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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.

FMI: Aero-Tips