1115 Oak Grove Rd.

Savannah TN. 38372

Ph. 731-926-2820

The ARC Navomatic 300A Autopilot

Please note its operation highlighted in “blue”
below:
The ARC 300A was an option on many single engine Cessna
aircraft available near the mid-seventies and stayed on the
option list up through the T210L. Cessna 172M, 180J, 182P, TU-
206F, these are but a few aircraft that had the 300A on their
option list. I’ve been told the 300A was available for the twin-
engine 337 Skymaster but I’ve never seen on installed in a twin.

The reason I’m writing this article is because for some reason
I’ve received a lot of tech calls about this product, even though
this autopilot has been out of production for well over a score.
Maybe this writing will answer some of your questions as to the
theory behind the 300A and what it takes to fix it.

The ARC 300A autopilot is what we call a “single axis”

system, meaning it only controls the roll axis of the aircraft, only
connected to the ailerons. This system has no effect on the
other two axis, pitch and yaw. All aircraft rotate around three
axis, roll, pitch and yaw; the 300A is only utilized in the roll axis.
This autopilot is all electric, thus if you lose vacuum the 300A
could keep the wings level. Yes, the vacuum directional gyro
would cease to function but still the little 300A could save your
bacon if you experienced a vacuum failure during IMC (more on
this later). In a nutshell, here’s how the 300A operates. Roll
and yaw changes in the aircraft are sensed by the G-300A Rate
Gyro (turn-coordinator). These changes are what we call error
signals which are then sent to the computer. The computer is
the box that is located in the bottom of your radio stack that has
300A Navomatic written on it. Also, when the heading or nav
function is selected, any deviation from the selected input is
transformed into an error signal, which is then processed by the
300A computer. On a good day, the computer figures out the
correction needed and moves the aileron via the actuator/servo.
Basically the autopilot computer looks for an error signal and
tries to null out that signal. The 300A is a very basic system and
normally works fairly well in the heading mode. The complete
system, including mounting brackets weighs around 10 lbs.

Lets Talk Components. The ARC 300A’s computer is called a

CA-395A.
Again, this
is the box
at the
bottom of
the radio
stack with
all the
switches
on it.
There’s a
lot of
electronics
inside but
nothing
modern, remember your 300A may be 25 years old. The panel
computer came in a black or gray color and the later model
computers had a wide faceplate. While they may be the same
electronics wise, the newer model computers may not fit in your
older Cessna due to the wide faceplate. The faceplate is a real
chore to change out, don’t go there. This computer receives
signals from the G-300A Rate Gyro, navigation indicators and G-
502A/B heading indicator and possibly your navigation
equipement. All controls of this system are located on the front
of the computer with the exception of the heading bug, which of
course is located on the directional gyro in the instrument panel.

The Control Switches located on the front of the

300A do the following:

A/P. turns the 300A on or off.

Pull Turn. When pulled out and centered in the detent,

the aircraft will fly what it thinks is wings level. When
turned left or right, the aircraft will respond by flying in
the selected direction. When properly adjusted, max
turn will not exceed standard rate. The exact turn is
determined by model of the aircraft and is spelled out
in the alignment procedure. In other words, I couldn’t
just yank a 300A computer out of a Cessna 185 and
install it in a Cardinal without performing the alignment
procedure, even though the computers are the same.

Trim. Move the white trim control to compensate for

variations in the aircraft trim or weight distribution. Be
SURE to adjust the rudder trim prior to messing around
with the Trim on this computer.

NAV. The pilot can select either a NAV 1 or NAV 2

tracking source. This source could be VHF Nav, Loran
or GPS, depending on avionics installed and how the
system is wired.

HDG SEL. The best part of the 300A is its capability to

track the heading bug. Push in this button (Pull Turn
pressed in) and the A/P will follow the heading bug.

NAV INT. In theory when the Nav Int button is pressed

in, the 300A should fly the intercept course to the Nav
data you have selected. In “Real World” it may do
anything or nothing, depending on the 300A’s mood at
that time.

NAV TRK. Now this is important, so listen up. When

the heading bug is set to the SELECTED course and the
PULL TURN knob is pressed in, the aircraft may fly the
selected course. I’ll go into this subject in detail later
but be advised the DG is part of the NAV TRK.

HI SENS. During NAV INT or NAV TRK usage, sensitivity

of the 300A is enhanced to provide more precise
operation. In the low-sensitivity position (button out),
response is somewhat dampened for smoother
operation. Normally when tracking a nav function,
you’ll want this button pressed in even though the POH
says different.

BACK CRS. This function is available in the Localizer

mode only. In a factory installation, when BACK CRS is
pressed in it will reverse the direction of the needle on
the selected navigation indicator. This allows the pilot
to shoot the back course without the needles deflecting
the wrong direction. For more information about back
course discuss this with your local CFII. The localizer
needle will only be reversed under the following
condition. Back CRS button is pressed in, a localizer
frequency is cranked in on the selected navigation
indicator and the A/P is turned on. Aircraft that have
upgraded radios may no longer have the BACK CRS
feature.
HDG. When this button is pressed in, the A/P is now
going to track the heading bug on the directional gyro.
If the heading bug is not centered under the lubber
line, then the A/P will turn in the proper direction, near
standard rate turn
to place the bug
under the lubber
line. As the bug
comes near the
lubber line located
at the top of the
directional gyro,
the bank angle
will decrease. Be
sure to set your
directional gyro to
your calibrated
wet compass on a
regular basis.
The knob on the right, bottom of the directional gyro
sets the heading bug; the knob on the bottom left sets
the compass card within the directional gyro when
pressed in and turned.

Actuator/Servo. Unlike the computer, the actuator/servo

voltage is sensitive. The PA295B is for 14Vdc operation and the
PA495A operates in the 28Vdc system; baring the voltage issue,
they are very similar. The purpose of the actuator is to take a
small electrical signal and convert it to a mechanical output that
will operate the aileron. This system doesn’t have slipping
clutch, the clutch is electronic and is easily overridden. The force
to override the actuator is only slightly higher than normal
control pressure. The PA495A actuator does incorporate a
thermostatic switch (built on the motor) that removes power
from the actuator should it over heat. It will reset itself once it
cools down, normally about 10 minutes. This thermo-protection
is a common problem with this system. Actuators are built for a
particular model aircraft and MUST be used in the aircraft they
were designed to operate in. Yes, you may find an actuator out
of a 210 will fit in a 182 but the torque settings are different and
should not be used under any condition. Normally the avionics
shop will verify you have the correct actuator installed if they are
working on the system, I couldn’t began to count the number of
times I’ve found the wrong servo in a Cessna; just because it will
bolt in place doesn’t make it the correct part. . The actuator
slides into a mount, thus allowing easy removal for
troubleshooting and repairs.

Actuator/Servo Mount. The computer sends the actuator a

command; the servo electronics develop a signal that turns a
gear on the end of the Servo Motor. The servo is mounted inside
the servo mount and the gear turns a larger gear within the
actuator mount. The gear ratio is determined by the model of
Cessna you may have. This large gear is connected to a chain
sprocket that moves a chain; a “shear pin” joins the large gear
and sprocket. The purpose of the shear pin is to shear should
actuator or actuator mount develop a problem. Lets say the
motor froze in the actuator; the pilot could give the controls a
brisk turn and the shear pin would shear, thus normal operation
of the controls would be restored. I’ve seen dozens of shear pins
in my 23 years of general aviation fail but none were due to an
actuator or mount failure. The shear
pin is often a failure point and we will
discuss why later in this article. Each
actuator mount has a tag on the side
stating the color and part number of
the pin that should be used in that
application. A good avionics shop will
verify the proper pin is in fact
installed in your mount if they are
working on the autopilot. If the
actuator mount is removed from the
aircraft, the bridle cable tension
should be reset after installation.

Rate Gyro (Turn Coordinator). Yes, the turn coordinator (TC)

you see in the panel is the main input to the 300A computer. All
ARC 300A A/P’s used what is called a G-300A gyro. There are
two different models available but functionally they are the
same. A lot of autopilot problems are generated from the output
of this gyro; but without the correct test equipment one may
start tweaking on the computer to cure the problem but chances
are they will chase their tail and never accomplish anything. It
all boils down to knowing what you’re doing and having the
correct test equipment. Rate of turn is displayed via a symbolic
airplane, which is driven by the precession of the gyro. It’s ironic
that Cessna used a TC that shows a low wing aircraft The
newer Cessna’s now have a high wing aircraft inside. A warning
flag is visible and if it’s red that’s a bad thing. Aircraft power
pulls the red warning flag. Even though the flag is out of view,
the gyro may not function properly. This warning device has
nothing to do with the proper operation of the TC; only tells the
pilot there is power to the indicator, nothing else.

Directional Gyro. The ARC 300A came with the G-502A DG or

the optional slaved G-502B. Normally with the slaved DG, you
set the heading once after the gyro is up and running, then
you’re done; the electronics will take care of precession. The G-
502B has a flux gate and slaving meter located somewhere in
the
aircraft.
You’ll
need
one of
these
two
heading
systems
for your
ARC 300A to operate properly. Few autopilot problems are
related to the G-502A; most gyros get changed out due to
excessive precession. We have had cases where the heading
bug internal adjustment was so far out of spec that we couldn’t
align it to the computer. Anytime the 300A computer has
Heading selected, the computer is looking at the DG and TC.
Both of the above model directional gyros are vacuum driven.

It’s a Matched System. Originally all the components

mentioned above were installed as a matched system. Someone
on the bench aligned all the pieces together prior to installation
in the aircraft. You should not “just” install a DG, turn
coordinator or other component in the aircraft and fly away;
chances are it will never fly as it was designed to. There’s a good
chance the aircraft will fly to one side of the heading bug, wing
rock or some other strange symptom. In my opinion, an AF-
395A autopilot (300A) should not be repaired or serviced unless
a H-42A test set is in the hands of an experienced repairman.
Normally the actuator/servo can be replaced without any
adjustments being made.

Here’s the Way I Align the 300A.

Using the H-42A test box and related extenders, the first
thing I do is measure the TC output and verify there is little or
no output when the TC is level and no movement. Often we see
a huge error signal from the TC when in fact there shouldn’t be;
often this signal is too large to adjust out using the adjustments
on the autopilot computer (these specs are called out in the
maintenance manual). If the TC output isn’t up to speed, then
an overhauled unit must replace it. We use a turntable to verify
proper operation of the instrument during turns. If the TC
generates excessive noise or the warning flag doesn’t function
properly, then it’s replaced, no questions asked.

Next I measure the output of the G-502A/B Directional

Gyro to verify the proper output is present. Often we find there
is excessive error from these gyros when the heading bug is
centered (nulled out), too much error to adjust out with the
adjustments on the computer. If so, the gyro should be
replaced. If the unit passes this test then we spool the gyro up
on a bench and check for precession. You see, the inputs to the
computer must be correct or the 300A will never fly the aircraft
as you would desire. Ever heard the saying “Garbage in,
garbage out”?
Normally a shop
will check the
female pins that
are in a connector
that attaches to the
G-502A; these
often spread or
show signs of wear
because of age.
Once the gyro has
spooled down, it is
removed from our
rate table and
carefully installed in the aircraft.

Inspection of the Actuator Mount. Did you know Cessna

recommended lubing the drive chain and gear teeth every 100
hours? When was the last time your autopilot mount gears and
chain were lubed? Move the ailerons up and down, if they feel
ratchety, then you’re way past service in this area. Old grease
should be removed prior to installing the new lube. I’d highly
recommend your A&P check the bridle cable tension at every
annual inspection along with lubing the actuator mount.
According to the installation manual, bridle cable tension should
be 10+3 lbs. While lubing the mount components, be sure to
check for loose hardware. We’ve had a few cases where the
posts that act as chain guards have become loose. Oh, don’t
just spray solvent on the servo mount and actuator. The
bearings in the actuator motor are not sealed, thus the solvent
will enter and cause all kinds of expensive problems.

Next is the 300A Computer Alignment. Believe it or not,

aligning this little computer isn’t a huge deal, well unless there’s
a problem on one of the internal boards. Using the H42A test
box makes life simple. I connect the TC and DG into the
autopilot computer while on the bench and align it as a system.
If all of your components are good and you align the 300A as
system on the bench, chances are it will fly the aircraft fairly well
in the heading mode.
VOR navigation
seldom is
satisfactory with the
300A. Why? Well,
it’s a cheap autopilot
and it’s getting old.

The Test Flight. In

theory, you could
slap the components
in the aircraft, grab a check from the aircraft owner and kick him
out the door but I always insisted on a test flight to verify proper
operation of the system. Yes, there are a couple of airframe
issues that will cause havoc with the 300A. We’ve got what is
called a “Break-Out” box that goes between the aircraft autopilot
connector in the radio stack and the back of the autopilot
computer. In other words, I’m able to have the computer in my
lap and perform adjustments while someone flies the aircraft.
Often minor adjustments are required during the test flight or in
some cases we find the system will not work at all due to
airframe problems.

Most of the Flight Test Problems are Related to VOR

Tracking. As mentioned earlier, the 300A is weak when
tracking a VOR but often the problem is PILOT INDUCED. Here
are the steps that must be done in order to track a VOR, GPS or
whatever. Let’s say you are tracking a VOR TO the station and
the needle centers up on 120 degrees. While still in the heading
mode, verify your DG reads close to your magnetic compass and
then set the heading bug to 120. Once the aircraft has turned to
the 120 heading, press Nav and verify you have the proper nav
indicator selected with the 1-2 switch. There’s a high probability
the system will now track the VOR, it may be somewhat sloppy
and if that’s the case, press the HI SENS button on the
computer, things may get better. You’ll probably notice the
300A will track a Loran or GPS much better than a VOR. Bottom
line is this…Whatever the Bearing TO the waypoint is, the
heading bug MUST be set to the same. In the NAV mode the
300A autopilot looks at both Heading and NAV left/right input.
Don’t expect miracles out of this system but if properly aligned it
does a fair job in the heading mode and may track the nav. For a
good description of the operation of your 300A Autopilot, read
the Supplement in the back of your Pilot’s Operating Handbook;
it explains the operation of the unit quite well. When the POH
says the word “Will”, substitute the word “Might”.

There are Several Aircraft Related Problems with regards

to the 300A. One of problems is at the top, right side of the
cabin. There’s a connector there that often corrodes or the pins
get loose. Normally when autopilot will only turn in one direction
when using the turn knob or the DG, this connector would be
the prime suspect. Another high failure area is the relays for the
left/right needle movement located in the original Cessna audio
control panel. If your audio panel has been updated, chances are
these relays are gone. The symptom of failure is the CDI needle
will constantly stay in the middle of the CDI (centered needle)
regardless there really is a deviation or not! I don’t need to tell
you how dangerous this could be. If you’re flying the localizer
and doing a better job than you think you should, the relays
could be the problem. If you still have the original Cessna panel
and a 300A autopilot, here’s something I recommend doing prior
to each flight. In the run up area, turn on the A/P and press the
nav button. Now select any localizer frequency (doesn’t matter if
you can receive it or not). Now, press the Back Course button in
and out several times. Repeat the process using Nav 2. When
you select Back Course under the above conditions, it activates
the relay in question and cleans the contacts. While this isn’t a
cure all fix, it might keep you from getting a centered needle
during an approach. This problem can arise on the front course
as well as the back course (A/P on or not) so if something seems
strange on the approach check the data with the other nav. By
the way, the last time I priced those relays they were in the
$300 dollar range. To be honest, I can’t imagine anyone flying
today with old ARC radios and the original audio panel still
installed.

Loose Bridle Tension. The bridle cable along with the main
aileron cable tension is very important. If the tension isn’t within
book spec chances are the wings will rock constantly. But then
again, maybe the wings rock constantly when you fly the
machine with the autopilot off anyway….Excessive friction within
the aileron system will cause all kinds of problems. If your
controls seem too tight, then have your favorite A&P check them
out. Cables being off the pulleys, frozen pulleys are just a few of
the aircraft related problems we run across from time to time.

Loose Sprocket On
the Actuator Mount.
I’ve found dozens of
cases where the
setscrew within the
sprocket has came
loose. In 50% of the
cases where the pin
has sheared I noted
this setscrew was
loose. To check the
tightness of this
setscrew, one would
have to remove the
actuator mount. I
wouldn’t recommend
removing the actuator
mount unless a
problem arises in that
area. If the autopilot
is turned on while on the ground and if the controls are
commanded a sharp turn, often this will shear the pin. I’d
recommend having your hand on the yoke anytime you turn the
system on while on the ground so the soft pin will not shear.
Lack of lubrication within the mount can cause problems; as
mentioned above, the mount should be lubricated every 100
hours as called out in the 300A Service Manual.

Can I add an ARC 300A to my Aircraft? You may if Cessna

certified your aircraft for the 300A. One would have to purchase
the components listed above, have them installed and wired.
You may find it would be cheaper to purchase a S-Tec System 20
than install a 25-year-old 300A. Some aircraft had mounting
structure built-in the wing that was completed during wing
assembly. If your aircraft falls within this rim (only a few do),
then the wing would have to be de-skinned and the properly
mounting structure installed. No doubt, this would not be cost
effective.

I’d Like to Keep My 300A but I Really Want Pitch; What

Can I Do? S-Tec Corporation makes a System 600PSS (Pitch
Stabilization) that can be added in conjunction with the 300A
that will allow the pitch axis of the aircraft to be controlled. The
60PSS is an additional box in the panel that allows the pilot to
select altitude hold, vertical speed and displays pitch trim
annunciation; the 60PSS will allow glideslope coupling. This
system weighs 7.3 pounds and list price of the “Boxes” is
$5,995.00. The S-Tec System THIRTY ALT only contains altitude
hold and pitch annunciation and lists for $3,895.00. You may find
the cost of upgrading to a modern autopilot wouldn’t be a lot
more than the PSS. The 300A does not allow for GPSS (GPS
Steering) that allows the autopilot to track your GPS flightplan,
you’ll need an S-Tec A/P to get that nice feature.

The ARC AF-295B AKA 200A Autopilot. The 200A does not
have a heading input, in fact instead of calling the mode
“Heading” Cessna calls it “Direction Hold”. In other words it
keeps the wings level but goes wherever it likes. Expect the
200A to be a wing leveler and nothing more. The ARC 200A is
about as brain-dead an autopilot as one could get; don’t spend a
cent on it or give it to a friend. The actuator/servo and wiring
are the same as with the 300A.

I Have a 200A Installed in my Cessna; can I Update to a

300A? In all but a few cases the answer is yes. Have your local
avionics shop verify that your aircraft serial number falls within
the rim of the 300A installation range. If so, you’ll need a G-
502A directional gyro, connector and a 300A computer the same
width as your present 200A. The shop will have to install the DG
and run a few wires from the new DG to the autopilot computer
connector. Of course the new 300A computer will need to be
aligned with the components you now have. Years past this was
a popular upgrade; there’s a lot of 300A autopilots available on
the used market but keep in mind they are old and parts could
be an issue if a repair is needed in the future.

During My Days as a “300A Man” I was very successful on

repairing the 300A and getting it to fly the Cessna in a
satisfactory manner. Basically I’d have my customers bring me
the aircraft and I’d align everything as a system as stated above,
the end results were usually favorable. Often some one would
want to ship me just the computer or the servo but I’d refuse to
touch it. My
method of
repairing and
aligning the
300A worked
and I wasn’t
about to take a
“shotgun”
approach. Now
there’s nothing
wrong with
sending out a
failed
component to
get repaired at
an authorized
repair center but once that repair is complete, the 300A needs to
be aligned as a system and if the alignment is done as
mentioned in this article, chances are you’ll be happy with the
results. I can’t express enough how important to look at the
300A as a system, not individual components and have the
proper test equipment. Of course working on the system for
over 20 years does help a little Seldom do I repair the 300A
anymore; few are willing to pay the price to get it working as it
should. If you want to breathe life back into your 300A, I’d
recommend calling a shop that has the proper test equipment as
mentioned in this article and has a good knowledge of 300A
autopilot. Others may have a different method of repairing this
system but this article explains what has worked well for me.
Hopefully I’ve given you some good pointers on how the ARC
300A works and what is required to fix it.