Refer to figure.
EASA learning objective 022.03.02.01.02 states: ‘Explain the principles of and the reasons for: compass swinging (determination of initial deviations); compass compensation (correction of deviations found); compass calibration (determination of residual deviations).’
EASA learning objective 022.03.02.01.03 states: ‘Explain how permanent magnetism within the aircraft structure and electromagnetism from the aircraft systems affect the accuracy of a compass.’

The direct reading compass (DRC) aligns itself with the Earths magnetic field and should theoretically point to magnetic North. Unfortunately an aircraft has its own magnetic field, generated by its component parts, which can cause a difference between where the compass points to and Magnetic North!  This direction is known as Compass North and the difference between Compass North and Magnetic North is called deviation.

Aircraft magnetic fields can be generated by ferromagentic materials used in airframe components or electromagnetism caused by electrical equipment.  There are two types of magnetism associated with ferromagnetic material:

• Permanent - refers to materials which once magnetised retain their magnetism.
• Electromagnetic (Temporary) - refers to materials which are easily magnetised but retain little or no magnetism when the magnetic field is removed.

Compass Swing

The deviations caused by aircraft magnetism need to be corrected for as much as possible to minimise the amount of deviation. Deviation will vary with aircraft heading due to the orientation of  the aircraft magnetic field in relation to the Earths magnetic field.

A compass swing compares the DRC compass heading with magnetic North, by taxying the aircraft in a circle onto various cardinal headings, to determine the amount of deviation.  Many airfields have a specific area for this called the ‘compass base’. The initial deviation is corrected by compensating magnets built into the DRC.  After the compass has been compensated it is then calibrated to determine any residual deviation.  Once the swing is complete the residual deviation is recorded on a deviation card which is placed near the DRC to allow pilots to fly the correct magnetic heading.

deviation compensation is noted before the compass swing procedure  → INCORRECT.  This is part of the compass swing procedure, initial deviations are determined then compensated for.  The compass card shows residual deviation that cannot be compensated for during the compass swing.

deviation compensation is noted after the compass swing procedure → INCORRECT.  Compensation is the act of correcting the initial deviation as much as possible with the adjustment magnets during the compass swing.

residual deviation is noted after the compass swing procedure → CORRECT.  After compensation the compass is calibrated to determine any residual deviation and this is noted on a compass deviation correction card after the compass swing.

residual deviation is noted before the compass swing procedure → INCORRECT.  Residual deviation only becomes apparent after determining the initial deviation and correcting for it during the compass swing.