How do you calibrate the compass of an aircraft on a compass rose?
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).


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:


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.

Turn the airplane so it is pointing toward Magnetic East (090).


Adjust the compass compensating magnets so that the compass reads East.

Turn the airplane so it is pointing toward Magnetic South (180).


Adjust the compass compensating magnets to remove one half of the "South" error.

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.

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.