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A follow-up from this question; one of the answers mentioned that the compass rose in the picture was used to calibrate an aircraft's compass.

But how does that actually work? Do you park the aircraft on top or something?

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  • $\begingroup$ @RonBeyer Yeah, that was my impression as well: the compass would be pre-calibrated outside of the aircraft, so the only thing you would really be able to do on the airfield is double check its right. But even with that, I don't see why you can't double check with just a hand-held compass $\endgroup$ – David says Reinstate Monica Feb 8 '16 at 22:33
  • $\begingroup$ For a magnetic a compass, a card is completed with the error for each cardinal point. The pilot takes that error into account when setting the heading. $\endgroup$ – Simon Feb 8 '16 at 22:36
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    $\begingroup$ Usually completing the card is better left to the A&P, not something I'd attempt to accomplish before a flight. I'm not sure that the pilot is allowed to adjust this anyway? (I was wrong in my comment before, it is adjustable). $\endgroup$ – Ron Beyer Feb 8 '16 at 22:37
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    $\begingroup$ @RonBeyer A compass swing is indeed an A&P maintenance item. Technically a non-A&P rated pilot can't even do the last step of checking the error and filling out the card without a sign-off from a certificated mechanic. The compass swing is generally a two-person job anyway though (one person taxiing the plane into position, the other verifying it's properly aligned on each heading from outside). $\endgroup$ – voretaq7 Feb 8 '16 at 22:44
  • $\begingroup$ @DavidGrinberg It's surprising (and a little depressing) how "wrong" a regular hand-held compass can be: If you take one up in a small aircraft and move it around the cabin you can find variations of 10 degrees or more depending on where you hold it. The flux-gate compasses in many modern cell phones (like the iPhone) can also give variable results (if anything they seem easier to upset, even after doing the "calibration" dance the device insists on). $\endgroup$ – voretaq7 Feb 8 '16 at 22:58
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Most aircraft compasses include compensating magnets to correct for installation error (due to natural magnetic fields in an aircraft from steel parts and electrical equipment). When installing a new compass or performing significant electrical work on an aircraft the compass may need to be re-calibrated and the compensating magnets adjusted (typically by turning small screws, like the ones shown below).
Compass compensating screws

There are several ways to calibrate (or "swing") an aircraft compass - the use of a properly-surveyed compass rose constructed to applicable FAA standards and marked on an airport is one of them, and ensures the compass swing is being performed in an area free of magnetic disturbances which should result in an accurate calibration.

The basic procedure is to taxi the aircraft to the compass rose and align it with the compass directions using a combination of the ground markings and a "gunsight" compass. A full description of the procedure can be found in AC 43-13.1, Chapter 12-37 ("Compass Swing"), but the quick-and-dirty version is:

  1. Taxi out to the compass rose and align the aircraft so it is pointing toward Magnetic North.
    • Adjust the compass compensating magnets so that the compass reads North.
  2. Turn the airplane so it is pointing toward Magnetic East (090).
    • Adjust the compass compensating magnets so that the compass reads East.
  3. Turn the airplane so it is pointing toward Magnetic South (180).
    • Adjust the compass compensating magnets to remove one half of the "South" error.
  4. Turn the airplane so it is pointing toward Magnetic West (270)
    • Adjust the compass compensating magnets to remove one half of the "West" error.

Once the compensating magnets have been adjusted a Compass Deviation Card is prepared to inform the pilot of the remaining compass error.
Beginning with Magnetic West (270) turn the aircraft to align with each of the 30-degree lines on the compass rose (270, 300, 330, North, 30, 60, 90, 120, 150, 180, 210, 240), and the error is recorded on a compass deviation card similar to the one shown below.
Compass Deviation Card
A compass reading within ±10 degrees on each 30-degree heading marker is considered acceptable per AC 43-13, and better results are usually achieved.

The compass swing is typically performed with the engine and electrical accessories (radios) running per AC 43-13. Some mechanics may also perform a second swing with radios off, and include an alternate deviation card for that configuration.

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    $\begingroup$ Note that not all aviation compasses have integral compensating magnets. For those that don't you generally skip right to the end and fill out the compass deviation card. If it's within acceptable limits no further action is necessary, if not external compensating hardware ("compass balls") can be used, or the compass can be replaced with one that has compensating magnets. Compass balls are also used with compensating compasses if they can not be brought into calibration using the compensating magnets. $\endgroup$ – voretaq7 Feb 8 '16 at 22:51
  • $\begingroup$ Are compass balls like mouse balls? $\endgroup$ – FreeMan Feb 9 '16 at 17:33
  • $\begingroup$ @FreeMan the ones for aircraft are about that size actually. They're the little cousins of Kelvin's Balls that you'll find on marine vessels for the same reason. $\endgroup$ – voretaq7 Feb 9 '16 at 22:21
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A clever way to alight the aircraft is to put both wheels on the desired radial line that is orthogonal to the desired heading. For example, if you put both main landing gear exactly on the 270 radial line with the right wheel on the inner part of the radial circle, then your aircraft is pointing north (orthogonal to the 27 radial line). You can repeat this procedure for each radial line to get the desired heading. This works particularly well for a high wing aircraft such as a 182. For a low wing aircraft you could use a spotter outside the aircraft to help determine when wheels are on the line.

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