Aircraft electronics has always been on the bleeding edge of technology
because of the ever-increasing need to fly in the widest range of
atmospheric conditions possible. Accordingly, skills needed by avionics
servicemen are amongst the highest required in any electronics field.
There are still many pieces of vintage equipment in service that
need to be maintained, but even 20- to 30-year-old airborne radars
and navigational units require top-notch techs to troubleshoot and
align. One topic in particular that plagues electronics operation
even in modern airframes is that of static electricity build-up
and lightning strikes. We all face those kinds of static discharge
hazards in non-aviation environments, but for the most part a failure
on the ground or water is not as imminently life-threatening as
a failure in the air. This article from a 1946 edition of Radio
News does a good job of outlining issues that are still relevant
today - maybe even more relevant with the replacement of relatively
high-voltage-immune vacuum tubes with semiconductors.
The Aircraft Radio Serviceman
By Tony Wayne*
the increasing demand for civilian operated planes, competent technicians
will be needed for the installation and maintenance of aircraft
Irrespective of the modern trends of present day inventions,
use of radio in light private planes doesn't seem to be popular.
True, the war had a lot to do with it, but the war is over now.
The principal reason for its unpopularity was, and is, installation
and maintenance problems. Unlike the car radio, aircraft radio presents
many problems, such as interference, bonding, electrolysis, and
While most everyone drives a car today, and is familiar with
its mechanics, this is not true of the airplane. True, most pilots
are familiar with their planes, and possess a certain amount of
the mechanical aptitude necessary; yet, relatively few servicemen
know enough about aircraft to install this equipment properly. The
experienced aircraft radio mechanics who were available prior to
war, were absorbed by the airlines. Then too, the little aircraft
radio business then existent was not sufficient to entice radio
This condition has been rectified largely during the past four
years, as thousands of men have been trained in Armed Forces' radio
Fig. 1. - Panel view of the Learadio
receiver tuning unit, Model RCBB.
The radio serviceman has always been a resourceful individual.
When home sets were not coming into the shop fast enough, and competition
was not only keen but rough, with radio shops in every block, in
attics, basements, and bedrooms, the serviceman was repairing toasters,
iron's, heaters, vacuum cleaners, and washing machines. With the
advent of new radios, owners of old "blooper" or "orphan" sets,
and recognized brands which have been holding out with a wire and
a prayer, will be reluctant to have them repaired now.
It scarcely seems the intelligent thing to do for servicemen
to return to toasters and washing machines; rather, they should
put their vast knowledge to work specializing in such fields as
electronic equipment, television installation, small boat radio
and aircraft radio maintenance and installation.
There are a number of public airports for light planes springing
up all over the country, and all of them are taking into consideration
the varieties of services the private flyer will demand; and are
building with the idea in mind of allocating various concessions
necessary. Aircraft radio servicemen-this should be your cue - get
your bid in now. These airports will furnish you the necessary space,
either on a reasonable percentage or rental basis.
All signs are definitely pointing toward an almost unbelievably
rapid growth of the use of aircraft radio. Light plane purchasers
can look forward to buying planes completely radio equipped. This
is not mere crystal gazing, as many large light plane and radio
manufacturers have made public this intention.
The subject, in general, should become of interest to the radio
serviceman. To meet possible demands for information concerning
general description, installation, and maintenance of aircraft radio
equipment, and some new and unusual accompanying problems, this
article is written. Its purpose is to make available such pertinent
information as will provide a general outline of aircraft radio;
its parallelisms to the radio serviceman's present activities, showing
that the slight departure from those activities will not be too
This article discusses the problems of aircraft radio, strictly
from an elementary viewpoint, with sufficient detail given to enable
the radio serviceman to answer questions intelligently. It is also
intended that he will gain sufficient information from the detail
herein so that he will be able to install and service aircraft radio.
There are a number of factors met in servicing aircraft radio
not common to average radio servicing. Mounting and installation
alone are of great importance; and when servicing is done on the
more complicated units, such as the automatic direction finder,
automatic pilot, etc., need of experience and knowledge is greatly
enhanced. Need for radio mechanics qualified to service aircraft
radio will undoubtedly become greater in the near future, and the
importance of sound and dependable training cannot be minimized.
Aircraft pilot's dependence upon his radio equipment is too important
to leave to the wiles of an untrained serviceman.
Fig. 2. - Transmitter tuning unit. Learadio
Model T-30 AB.
Before actually installing radio equipment, it is imperative
that you understand the construction of light planes.
For this purpose, we will use illustrations of one of the most
popular light planes - the Piper Cub. These illustrations will give
the necessary information concerning not only the Piper Cub, but
many similar models as built by other manufacturers.
Fig. 5 shows a cutaway view, clearly indicating the various parts
of the airplane, together with their correct names. Recognition
of these parts will enable you to talk "pilot's language" about
any light plane.
Fig. 3 shows the control system of the light plane.
As a model for our aircraft radio discussion, we will use the
model T-30, RCBB two-way aircraft radio equipment as manufactured
by Lear, Incorporated. Front view of the receiver is shown in Fig.
To require a minimum of instrument panel control space, and to
allow better weight distribution, this receiver has been divided
electrically and mechanically into two units. The tuning control
unit incorporates i.f. amplifier, detector, and oscillator; while
Model G-3-AB unit contains dynamotor power supply, i.f. amplifier,
second detector, a.f. amplifier, and range filter. In the case of
the transmitter-receiver power supply, Model G-30-AB, the modulator
unit of the T-30 transmitter is also built on this chassis; and
it is only necessary to add the i.f. section of the transmitter
and the necessary cables to have the T-30, RCBB two-way combination.
A front view of the model T-30-AB transmitter tuning unit is
shown in Fig. 2.
The RCBB is a three band receiver covering the 200 to 400 kc.
beacon band; the 500 to 1200 kc. broadcast band; and the 2800 to
6700 kc. aircraft communications band.
Prior to the actual installation of any aircraft radio equipment,
make a "bench" test of its operation. This will serve as a basis
for trouble shooting the installation, should it not operate upon
Actual placement of the tuning control unit and the power supply
in an aircraft installation is dictated by several considerations.
Fig. 3. - Conventional control equipment
employed in a Piper Cub Plane.
Whether the receiver tuning and control section is mounted in
the instrument panel, overhead or elsewhere within arm's reach,
must be determined by the user of the equipment and may be limited
by space available in the instrument panel, vision, movement of
controls or other considerations. The wide variety of individual
demands and limitations imposed by previously installed equipment
makes it difficult to recommend standard installations in aircraft
already in use.
Center of Gravity
While an aircraft radio mechanic can scarcely be expected to
be responsible for the weight and balance of the aircraft in which
he is installing or servicing radio equipment, he should be concerned
when his installation or the possible addition of radio equipment
will affect the center-of-gravity consideration (Fig. 6A).
Every particular airplane is designed to do a certain job and
fly within stipulated load limits. The top limit is figured with
a factor of safety sufficient to allow for stresses and strains
of all ordinary operations. Attempting to fly an overloaded airplane
will definitely decrease its range, increase its stalling speed,
and require a longer runway to take off and land. Should an overloaded
airplane run into rough weather or experience any other unforeseen
trouble, the margin of safety due to high gross weight will be reduced
to such an extent that the pilot may not pull through.
Balance will affect an airplane's performance more than overload.
An airplane slightly out of balance will not only change the spin
and stall characteristics of the airplane, but will render it uncontrollable,
and if a pilot encountered trouble in the air, he could not recover.
- Fig. 6B.
The center-of-gravity (CG) for a loaded airplane should be that
point at which it would balance itself were it suspended, as shown
in Fig. 6A.
Shock and vibration are of relatively great amplitude and of
three-dimensional character. The impact experienced when an airplane
"bumps" in for a landing on rough terrain, is often of considerable
magnitude and may cause serious damage to radio equipment if adequate
cushioning of shocks is not afforded.
Fig. 4. - Cloud layers that are electrically
charged cause corona discharge. This charge Introduces severe
electrical pulses in the radio receiver.
Also, it is important that radio equipment, in aircraft, be properly
protected from the weather, as well as from such trouble makers
as corrosion, commonly known as electrolysis. Two dissimilar metal
bodies making contact with each other in salt air causes an electrolytic
action to set in. This not only corrodes the parts, but results
in a difference of potential, which causes noise in the radio.
When installed in an instrument panel and when using rubber mounted
brackets, sufficient clearance must be allowed in the size of the
hole cut in the panel to prevent the receiver from vibrating and
rubbing the instrument panel. Mount the receiver from the front
of the panel. Mount the power unit wherever space is available -
baggage compartment, cabin floor, or on a special mounting in the
rear of the fuselage. Because the power unit need not be accessible
for operation, it may be installed forward or aft in the fuselage
to bring the center-of-gravity to a desired point.
The various switches and controls may be installed wherever convenient
and each cable to them should be securely taped and laced into place.
Danger! Be certain that in lacing any electrical or mechanical cables
they are not fastened to any of the aircraft controls. Double check
all controls upon completion of the installation to be sure that
they are free!
Don't fasten the unshielded antenna lead parallel to any metal parts
of the aircraft, such as the fuselage tubing, for a greater distance
than necessary. The capacity between the antenna lead and such metal
surfaces reduces signal strength. The antenna lead-in should not
be shielded, except where transmission lines or coaxial cables are
Don't run the antenna lead near any of the ship's ignition, or
electrical wiring. This includes the primary of the ignition system
to the ignition switch even though these leads themselves are shielded.
Don't route the battery lead near the aircraft's magnetic compass.
Check this by switching on the receiver and noting its effect on
the magnetic compass.
CAA and FCC Rules
A mechanic installing radio equipment in aircraft need not be
a certified CAA mechanic. However, his work must be examined by
one, to be sure it does not provide a hazard in plane operation.
This is the only requirement in the Civil Air Regulations on this
Fig. 5. - Cutaway view indicating the
various parts of a Piper Cub plane.
According to the CAA, " ... Anyone using an aircraft radio transmitter,
whether in checking frequencies or contacting the tower, according
to FCC rules, must have at least a Class 3 operator's license."
The FCC has no specific requirements for radio mechanics performing
work on aircraft radio equipment. However, personnel making adjustments
to non-governmental radio transmitting equipment which involves
the radiation of energy, should hold a radio operator's license
of the second class or higher, either radio-telephone or telegraph.
Now, the $64 question is - if it only takes a restricted phone
ticket (3rd class phone), to operate the equipment in a plane, why
make the poor mechanic take that "engineering course" - 2nd class
phone or telegraph exam?
Aircraft radio servicemen should be cognizant of that type of
static which is so common to aircraft radio-Precipitation Static
- new pilots experiencing the brilliant fiery display on their prop
tips, on the windshield, or along the wing and tail surfaces (as
shown in Fig. 7), and have experienced the difficulties of the accompanying
static, will expect the serviceman to do something about it. Actually,
there is very little you can do, except give the customer an explanation
of precipitation static, and then install a loop antenna, instructing
the customer in its use.
It has been discovered that certain types of aircraft antennas
are definite sources of precipitation static, since the antenna
is directly exposed to extremely high electric fields.
These high fields cause corona discharge, which introduces electrical
pulses directly into the radio receiver, shock exciting it, resulting
in noisy precipitation static, Fig. 4.
Some of the present day static dischargers are not successful
in eliminating this interference at the antenna. More desirable
methods are to use an insulating plastic covering on the antenna
wire or to use a bare wire of a very large diameter. All metallic
connections to the antenna and lead-in should be covered with an
insulating dielectric material, preferably plastic. However, even
these methods are not effective, since it is possible on first encounter
with an electrical field of sufficient intensity, the insulation
will be punctured and you will be right back where you started.
Fig. 6. - (A) The stipulated center of
gravity of any particular plane should always be maintained.
If, when adding radio equipment, the center of gravity shifts
as depicted in (B), the plane will be rendered uncontrollable.
(C) The maximum load limit of the plane should never be
exceeded. In designing airplanes, the specified maximum
load limit is considered when allowing for safety factor
sufficient to cover all stresses and strains.
It has been common practice for quite a few pilots to reel out
their trailing antenna, grounding it to some metal part of their
airplane, in an effort to provide a discharge path for the charge.
This is a waste of energy since the discharge noise is worse than
that which the pilot is endeavoring to eliminate, and is especially
dangerous in storm areas, as the trailing wire will function as
a lightning rod, inviting lightning to strike and ruin the radio
One of the primary requisites for noise-free reception is proper
grounding or so-called bonding of individual units of an installation.
Each unit should have an individual grounding strap securely fastened
to the nearest metal part of the aircraft structure. When this structure
is steel tubing, the plated copper braiding can be soldered directly
to the tubing after paint is cleaned away. This operation requires
a heavy-duty soldering iron and the connection should be cleaned
off and repainted or shellacked to prevent corrosion or rust.
Non-metal fuselage aircraft present a special problem in this
respect, and the aircraft manufacturer should be consulted for the
right method of bonding, if the usual method of bonding the motor
mounts, the motor, and all metal braces, etc., does not prove adequate.
It is important in any aircraft, that the primary, as well as
the secondary of the ignition system be completely shielded.
In some cases, control surfaces or moving metallic parts will
produce noise in the receiver. Bonding of such offending points
will usually eliminate this interference.
Antenna selection is a matter controlled by the flight operations
of the ship in which the radio equipment is to be installed. Generally
speaking, the results obtained with various antennas are as follows:
A whip antenna, while it gives the least signal pickup, does
have the desirable characteristic for range or instrument flying
of accurate and true radio range signals. It also gives a sharp
"cone of silence" indication.
The fixed antenna, mounted either overhead from the vertical
stabilizer to a mounting or mast, forward on the top of the fuselage
or the "belly" version mounted underneath the fuselage to another
mast or to the landing gear struts, is called a "T" antenna when
the lead-in is taken off the center. When the lead-in is taken from
the forward end, this is known as the "L" antenna.
When the receiver is mounted so that the antenna lead will be
run nearly vertical to the "T" antenna, a truer range signal may
be generally expected than with the "L" type. The signal pickup
with both of these antennas is good.
The trailing antenna, whether of the retractable reel or of the
permanent trailing type, will give the maximum signal, and consequently
should be used when trying to receive long distance or weak stations.
The use of the trailing wire antenna, when an accurate range signal
is desired, is dangerous. This is especially true when "close in"
or "on instruments."
It is feasible to have the benefits of all of the aforementioned
antennas by the proper installation of a switching system. A suitable
switch, or remote operated relays may be used for switching from
whip to trailing.
The selection of antennas on such a switching system should be
clearly marked to prevent using the wrong antenna and possibly receiving
erroneous range signals.
It is desirable to mount the antenna so that the lead-in will
be kept to a minimum. Insulated ignition wire (unshielded) makes
good lead-in wire.
Most aircraft radio equipment is operated from a 12-volt storage
battery, with a generator being employed to charge the radio battery
during periods in which the aircraft engine is in operation. Voltage
regulators are used to maintain proper charging rate, as in the
conventional automobile electrical systems.
The open lead from the volume ontrol switch should be connected
through a 10 ampere fuse to the nearest source of +12 volts. Most
aircraft have a master switch or master fuse panel, with an additional
fuse and connector provided for a radio circuit. All wire in this
circuit should be number 14 or larger, copper stranded, and well
insulated. Don't route this lead near any receiver antenna lead
or near the magnetic compass.
The shield of this lead is the negative return and should be
securely fastened to the nearest metal portion of the ship's fuselage.
Upon completion of installation, check the operation of the receiver
and the dial calibration, with the ship's engine running.
Failure to operate or improper performance means that the installation
wiring must be checked. Refer to troubleshooting recommendations
on page 30.
Fig. 7. - Illustration of precipitation static collected
on plane in flight.
The on and off switch is incorporated in the receiver volume
control and the battery power is supplied to the receiver through
One or more pairs of low impedance (200 ohms) headphones may
be used, plugging them into either of the two headphone jacks, which
are connected electrically in parallel.
The band selector switch is set to the number of the band to
be tuned as indicated on the main tuning dial.
Control tower reception is facilitated on band 1 by the 278 kcs.
frequency being "spotted" on the dial.
Sidetone connection is provided through a binding post on the
power supply, permitting the pilot to monitor his transmissions
over and above the receiver reception.
Interphone communication is possible through the INT. connection
on either of the power units.
Code reception is possible by the use of the telephone-telegraph
Servicing, with the exception of the aircraft itself or mechanical
troubles, should be done by competent radio service personnel having
technical ability and the necessary test equipment to perform such
Faulty operation should be localized in (1) the equipment, (2)
attached accessories, such as headphones, antennas, etc., or (3)
in the aircraft, its battery, the engine, or the installation.
Trouble in the equipment itself should likewise be isolated in
the following order - in power supply, audio and output section,
second detector, intermediate frequency amplifier, or in radio frequency
Unless it is definitely known that radio frequency circuits are
in need of realignment and the necessary test equipment and tools
are at hand, "tinkering" with these circuits should not be tolerated.
Shorted armatures are indicated by low output voltages and high
input current. By disconnecting the output leads and running the
dynamotor at its rated input voltage, if the input current of the
G-3-AB dynamotor (without load) is in excess of 1.5 amperes, a shorted
armature is usually indicated. Input current in excess of 3 amperes
(without load) on the G-30-AB dynamotor is also the usual indication
of a shorted armature. The armature should be removed and tested
on a growler.
Trouble shooting noise in aircraft equipment is difficult because
of the many possible sources. Noises must be isolated, such as (1)
in the equipment; (2) in the installation; (3) at the antenna, battery,
or other accessories; (4) in the aircraft or its engines, or finally,
(5) whether the interference is of non-man-made sources, such as
the various static conditions encountered.
1. A good signal generator covering the range of frequencies
used in the receiver.
2. An output meter, 0-50 milliwatts, 200 ohms impedance.
3. An insulated, non-metallic screwdriver for condenser trimmers.
4. A small metal screwdriver for r.f. coil core screws.
5. One 0.1 μfd., and one 100 μμfd. condensers.
1. Connect signal generator ground on output lead to receiver
2. Connect output meter in parallel with audio output.
3.Allow receiver and signal generator to "heat up" for accurate
4. Connect recommended dummy antenna condenser in series with
signal generator output lead.
5. Before aligning, set dial pointer to small dot at low frequency
end of dial scale when rotor plates of tuning condenser are closed.
6. Receiver volume set to maximum unless noise level is excessive.
7. Signal generator output set to limit receiver output to 50
milliwatts or less.
8. 30% modulation on signal generator output.
9. BFO switch set to "phone."
10. Sensitivity - 50 milliwatts output with 2 microvolts input;
4:1 signal-to-noise ratio.
The length of time between periodic maintenance checks on aircraft
radio equipment will vary under the particular conditions of usage,
i.e., temperature, humidity, and corrosive elements, etc. An arbitrary
period of 200 actual-equipment-operating hours may be used, checking
When lubrication is needed, remove the end covers and bearing
plates, clean with gasoline and apply enough Royco 6A grease (or
equivalent) to cover bearings. Do not pack bearings. Keep grease
The dynamotor should be given a periodic visual inspection for
armature shaft alignment; smooth, free-running bearings and end
play in armature.
The commutators, especially, should be inspected. Keep them clean
by the use of "wet-or-dry" sandpaper. (Don't use emery cloth.) When
commutators show signs of excessive wear, the armature should be
removed and commutators "dressed."
Brushes must be kept well seated and not allowed to wear so short
that the spring pressure is insufficient. Any excessive sparking
of the commutator should be investigated as to cause and the trouble
All relays should be checked and kept clean and the contacts
trimmed only when necessary. Fine sandpaper should be used on heavy
duty relay contacts, followed by carbon tetra-chloride, to remove
sand grit. When low current, small contact relays, are cleaned,
special relay tools should be used whenever possible. Don't adjust
relay arms and springs unless necessary.
While the user will usually detect any error in dial calibration,
it is well to check it closely during any inspection. This should
include mechanical inspection of pointer and knob, as well as circuit
alignment check. The 278 kc. control tower spot on the dial should
not be overlooked.
When equipment is at hand, the receiver sensitivity should be
checked and when low, proper service be effected.
The aircraft radio serviceman will find it necessary, on many
occasions, to check frequencies with the control tower while he
is working on a ship, either on the line, or in the parking area.
He should familiarize himself with correct control tower and voice
Avoid personal remarks or "rag chewing," and limit each transmission
to strictly business. Don't use a normal tone when speaking into
the microphone; raise your voice slightly and make pronunciation
distinct. Knowledge of the standard phonetic alphabet is very useful.
The control tower may, at times, find it necessary to resort to
use of a signal light gun to flash you a warning or instructions.
Learn to interpret signals properly and obey them, as your safety
or that of someone else may be at stake.
For the "wide awake" radio serviceman, the foregoing will serve
to reaffirm his belief in the future-a future filled with opportunities
The era of the business lull being filled with toaster, washing
machine, and vacuum cleaner fixing, is strictly passe. The radio
serviceman is a bona fide member of the electronics fraternity,
and as such, he should devote his time and energy to furthering
his accomplishments there, leaving the maintenance of mechanical
"household gadgets" to others.
Installing and servicing radio aircraft equipment is vital to
air travel, destined to become even more so in the future. Being
in on the "ground floor," established at a field with a "future"
and making it pay, will be the reward of those who are farsighted
enough to reserve a "front seat center" for themselves, in the drama
Contrary to widespread opinion, installation and servicing of
aircraft radio is not as complicated or intricate as it might seem.
Although there are problems, such as precipitation static, bonding,
electrolysis, vibration, and a few others encountered in servicing
aircraft radio, there is not a great deal of difference between
its installation and maintenance and that of the household radio
Many radio servicemen have veered from familiarizing themselves
with the aircraft radio field - why, we don't know, as it is definitely
a lucrative field, and one in which every "go-getting" serviceman
should become interested. Of course, all servicemen will not be
interested in changing their clientele - either because they are
firmly entrenched in household radio servicing, or because of other
interests. However, we have endeavored to awaken and kindle an interest
for this vital field by showing that it is not only a profitable,
but a long-lived enterprise.
Qualified radio men are needed now, and servicemen who have both
sound and dependable training behind them should "climb on the bandwagon,"
in order to be assured of not "missing the boat" in the bumper field
The author is indebted to a number of individuals and organizations
for assistance rendered in the preparation of this article. Mr.
Henry J. Hamm, of Lear, Incorporated, made many of the photographs,
circuit diagrams, and much of the data on aircraft radio equipment
available. Thanks are also due Piper Aircraft Corporation for the
use of photographs, diagrams, and data.
*Formerly associated with the U. S. Navy Bureau of Aeronautics,
and Aircraft Radio Laboratory. Wright Field. Dayton, Ohio.
Posted April 27, 2015