Compass adjusting
Richard L. Watson
I own a 1959 C-172 and have been a marine compass adjustor
on yachts, commercial vessels, coast guard vessels, naval
ships and civilian ships for over 20 years. Degaussing will
do nothing on a Cessna 172 since there is very little
steel or iron in the airframe. There is nothing to become
magnetic. This can however be a problem on steel framed
planes (fabric) and I understand in some planes that are
both steel and aluminum. Someone suggested a Narco
indicator. I just don't know about that, but if it is a
problem in other planes that could be it. Have you tried a
new compass? Have you had someone that knows what they are
doing compensate it? Have you checked the condition of the
bearing in the compass (move it slightly with something
slightly magnetic, just a degree or two and see if it
returns to the original reading). Have you tried removing
the compass from the plane to see how it behaves away from
the plane? Is the compass mounted in the middle of the
windshield? Does the problem occur with the master switch
off? Are the compass wires twisted to eliminate magnetism
from them? Are any high current wires near the compass in a
tightly twisted pair to eliminate magnetism? Are there any
items which are magnetic near the compass such as another
compass, a speaker in a radio (including handhelds) or in
the glove box. Magnetic or steel tools. Once I had a
problem adjusting the compass on a Mexican commercial
fishing vessel until I moved the magnetic dashboard Mary
from near the compass? This problem should be easy to solve
in a 172.
Following are directions for adjusting a compass.
Richard L. Watson
Compass adjustor
THE MAGNETIC COMPASS: INSTALLATION, ADJUSTMENT AND
PREPARATION OF DEVIATION CARD.
1. The compass should be installed as far away from
magnetic influences as possible. Loudspeakers have strong
magnets as do some VOR heads. Wires which carry a lot of
current such as main battery cables and alternator or
generator output wires can produce a strong magnetic field.
Any ammeters should be the shunt type, and not the full
current type such as are used in automobiles. Finally any
current carrying wires in the immediate vicinity of the
compass and especially the compass light wiring should be a
twisted pair to reduce the magnetic field.
2. The compass must be mounted so that it is aligned
exactly in a fore/aft direction. It cannot be twisted for
an easier view for the pilot. The face of the compass
should be vertical. In summary, the three axes of the
compass should be parallel with the three axes of the
airplane. No portable avionics should be placed near the
compass unless you have determined that it has no effect on
the compass either turned on or turned off. With the plane
parked, move it around in the vicinity of the compass. If
the compass card moves, it is too close.
3. With the plane parked, turn on each item of electrical
equipment one at a time. If the compass card moves, you
have a problem. I believe that it is best to adjust the
compass with everything off. You may have to navigate that
way with a total electrical failure and will be really
dependent upon the compass. If any of the electrical or
electronic equipment significantly affects the compass
prepare two deviation cards, one with the equipment on and
another with the equipment off. The FAA is correct in
believing that the magnetic compass is your most important
navigation instrument, it is the only one that is required!
4. Proper procedure to correct the compass.
Even a brand new magnetic compass needs to be checked for
accuracy and perhaps adjusted. The compass contains
powerful adjusting magnets which are probably not set to
zero effect when the compass is new, therefore even if there
is no magnetic field present due to equipment in your
aircraft, the compass will still probably be in error. It
is also unlikely that your aircraft is free of any magnetic
field in the vicinity of the compass.
The magnetic error due to deviation is approximately a sine
curve. There will be two compass directions in which there
is no error and two other directions approximately 90
degrees away from the zero error headings where the error
will be maximum. One will generally be easterly error and
the other westerly error. If all of your errors are in one
direction (either easterly or westerly), then either the
compass is not correctly aligned with the axis of the
airplane, or your compass rose on the pavement is not
correct. If the compass rose on the pavement is very old,
then it may be off by several degrees due to the slow annual
change in the earth's magnetic field.
Check the accuracy of the compass by aligning the airplane
on the compass rose in the N, E, S, and W directions. The
compass rose on the pavement MUST be a magnetic compass rose
and not a TRUE compass rose. If there is little or no error
on these cardinal directions it does not need to be
adjusted. Use an outside observer to be sure that the
airplane is very accurately placed on the compass rose. One
degree of mis-alignment here will result in 1 degree of
error in the compass.
If it does need to be adjusted do the following:
Align the airplane on the east magnetic line of the rose.
Adjust the compass to read 090 with the adjusting screw for
east/west.
Align the airplane on the 180 magnetic line. Adjust the
compass to read 180 with the adjusting screw for
north/south.
Align the airplane on 270. Read the compass. If there is
any error, remove 1/2 of the error with the east/west
adjusting screw. This will divide the error for east-west
equally between east and west.
Align the airplane on 360. Read the compass. If there is
any error remove 1/2 of the error with the north/south
adjusting screw.
It is critical that the airplane be very accurately aligned
with the compass rose. Do not use runway alignments, as the
runway numbers are accuate only to the nearest 10 degrees.
If you obtain over 2 degrees of error, try again if you wish
higher accuracy. I generally would adjust on the east and
if there was an error when I got to west, would again adjust
the compass to zero error on west. Only if I then got back
to the east for the second time and an error still remained
would I divide it by removing 1/2 of the error. This is
because my first adjustment may not have been exactly
correct. Use the same procedure for north/south.
After as much of the deviation is removed as possible, Then
orient the plane on each 30 degree heading and record the
compass reading and compute the deviation for each heading
for use on the deviation card.
It is helpful to remember the following
Error east, compass least.
Error west, compass best.
This is just a gimmick to determine whether the error is
easterly or westerly. That is if the compass reads greater
than the actual magnetic direction the error is westerly.
Example: Plane is oriented 090 on the pavement rose and
compass reads 093, error is 3 degrees west.
It is important to be sure that there is no movable magnetic
material within 3 feet of the compass. This would include
some portable electronics, all portable radios with
speakers, some earphones, magnetic screwdrivers, some tools,
etc.
Look at what is in the glove box.
Richard L. Watson
Compass Adjustor for ships and boats.
rwa...@centuryinter.net
Rod Farlee
"Richard L. Watson" <rwa...@centuryinter.net> writes:
>...Degaussing will do nothing on a Cessna 172 since there is very little
>steel or iron in the airframe.
Well, some older radios and indicators have steel shield cages. The
compass is mounted right above them.
Compare compass readings with the radios on/off, and master switch
on/off. Sometimes a radio is grounded to the firewall, too close to the
compass. Relocating the ground can solve that. Or if the ground has
broken, current might flow through the radio rack and glareshield itself.
Check the wiring to the compass light. The two wires should be
twisted together. Remove the compass from it's mount to check
inside. If they aren't twisted, the current to the light might cause a
deviation (it's a small current, but very close).
Aircraft speakers have shielded magnets. You'll see the soft iron
cylinder shield around the outside of the driver, behind the speaker cone.
Might check that someone hasn't replaced it with an unshielded
automotive or hi-fi speaker.
There are (rare) reports of the engine crankshaft itself being magnetized.
(Can this be caused by improperly performing the magnaflux inspection
during overhaul?) The engine mount is also steel. On older 172s with
O-300 engines, the generator is at the top rear of the engine, not that
far from the compass. It should be easy to detect the source of any
problem by hunting around the engine compartment and instrument
panel with a compass in hand.
- Rod Farlee
Mark Mallory
: There are (rare) reports of the engine crankshaft itself being magnetized.
: (Can this be caused by improperly performing the magnaflux inspection
: during overhaul?) The engine mount is also steel. On older 172s with
: O-300 engines, the generator is at the top rear of the engine, not that
: far from the compass. It should be easy to detect the source of any
: problem by hunting around the engine compartment and instrument
: panel with a compass in hand.
: - Rod Farlee
On my '182 the alternator is mounted at the rear on the engine.
Switching the (nominal) 2 amp field current on and off produces a 10 degree
change in the compass reading on certain headings. Because the field
current will change with changes in the electrical load and RPM, this
will result in a *varying* compass error.
I'm not quite sure how to solve this problem. I don't believe it's
practical to shield the alternator, since such would interfere with the
cooling airflow. I'm wondering if a small "compensating coil" mounted a
few inches from the compass (connected in series with the alt field and
producing an *opposing* magnetic field) might possibly work?