment. Note the sending
key in the right center of the photograph.
Aircraft radio's future is tied up with aviation. If aviation
rises or falls so will its electronic offspring.
Do not be deceived into thinking that aviation is the industry
open to a chosen few booted and goggled supermen where everything
is booming and everybody carries off a bucket of gold for his daily
It is a new industry; in its short life it has had its share
of depressing times when many employees were furloughed. In the
years since the war most of the companies have lost money. Those
that survived are beginning to get into the black and show signs
of an upward trend.
Showing the positioning of the radio compass loop in relation
to the fuselage.
What can be definitely stated is that aviation is a young industry
and that aviation is here to stay.
There is a tremendous amount of wealth, both government and private,
invested in aviation.
In spite of all its fine achievements aircraft radio has its
shortcomings; it still has vast room for improvement.
Much of aviation's future is dependent upon the advancement
How a radio signal is used to "home" a plane during adverse weather
Loop position characteristics.
The administrator of Civil Aeronautics, D. W. Rentzel, recently
made the following statement:
"The most urgent need of aviation today - civil and military
- is a reliable, all-weather navigation and landing system... The
United States already has approximately six billion dollars invested
in civil airports. Because of weather these airports are closed
fifteen percent of the time ... The gravity of this situation, which
is a bottleneck to commercial aviation and a weak link in our national
defense, has been recognized in every group which has studied air
transportation problems. Both the President's Air Policy Commission
report and the report of the Congressional Aviation Policy Board
emphasize the need for a safe, efficient, all-weather navigation
system, estimated at one hundred million dollars to implement and
requiring fifteen years until 1963 - to be placed in operation.
This represents about five percent of our present investment in
aviation in the United States ... The system must accomplish a task
of almost fantastic complexity. Before the war, such a system could
not have been established. But new developments, such as radar and
other electronic devices using extremely high frequencies, have
given us the tools which make such a system possible."
*The author is a radioman who became a pilot in the USAAF during
the war. At the end of the war he was placed in charge of radio
and radar maintenance at Rapid City Army Air Base. Since separation
from service he has been in the aircraft radio field and is employed
by Air Associates Inc.
Posted September 3, 2015