Refer to figure.
The direct reading magnetic compass is designed to align itself with the horizontal component of the Earths magnetic field, so in an ideal world the compass should point to magnetic North. Unfortunately each aircraft has its own magnetic field, caused by aircraft components and electrical systems, which can cause the compass heading to deviate from Magnetic North. This deviation can usually be corrected by adjusting compensating magnets in the direct reading compass during a procedure known as a 'Compass Swing'.

During a compass swing the aircraft will be parked in an allocated area away from external magnetic influences and manoeuvred onto each cardinal heading to compare the compass heading with known magnetic headings. The deviation is recorded and compensated for while the engines are running and all electronic equipment switched on. It's not always possible to remove the deviation on every heading so any residual deviation is recorded a deviation card which is mounted near the compass.

Occasions when a compass swing is required are:

• After a check inspection if required by the approved Maintenance Schedule.
• Whenever inaccuracies in heading indications are reported.
• After any modification, repair or major replacement involving magnetic material, particularly in aircraft the engines of which are mounted in the fuselage or wing nacelles.
• Whenever a compass has been subjected to shock, e.g. after a heavy landing.
• After the aircraft has passed through a severe electrical storm, or has been struck by lightning.
• Whenever the aircraft has been subjected to a magnetic crack detection examination.
• Whenever the sphere of operation of the aircraft is changed to one of different magnetic latitude.
• Whenever a significant change is made to the electrical or radio installation, particularly to circuits in the vicinity of the compass.
• Whenever a freight load is likely to cause magnetic influence and thereby affect compass readings.
• After the aircraft has been in long term storage