Pilot educational curriculum includes learning about magnetic compass errors intrinsic to their construction.

A standard magnetic compass in an airplane erroneously shows turns when accelerating or decelerating on an east or west heading. They also read incorrectly during turns from the north or the south.

My question is about more advanced instruments like a fluxgate compass found slaved to an HSI or a magnetometer used in an AHRS. Are these systems impervious to the compass errors present in a magnetic compass?


The short answer is yes, but for different reasons.

The analog HSI in most aircraft that have them either has an internal directional gyroscope, or it is slaved to the directional gyro (more often the former than the latter as the HSI often replaces the DG). An analog DG/HSI must still be periodically corrected for precession error by levelling off, letting both instruments stabilize and then adjusting the HSI's card to the magnetic compass.

AHRS systems that power glass cockpits use a solid-state directional gyro that is corrected by a magnetometer (an antenna device that directly senses the relative direction of the magnetic field) rather than a floating magnet. This makes them impervious to the types of error commonly seen in floating compasses such as acceleration bias (which causes all the behaviors you mentioned). They can still be sensitive to nearby ferrous metal structural members and to electrical currents, requiring a digital version of "spinning the compass" with corrections input into the software that uses the magnetometer readings, and magnetic declination must also be accounted for (either mentally by the pilot, manually based on declination charts or in more sophisticated systems by directly incorporating position data from the GPS).

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    $\begingroup$ In other industries, one would usually still use a gyro for something like an HSI, because magnetometers tend to be noisy. You can filter out the noise, but doing that makes the instrument slow (at least relative to a gyroscope). Instead the magnetometer is often best used to slowly apply automatic drift correction to a gyro's output. I would not be surprised if many/most glass cockpits actually did that (using a solid state gyro, of course). $\endgroup$ – Kevin Cathcart Aug 3 '15 at 21:50
  • $\begingroup$ Most of the time, magnetic declination does not have to be accounted for, because all headings are normally given in reference to magnetic north. The only exception is near the magnetic poles where true headings are used. $\endgroup$ – Jan Hudec Aug 4 '15 at 8:01

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