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Aero-Tips 03.31.06
Most lightplane pilots
still fly with heading indicators that must be set manually against
the "mag compass." All pilots face scenarios where the magnetic
compass may be the only heading indicator available.
You can certainly see why you’d want magnetic compass
information to be accurate.
Mag compasses are subject errors, one being deviation, the
influence of local magnetic fields on the compass indication. Put a
metal kneeboard or headset on the glareshield and you’ll see
the effect it has on the compass. Radios and other equipment
generate magnetic fields. Portable GPS antennae need to be as far
from the mag compass as possible. Metal-tube structure airplanes
(Mooneys, Bellanca Super Vikings, etc.) also have attractive
effects on the compass.
(Note: these tubes can become magnetized,
making the compass even less accurate. It this case the airplane
needs to be degaussed or demagnetized.)
Swinging the Compass
Normally the effect of local magnetic fields is measurable. The
usual procedure is to run the airplane with everything turned ON,
aligning the airplane with known magnetic headings (a "compass
rose" painted on a ramp helps immensely). With the airplane
precisely lined up on a known heading, note the magnetic compass
indication—a process called "swinging the compass."
You might determine, for instance, that on a magnetic heading of
270 degrees, the mag compass reads 273 degrees. Log this three
degree error -- and use that error when setting the
directional gyro or when using the mag compass in an
emergency:
- Set the gyro to 270 degrees when you see 273 degrees on the
compass.
- Fly 273 degrees indicated when you need to fly 270 degrees
using the mag compass.
It’s also possible to turn compensating magnets in the
compass to adjust for the error. Use a brass screwdriver to avoid
more deviation from a ferrous tool as you make adjustments.
Since deviation varies with airplane heading, you’ll need
to turn the airplane to each 30 degree interval around the
compass and log the indicated error for each. It’s likely not
possible to "cancel out" the deviation error on all headings using
the compensating magnets.
You’ll end up with a series of magnetic headings and the
compass deviation error for each. This must be noted on a compass
correction card adjacent to the compass.
A Second Thought
If you fly an airplane with an air-driven directional gyro, you
should know compass errors effective when all electrical equipment
is turned ON. If your heading indicator is electrically driven,
however, you need to know deviation when electrical equipment is ON
(in the case of a compass slaving failure), but also when all
electrical equipment is OFF (flying the mag compass during an
electrical power failure) and when "essential equipment only" is ON
(in the case of an alternator failure and operation on a backup
system or battery power alone, when you need to conserve electrical
power). Each scenario provides different compass deviation values
you may encounter. You might consider swinging the compass in all
three power states, and logging deviation for each somewhere where
you can get to it in the cockpit.
Aero-tip of the day: Don’t overlook the
vital need to know deviation’s effect on your magnetic
compass.